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Introduction to Java programming. This tutorial explains the installation and usage of the Java programming language. It also contains examples for standard programming tasks.

1. Introduction to Java

1.1. A small history of Java

Java is a programming language created by James Gosling from Sun Microsystems (Sun) in 1991. The target of Java is to write a program once and then run this program on multiple operating systems. The first publicly available version of Java (Java 1.0) was released in 1995. Sun Microsystems was acquired by the Oracle Corporation in 2010. Oracle has now the steermanship for Java. In 2006 Sun started to make Java available under the GNU General Public License (GPL). Oracle continues this project called OpenJDK.

Over time new enhanced versions of Java have been released. The current version of Java is Java 1.8 which is also known as Java 8.

Java is defined by a specification and consists of a programming language, a compiler, core libraries and a runtime (Java virtual machine) The Java runtime allows software developers to write program code in other languages than the Java programming language which still runs on the Java virtual machine. The Java platform is usually associated with the Java virtual machine and the Java core libraries.

The Java language was designed with the following properties:

  • Platform independent: Java programs use the Java virtual machine as abstraction and do not access the operating system directly. This makes Java programs highly portable. A Java program (which is standard-compliant and follows certain rules) can run unmodified on all supported platforms, e.g., Windows or Linux.

  • Object-orientated programming language: Except the primitive data types, all elements in Java are objects.

  • Strongly-typed programming language: Java is strongly-typed, e.g., the types of the used variables must be pre-defined and conversion to other objects is relatively strict, e.g., must be done in most cases by the programmer.

  • Interpreted and compiled language: Java source code is transferred into the bytecode format which does not depend on the target platform. These bytecode instructions will be interpreted by the Java Virtual machine (JVM). The JVM contains a so called Hotspot-Compiler which translates performance critical bytecode instructions into native code instructions.

  • Automatic memory management: Java manages the memory allocation and de-allocation for creating new objects. The program does not have direct access to the memory. The so-called garbage collector automatically deletes objects to which no active pointer exists.

The Java syntax is similar to C++. Java is case-sensitive, e.g., variables called myValue and myvalue are treated as different variables.

1.2. Hello world Java program

// a small Java program
public class HelloWorld {
        public static void main(String[] args) {
                System.out.println("Hello World");
        }
}

1.3. Java virtual machine

The Java virtual machine (JVM) is a software implementation of a computer that executes programs like a real machine.

The Java virtual machine is written specifically for a specific operating system, e.g., for Linux a special implementation is required as well as for Windows.

jvmintroduction10

Java programs are compiled by the Java compiler into bytecode. The Java virtual machine interprets this bytecode and executes the Java program.

1.4. Java Runtime Environment vs. Java Development Kit

A Java distribution typically comes in two flavors, the Java Runtime Environment (JRE) and the Java Development Kit (JDK).

The JRE consists of the JVM and the Java class libraries. Those contain the necessary functionality to start Java programs.

The JDK additionally contains the development tools necessary to create Java programs. The JDK therefore consists of a Java compiler, the Java virtual machine and the Java class libraries.

1.5. Development Process with Java

Java source files are written as plain text documents. The programmer typically writes Java source code in an Integrated Development Environment (IDE) for programming. An IDE supports the programmer in the task of writing code, e.g., it provides auto-formating of the source code, highlighting of the important keywords, etc.

At some point the programmer (or the IDE) calls the Java compiler ( javac ). The Java compiler creates the bytecode instructions. These instructions are stored in .class files and can be executed by the Java Virtual Machine.

1.6. Garbage collector

The JVM automatically re-collects the memory which is not referred to by other objects. The Java garbage collector checks all object references and finds the objects which can be automatically released.

While the garbage collector relieves the programmer from the need to explicitly manage memory, the programmer still need to ensure that he does not keep unneeded object references, otherwise the garbage collector cannot release the associated memory. Keeping unneeded object references are typically called memory leaks.

1.7. Classpath

The classpath defines where the Java compiler and Java runtime look for .class files to load. These instructions can be used in the Java program.

For example, if you want to use an external Java library you have to add this library to your classpath to use it in your program.

2. Installation of Java

2.1. Check installation

To run Java programs on your computer you must at least have the Java runtime environment (JRE) installed. This might already be the case on your machine. You can test is the JRE is installed and in your current path by opening a console (if you are using Windows: Win+R, enter cmd and press Enter) and by typing in the following command:

java -version

If the JRE is installed and within your path, this commands print information about your Java installation. In this case you can skip the Java installation description. You may want to note down if you have a 32 bit or 64 bit version of Java, see How can you tell you are using a 32 bit or 64 bit version of Java? .

If the command line returns the information that the program could not be found, you have to install Java.

2.2. Install Java on Ubuntu

On Ubuntu you can install Java 8 via the following command on the command line. These commands might change over time, if they do not work anymore please Google for the installation.

# install Java from the OpenJDK project
sudo apt-get install openjdk-8-jdk

# in case you prefer the Oracle Java distribution
# you can alternatively use the following commands
sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update
sudo apt-get install oracle-java8-installer

2.3. Install Java on MS Windows

For Microsofts Windows, Oracle provides a native installer which can be found on the Oracle website. The central website for installing Java is located under the following URL and also contains instructions how to install Java for other platforms: http://java.com.

2.4. Installation problems and other operating systems

If you have problems installing Java on your system, search via Google for How to install JDK on YOUR OS. This should result in helpful links. Replace YOUR OS with your operating system, e.g., Windows, Ubuntu, Mac OS X, etc.

2.5. Validate installation

Switch again to the command line and run the following command.

java -version

The output should be similar to the following output.

openjdk version "1.8.0_45-internal"
OpenJDK Runtime Environment (build 1.8.0_45-internal-b14)
OpenJDK 64-Bit Server VM (build 25.45-b02, mixed mode)

2.6. How can you tell you are using a 32 bit or 64 bit version of Java?

You can run a 32 bit or a 64 bit version of Java on a 64 bit system. If you use java -version and the output contains the "64-Bit" string you are using the 64 bit version of Java otherwise your are using the 32 bit version. The following is the output of a 64-bit version.

openjdk version "1.8.0_45-internal"
OpenJDK Runtime Environment (build 1.8.0_45-internal-b14)
OpenJDK 64-Bit Server VM (build 25.45-b02, mixed mode)

3. Exercise: Write, compile and run a Java program

3.1. Write source code

The following Java program is developed under Linux using a text editor and the command line. The process on other operating system should be similar, but is not covered in this description.

Select or create a new directory which will be used for your Java development. In this description the path \home\vogella\javastarter is used. On Microsoft Windows you might want to use c:\temp\javastarter. This path is called javadir in the following description.

Open a text editor which supports plain text, e.g., gedit under Linux or Notepad under Windows and write the following source code.

// a small Java program
public class HelloWorld {
        public static void main(String[] args) {
                System.out.println("Hello World");
        }
}
Do not use a rich editor like Microsoft Word or LibreOffice for writing Java code. If in doubt, google for "Plain text editor for [your_OS]".
Writing Java programs in a text editor

Save the source code in your javadir directory with the HelloWorld.java filename. The name of a Java source file must always equal the class name (within the source code) and end with the .java extension. In this example the filename must be HelloWorld.java, because the class is called HelloWorld.

3.2. Compile and run your Java program

Open a shell for command line access.

If you don’t know how to do this, google for "How to open a shell under [your_OS]".

Switch to the javadir directory with the command cd javadir, for example, in the above example via the cd \home\vogella\javastarter command. Use the ls command (dir under Microsoft Windows) to verify that the source file is in the directory.

Compile your Java source file into a class file with the following command.

javac HelloWorld.java

Afterwards list again the content of the directory with the ls or dir command. The directory contains now a file HelloWorld.class. If you see this file, you have successfully compiled your first Java source code into bytecode.

By default, the compiler puts each class file in the same directory as its source file. You can specify a separate destination directory with the -d compiler flag.

You can now start your compiled Java program. Ensure that you are still in the jardir directory and enter the following command to start your Java program.

java HelloWorld

The system should write "Hello World" on the command line.

Compiling and running Java programs on the command line

3.3. Using the classpath

You can use the classpath to run the program from another place in your directory.

Switch to the command line, e.g., under Windows Start ▸ Run cmd. Switch to any directory you want. Type:

java HelloWorld

If you are not in the directory in which the compiled class is stored, then the system will show an error message: Exception in thread "main" java.lang.NoClassDefFoundError: test/TestClass

To use the class, type the following command. Replace "mydirectory" with the directory which contains the test directory. You should again see the "HelloWorld" output.

java -classpath "mydirectory" HelloWorld

4. Base Java language structure

4.1. Class

A class is a template that describes the data and behavior associated with an instance of that class.

A class is defined by the class keyword and must start with a capital letter. The body of a class is surrounded by {}.

package test;

class MyClass {

}

The data associated with a class is stored in variables ; the behavior associated to a class or object is implemented with methods.

A class is contained in a text file with the same name as the class plus the .java extension. It is also possible to define inner classes, these are classes defined within another class, in this case you do not need a separate file for this class.

4.2. Object

An object is an instance of a class.

The object is the real element which has data and can perform actions. Each object is created based on the class definition. The class can be seen as the blueprint of an object, i.e., it describes how an object is created.

4.3. Package

Java groups classes into functional packages.

Packages are typically used to group classes into logical units. For example, all graphical views of an application might be placed in the same package called com.vogella.webapplication.views.

It is common practice to use the reverse domain name of the company as top level package. For example, the company might own the domain, vogella.com and in this example the Java packages of this company starts with com.vogella.

Other main reason for the usage of packages is to avoid name collisions of classes. A name collision occurs if two programmers give the same fully qualified name to a class. The fully qualified name of a class in Java consists of the package name followed by a dot (.) and the class name.

Without packages, a programmer may create a Java class called Test. Another programmer may create a class with the same name. With the usage of packages you can tell the system which class to call. For example, if the first programmer puts the Test class into package report and the second programmer puts his class into package xmlreader you can distinguish between these classes by using the fully qualified name , e.g, xmlreader.Test or report.Test.

4.4. Inheritance

A class can be derived from another class. In this case this class is called a subclass. Another common phrase is that a class extends another class.

The class from which the subclass is derived is called a superclass.

Inheritance allows a class to inherit the behavior and data definitions of another class.

The following codes demonstrates how a class can extend another class. In Java a class can only extend a maximum of one class.

package com.vogella.javaintro.base;

class MyBaseClass {

        public void hello() {
                System.out.println("Hello from MyBaseClass");
        }
}
package com.vogella.javaintro.base;

class MyExtensionClass extends MyBaseClass {
}

4.5. Object as superclass

Every object in Java implicitly extends the Object class. The class defines the following methods for every Java object: * equals(o1) allows checking if the current object is equal to o1

  • getClass() returns the class of the object

  • hashCode() returns an identifier of the current object

  • toString() gives a string representation of the current object

4.6. Exception handling in Java

In Java an exception is an event to indicate an error during the runtime of an application. So this disrupts the usual flow of the application’s instructions.

In general exceptions are thrown up in the call hierarchy until they get catched.

exception throw callstack

4.7. Checked Exceptions

Checked Exceptions are explicitly thrown by methods, which might cause the exception or re-thrown by methods in case a thrown Exception is not caught.

So when calling methods, which throw checked Exceptions the Exceptions have either to be caught or to be re-thrown.

public void fileNotFoundExceptionIsCaughtInside() {
        try {
                createFileReader();
        } catch (FileNotFoundException e) {
                logger.error(e.getMessage(), e);
        }
}

public void fileNotFoundExceptionIsReThrown() throws FileNotFoundException {
        createFileReader();
}

public void createFileReader() throws FileNotFoundException {
        File file = new File("/home/Documents/JavaTraining.txt");

        // creating a new FileReader can cause a FileNotFoundException
        new FileReader(file);
}

Checked Exceptions are used when an error can be predicted under certain circumstances, e.g., a file which cannot be found.

4.8. Runtime Exceptions

Runtime Exceptions are Exceptions, which are not explicitly mentioned in the method signature and therefore also do not have to be catched explicitly.

The most famous runtime exception is the NullPointerException, which occurs during runtime, when a method is invoked on an object, which actually is null.

public void causeANullPointerException() {
        String thisStringIsNull = getMessage(false);

        // because the thisStringIsNull object is null
        // this will cause a NullPointerException
        thisStringIsNull.toLowerCase();
}

public String getMessage(boolean messageIsAvailable) {
        if(messageIsAvailable) {
                return message;
        }

        return null;
}

5. Java interfaces

5.1. What is an interface in Java?

An interfaces is a type similar to a class and is defined via the interface keyword. Interfaces are used to define common behavior of implementing classes. If two classes implement the same interface, other code which work on the interface level, can use objects of both classes.

Like a class an interface defines methods. Classes can implement one or several interfaces. A class which implements an interface must provide implementation for all abstract methods defined in the interface.

5.2. Abstract, default and static methods in Interfaces

An interface can have abstract methods and default methods. A default method is defined via the default keyword at the beginning of the method signature. All other methods defined in an interfaces are public and abstract – explicit declaration of these modifiers is optional.

Interfaces can have constants which are always implicitly public, static and final.

The following code shows an example implementation of an interface.

package testing;

public interface MyInterface {

                // constant definition
                String URL = "http://www.vogella.com";

                // public abstract methods
                void test();
                void write(String s);

                // default method
                default String reserveString(String s){
                        return new StringBuilder(s).reverse().toString();
                }
}

5.3. Implementing Interfaces

A class can implement an interface. In this case it must provide concrete implementations of the abstract interface methods. If you implement a method defined by an interface, you can use @Override annotation to indicate to the Java compiler that you actually want to implement a method defined by this interface. This way the compiler can give you an error in you mis-typed the name of the method or in the number of arguments. The following class implements the MyInterface interface, its must therefore implement the abstract method and can use the default methods.

package com.vogella.javaintro.base;

public class MyClassImpl implements MyInterface {

        @Override
        public void test() {
        }

        @Override
        public void write(String s) {
        }

        public static void main(String[] args) {
                MyClassImpl impl = new MyClassImpl();
                System.out.println(impl.reserveString("Lars Vogel"));
        }

}

5.4. Evolving interfaces with default methods

Before Java 8 evolving interfaces, e.g., adding new methods to an interface, was not possible because such a change would break existing implementations. Java 8 introduced default methods, now you can extend an interface without breaking clients by simply suppling a default implementation with it. Adding such a default method is a source an binary compatible change.

A class can always override a default method to supply a better behavior.

5.5. Multiple inheritance of methods

If a class implements two interfaces and if these interfaces provide the same default method, Java resolves the correct method for the class by the following rules:

  • Superclass wins always against the superinterface - If a class can inherit a method from a superclass and a superinterface, the class inherts the superclass method. This is true for concrete and abstract superclass methods. This rule implies that default methods are not used if this method is also declared in the superclass chain.

  • Subtypes win over Supertypes - If a class can inherit a method from two interfaces, and one is a subtype of the other, the class inherts the method from the subtype

  • In all other cases the class needs to implement the default method

The following listing demonstrates listing number 3.

public interface A {
        default void m() {}
}

public interface B {
        default void m() {}
}

public class C implements A, B {
        @Override
        public void m() {}
}

In your implementation you can also call the super method you prefer.

public class C implements A, B {
        @Override
        public void m() {A.super.m();}
}

5.6. Functional interfaces

All interfaces that have only one method are called functional interfaces. Functional interfaces have the advantage that they can be used together with lambda expressions. See What are lambdas? to learn more about lambdas, e.g., the type of lambdas is a functional interface.

The Java compiler automatically identifies functional interfaces. The only requirement is that they have only one abstract method. However, is possible to capture the design intent with a @FunctionalInterface annotation.

Several default Java interfaces are functional interfaces:

  • java.lang.Runnable

  • java.util.concurrent.Callable

  • java.io.FileFilter

  • java.util.Comparator

  • java.beans.PropertyChangeListener

Java also contains the java.util.function package which contains functional interfaces which are frequently used such as:

  • Predicate<T> - a boolean-valued property of an object

  • Consumer<T> - an action to be performed on an object

  • Function<T , R> - a function transforming a T to a R

  • Supplier<T> - provides an instance of T (such as a factory)

  • UnaryOperator<T> - a function from T to T

  • BinaryOperator<T> - a function from (T, T) to T

6. Annotations in Java

6.1. Annotations in Java

Annotations provide data about a class that is not part of the programming logic itself. They have no direct effect on the code they annotate. Other components can use this information.

Annotations can be preserved at runtime (RetentionPolicy.RUNTIME) or are only available at development time (RetentionPolicy.SOURCE).

6.2. Override methods and the @Override annotation

If a class extends another class, it inherits the methods from its superclass. If it wants to change these methods, it can override these methods, i.e., redeclare the methods. This is necessary for an abstract method unless the class itself is defined as abstract.

The @Override annotation can be added to such a method. It is used by the Java compiler to check if the annotated method really overrides a method of an interface or the extended class.

To override a method, you use the same method signature in the source code of the subclass.

To indicate to the reader of the source code and the Java compiler that you have the intention to override a method, you can use the @Override annotation.

The following code demonstrates how you can override a method from a superclass.

package com.vogella.javaintro.base;

class MyBaseClass {

        public void hello() {
                System.out.println("Hello from MyBaseClass");
        }
}
package com.vogella.javaintro.base;

class MyExtensionClass2 extends MyBaseClass {

        @Override
        public void hello() {
                System.out.println("Hello from MyExtensionClass2");
        }
}

It is good practice to always use the @Override annotation. This way the Java compiler validates if you did override all methods as intended and prevents errors.

6.3. The @Deprecated annotations

The @Deprecated annotation can be used on a field, method, constructor or class and indicates that this element is outdated and should not be used anymore. Adding @Deprecated to the class does not deprecate automatically all its fields and methods.

6.4. Type annotations

Java supports that annotations can be placed on any type. The following gives several examples assuming the annotations exists.

@NonNull String name;
List<@NonNull String> names;
class UnmodifiableList<T> implements @Readonly List<@Readonly T> {...}
email = (@Email String) input;
new @Information MyObject();
void doSomething() throws @ImportantForMe MyException { ... }

7. Variables and methods

7.1. Variable

Variables allow the Java program to store values during the runtime of the program.

A variable can either be a primitive variable or a reference variable. A primitive variable contains the value while the reference variable contains a reference (pointer) to the object. Hence, if you compare two reference variables, you compare if both point to the same object. To compare objects, use the object1.equals(object2) method call.

7.2. Instance variable

Instance variable is associated with an instance of the class (also called object). Access works over these objects.

Instance variables can have any access control and can be marked final or transient. Instance variables marked as final cannot be changed after a value has been assigned to them.

7.3. Local variable

Local (stack) variable declarations cannot have access modifiers.

final is the only modifier available to local variables. This modifier defines that the variable cannot be changed after the first assignment.

Local variables do not get default values, so they must be initialized before use.

7.4. Methods

A method is a block of code with parameters and a return value. It can be called on the object.

package com.vogella.javaintro.base;

public class MyMethodExample {
        void tester(String s) {
                System.out.println("Hello World");
        }
}

Methods can be declared with var-args. In this case the method declares a parameter which accepts everything from zero to many arguments (syntax: type …​ name;) A method can only have one var-args parameter and this must be the last parameter in the method.

Overwrite of a superclass method: A method must be of the exact same return parameter and the same arguments. Also the return parameter must be the same. Overload methods: An overloaded method is a method with the same name, but different arguments. The return type can not be used to overload a method.

7.5. Main method

A public static method with the following signature can be used to start a Java application. Such a method is typically called main method.

public static void main(String[] args) {

}

7.6. Constructor

A class contains constructors that are invoked to create objects based on the class definition.

Constructor declarations look like method declarations except that they use the name of the class and have no return type.

A class can have several constructors with different parameters. Each class must define at least one constructor.

In the following example the constructor of the class expects a parameter.

package com.vogella.javaintro.base;

public class MyConstructorExample2 {

        String s;

        public MyConstructorExample2(String s) {
                this.s = s;
        }
}

If no explicit constructor is defined, the compiler implicitly adds a constructor. If the class is sub-classed, then the constructor of the super class is always called implicitly in this case.

In the following example the definition of the constructor without parameters (also known as the empty constructor) is unnecessary. If not specified, the compiler would create one.

package com.vogella.javaintro.base;

public class MyConstructorExample {

        // unnecessary: would be created by the compiler if left out
        public MyConstructorExample() {
        }
}

The naming convention for creating a constructor is the following: `classname (Parameter p1, …​) { } `.

Every object is created based on a constructor. This constructor method is the first statement called before anything else can be done with the object.

8. Modifiers

8.1. Access modifiers

There are three access modifiers keywords available in Java: public, protected and private.

There are four access levels: public, protected, default and private. They define how the corresponding element is visible to other components.

If something is declared public, e.g., classes or methods can be freely created or called by other Java objects. If something is declared private, e.g., a method, it can only be accessed within the class in which it is declared.

The access levels protected and default are similar. A protected class can be accessed from the package and sub-classes outside the package, while a default class can get accessed only via the same package.

The following table describes the visibility:

Table 1. Access Level
Modifier Class Package Subclass World

public

Y

Y

Y

Y

protected

Y

Y

Y

N

no modifier

Y

Y

N

N

private

Y

N

N

N

8.2. Other modifiers

final methods: cannot be overwritten in a subclass abstract method: no method body

synchronized method: thread safe, can be final and have any access control

native methods: platform dependent code, apply only to methods

strictfp: class or method

9. Import statements

9.1. Usage of import statements

In Java you have to access a class always via its full-qualified name, i.e., the package name and the class name.

You can add import statements for classes or packages into your class file, which allow you to use the related classes in your code without the package qualifier.

9.2. Static imports

Static import is a feature that allows members (fields and methods) which are defined in a class with the public static access modifier to be used in Java code without specifying the class in which the member is defined.

The feature provides a typesafe mechanism to include constants into code without having to reference the class that originally defined the field.

10. More Java language constructs

10.1. Class methods and class variables

Class methods and class variables are associated with the class and not an instance of the class, i.e., objects. To refer to these elements, you can use the classname and a dot (".") followed by the class method or class variable name.

Class methods and class variables are declared with the static keyword. Class methods are also called static methods and class variables are also called static variables or static fields.

An example for the usage of a static field is println of the following statement: System.out.println("Hello World"). Hereby out is a static field, an object of type PrintStream and you call the println() method on this object.

If you define a static variable, the Java runtime environment associates one class variable for a class no matter how many instances (objects) exist. The static variable can therefore be seen as a global variable.

The following code demonstrates the usage of static fields.

package com.vogella.javaintro.base;

public class MyStaticExample {

        static String PLACEHOLDER = "TEST";

        static void test() {
                System.out.println("Hello");
        }
}
package com.vogella.javaintro.base;

public class Tester {

        public static void main(String[] args) {
                System.out.println(MyStaticExample.PLACEHOLDER);
                MyStaticExample.test();
        }

}

If a variable should be defined as constant, you declare it with the static and the final keyword.

The static method runs without any instance of the class, it cannot directly access non-static variables or methods.

10.2. Abstract class and methods

A class and method can be declared as abstract. An abstract class can not be directly instantiated.

If a class has at least one method, which only contains the declaration of the method, but not the implementation, then this class is abstract and can not be instantiated. Sub-classes need then to define the methods except if they are also declared as abstract.

If a class contains an abstract method, it also needs to get defined with the keyword abstract.

The following example shows an abstract class.

package com.vogella.javaintro.base;

public abstract class MyAbstractClass {
        abstract double returnDouble();
}

11. Cheat Sheets

The following can be used as a reference for certain task which you have to do.

11.1. Working with classes

While programming Java you have to create several classes, methods, instance variables. The following uses the package test.

Table 2. Table Title

What to do

How to do it

Create a new class called MyNewClass.

package test;

public class MyNewClass {

}

Create a new attribute (instance variable) called var1 of type String in the MyNewClass class

package test;

public class MyNewClass {
        private String var1;
}

Create a Constructor for your MyNewClass class which has a String parameter and assigns the value of it to the var1 instance variable.

package test;

public class MyNewClass {
        private String var1;

        public MyNewClass(String para1) {
                var1 = para1;
                // or this.var1= para1;
        }
}

Create a new method called doSomeThing in your class which does not return a value and has no parameters.

package test;

public class MyNewClass {
        private String var1;

        public MyNewClass(String para1) {
                var1 = para1;
                // or this.var1= para1;
        }

        public void doSomeThing() {

        }

}

Create a new method called doSomeThing2 in your class which does not return a value and has two parameters, an int and a Person

package test;

public class MyNewClass {
        private String var1;

        public MyNewClass(String para1) {
                var1 = para1;
                // or this.var1= para1;
        }

        public void doSomeThing() {

        }

        public void doSomeThing2(int a, Person person) {

        }

}

Create a new method called doSomeThing2 in your class which returns an int value and has three parameters, two Strings and a Person.

package test;

public class MyNewClass {
        private String var1;

        public MyNewClass(String para1) {
                var1 = para1;
                // or this.var1= para1;
        }

        public void doSomeThing() {

        }

        public void doSomeThing2(int a, Person person) {

        }

        public int doSomeThing3(String a, String b, Person person) {
                return 5; // any value will do for this example
        }

}

Create a class called MyOtherClass with two instance variables. One will store a String, the other will store a Dog. Create getter and setter for these variables.

package test;

public class MyOtherClass {
        String myvalue;
        Dog dog;

        public String getMyvalue() {
                return myvalue;
        }

        public void setMyvalue(String myvalue) {
                this.myvalue = myvalue;
        }

        public Dog getDog() {
                return dog;
        }

        public void setDog(Dog dog) {
                this.dog = dog;
        }
}

11.2. Working with local variable

A local variable must always be declared in a method.

Table 3. Table Title
What to do How to do it

Declare a (local) variable of type String.

String variable1;

Declare a (local) variable of type String and assign "Test" to it.

String variable2 = "Test";

Declare a (local) variable of type Person

Person person;

Declare a (local) variable of type Person, create a new Object and assign the variable to this object.

Person person = new Person();

Declare an array of type String

String array[];

Declare an array of type Person and create an array for this variable which can hold 5 persons.

Person array[]= new Person[5];

Assign 5 to the int variable var1 (which was already declared);

var1 = 5;

Assign the existing variable pers2 to the existing variable pers1;

pers1 = pers2;

Declare an ArrayList variable which can hold objects of type Person

ArrayList<Person> persons;>

Create a new ArrayList with objects of type Person and assign it to the existing variable persons.

<_persons = new ArrayList<Person>();_>

Declare an ArrayList variable which can hold objects of type Person and create a new Object for it.

ArrayList<Person> persons = new ArrayList<Person>();>

12. Integrated Development Environment

The previous chapter explained how to create and compile a Java program on the command line. A Java Integrated Development Environment (IDE) provides lots of ease of use functionality for creating Java programs. There are other very powerful IDEs available, for example, the Eclipse IDE.

For an introduction on how to use the Eclipse IDE please see Eclipse IDE Tutorial.

The remaining description uses the phrase: "Create a Java project called…​". This refers to creating a Java project in Eclipse. If you are using a different IDE, please follow the required steps in that IDE.

13. Exercises - Creating Java objects and methods

13.1. Create a Person class and instantiate it

Create a new Java project called com.vogella.javastarter.exercises1 and a package with the same name.

Create a class called Person.

Add three instance variables to it, one for storing the first name of the person, one for storing the last name and one for storing the age of the Person.

Use the constructor of the Person object to set the values to some default value.

Add a toString method as described by the following codeing and solve the TODO. This method is used to convert the object to a String representation.

@Override
public String toString() {
    // TODO replace "" with the following:
    // firstName + " " + lastName
        return "";
}

Create a new class called Main with a public static void main(String[] args). In this method create an instance of the Person class.

13.2. Use constructor

Add a constructor to your Person class which takes first name, last name and age as parameter. Assign the values to your instance variables.

In your main method create at least one object of type Person and use System.out.println() with the object as parameter.

13.3. Define getter and setter methods

Define methods which allow you to read the values of the instance variables and to set them. These methods are called setter and getter.

Getters should start with get followed by the variable name whereby the first letter of the variable is capitalized.

Setter should start with set followed by the variable name whereby the first letter of the variable is capitalized.

For example, the variable called firstName would have the getFirstName() getter method and the setFirstName(String s) setter method.

Change your main method so that you create one Person object and use the setter method to change the last name.

13.4. Create an Address object

Create a new object called Address. The Address should allow you to store the address of a person.

Add a new instance variable of this type in the Person object. Also, create a getter and setter for the Address object in the Person object.

14. Solution - Creating Java objects and methods

14.1. Create a Person class and instantiate it

The following is the expected result after Create a Person class and instantiate it .

package exercises.exercise04;

class Person {
        String firstname = "Jim";
        String lastname = "Knopf";
        int age = 12;

        @Override
        public String toString() {
                return firstName + " " + lastName;
        }

}
package exercises.exercise04;

public class Main {
        public static void main(String[] args) {
                Person person = new Person();

                //  this calls the toString method on the pers object
                System.out.println(pers);
        }
}

14.2. Use constructor

The following is the expected result after Use constructor .

package com.vogella.javastarter.exercises1;


class Person {
        String firstName;
        String lastName;
        int age;

        public Person(String a, String b, int value) {
                firstName = a;
                lastName = b;
                age=value;
        }

        @Override
        public String toString() {
                return firstName + " " + lastName;
        }

}
package com.vogella.javastarter.exercises1;

public class Main {
        public static void main(String[] args) {
                Person p1 = new Person("Jim", "Knopf" , 12 );
                System.out.println(p1);

                // reuse the same variable and assign a new object to it
                Person p2 = new Person("Henry", "Ford", 104);
                System.out.println(p2);
        }
}

The following is the expected result after Define getter and setter methods .

14.3. Define getter and setter methods

package com.vogella.javastarter.exercises1;

class Person {
        String firstName;
        String lastName;
        int age;

        public Person(String a, String b, int value) {
                firstName = a;
                lastName = b;
                age = value;
        }

        public String getFirstName() {
                return firstName;
        }

        public void setFirstName(String firstName) {
                this.firstName = firstName;
        }

        public String getLastName() {
                return lastName;
        }

        public void setLastName(String lastName) {
                this.lastName = lastName;
        }

        public int getAge() {
                return age;
        }

        public void setAge(int age) {
                this.age = age;
        }

        @Override
        public String toString() {
                return firstName + " " + lastName;
        }


}
package com.vogella.javastarter.exercises1;

public class Main {
        public static void main(String[] args) {
                Person person = new Person("Jim", "Knopf", 21);
                Person p2 = new Person("Jill", "Sanders", 20);
                // Jill gets married to Jim
                // and takes his name
                p2.setLastName("Knopf");
                System.out.println(p2);
        }
}

14.4. Solution - Create an Address object

The following is the expected result after Create an Address object.

package com.vogella.javastarter.exercises1;

public class Address {

        private String street;
        private String number;
        private String postalCode;
        private String city;
        private String country;

        public String getStreet() {
                return street;
        }

        public void setStreet(String street) {
                this.street = street;
        }

        public String getNumber() {
                return number;
        }

        public void setNumber(String number) {
                this.number = number;
        }

        public String getPostalCode() {
                return postalCode;
        }

        public void setPostalCode(String postalCode) {
                this.postalCode = postalCode;
        }

        public String getCity() {
                return city;
        }

        public void setCity(String city) {
                this.city = city;
        }

        public String getCountry() {
                return country;
        }

        public void setCountry(String country) {
                this.country = country;
        }

        public String toString() {
                return street + " " + number + " " + postalCode + " " + city + " "
                                + country;
        }

}
package com.vogella.javastarter.exercises1;


class Person {
        String firstName;
        String lastName;
        int age;
        private Address address;

        public Person(String a, String b, int value) {
                firstName = a;
                lastName = b;
                age=value;
        }

        public String getFirstName() {
                return firstName;
        }

        public void setFirstName(String firstName) {
                this.firstName = firstName;
        }

        public String getLastName() {
                return lastName;
        }

        public void setLastName(String lastName) {
                this.lastName = lastName;
        }

        public int getAge() {
                return age;
        }

        public void setAge(int age) {
                this.age = age;
        }


        public Address getAddress() {
                return address;
        }

        public void setAddress(Address address) {
                this.address = address;
        }

        @Override
        public String toString() {
                return firstName + " " + lastName;
        }

}
package com.vogella.javastarter.exercises1;

public class Main {
        public static void main(String[] args) {
                // I create a person
                Person pers = new Person("Jim", "Knopf", 31);
                // set the age of the person to 32
                pers.setAge(32);

                // just for testing I write this to the console
                System.out.println(pers);
                /*
                 * actually System.out.println always calls toString, if you do not
                 * specify it so you could also have written System.out.println(pers);
                 */
                // create an address
                Address address = new Address();
                // set the values for the address
                address.setCity("Heidelberg");
                address.setCountry("Germany");
                address.setNumber("104");
                address.setPostalCode("69214");
                address.setStreet("Musterstr.");

                // assign the address to the person
                pers.setAddress(address);

                // dispose reference to address object
                address = null;

                // person is moving to the next house in the same street
                pers.getAddress().setNumber("105");

        }

}

15. Java statements

The Java language defines certain statements with a predefined meaning. The following description lists some of them.

15.1. if-then and if-then-else

The if-then statement is a control flow statement. A block of code is only executed when the test specified by the if part evaluates to true. The optional else block is executed when the if part evaluates to false.

The following example code shows a class with two methods. The first method demonstrates the usage of if-then and the second method demonstrates the usage of if-then-else.

15.2. Switch

The switch statement can be used to handle several alternatives if they are based on the same constant value.

switch (expression) {
        case constant1:
                command;
                break; // will prevent that the other cases or also executed
        case constant2:
                command;
                break;
                ...
        default:
}

// Example:

switch (cat.getLevel()) {
        case 0:
                return true;
        case 1:
                if (cat.getLevel() == 1) {
                        if (cat.getName().equalsIgnoreCase(req.getCategory())) {
                                return true;
                        }
                }
        case 2:
                if (cat.getName().equalsIgnoreCase(req.getSubCategory())) {
                        return true;
                }
}

// you can also use the same logic for different constants

        public static void main(String[] args) {
                for (int i = 0; i < 6; i++) {
                        switch (i) {
                        case 1:
                        case 5:
                                System.out.println("Hello");
                                break;
                        default:
                                System.out.println("Default");
                                break;
                        }
                }
        }

}

15.3. Boolean Operations

Use == to compare two primitives or to see if two references refer to the same object. Use the equals() method to see if two different objects are equal.

&& and || are both Short Circuit Methods which means that they terminate once the result of an evaluation is already clear. Example (true || …​) is always true while (false && …​) always is always interpreted as false. Usage:

(var !=null && var.method1() …​) ensures that var is not null before doing the real check.

Table 4. Boolean
Operations Description

==

Is equal, in case of objects the system checks if the reference variable point to the same object. It will not compare the content of the objects!

&&

And

!=

is not equal, similar to ==.

a.equals(b)

Checks if string a equals b.

a.equalsIgnoreCase(b)

Checks if string a equals b while ignoring lower cases.

If (value ? false : true) {}

Negotiation: return true if value is not true.

16. Loops in Java

16.1. The for loop

A for loop is a repetition control structure that allows you to write a block of code which is executed a specific number of times. The syntax is the following.

for(initialization; expression; update_statement)
{
   //block of code to run
}

The following shows an example for a for loop.

public class ForTest {

        public static void main(String args[]) {

        for(int i = 1; i < 10; i = i+1) {
                System.out.println("value of i : " + i );
                }
        }
}

TIP:For arrays and collections there is also an enhanced for loop available. This loop is covered in the Array description.

16.2. The while loop

A while loop is a repetition control structure that allows you to write a block of code which is executed until a specific condition evaluates to false. The syntax is the following.

while(expression)
{
        // block of code to run
}

The following shows an example for a while loop.

public class WhileTest {

        public static void main(String args[]) {
                int x = 1;

                while (x < 10) {
                        System.out.println("value of x : " + x);
                        x++;
                }
        }
}

16.3. The do while loop

The do-while loop is similar to the while loop, with the exception that the condition is checked after the execution. The syntax is the following.

do
{
        // block of code to run
} while(expression);

The following shows an example for a do-while loop.

public class DoTest {

        public static void main(String args[]) {
                int x = 1;

                do {
                        System.out.println("value of x : " + x);
                        x++;
                } while (x < 10);
        }
}

17. Arrays

17.1. Arrays in Java

An array is a container object that holds a fixed number of values of a single type. An item in an array is called an element. Every element can be accessed via an index. The first element in an array is addressed via the 0 index, the second via 1, etc.

package com.vogella.javaintro.array;

public class TestMain {
        public static void main(String[] args) {
                // declares an array of integers
                int[] array;

                // allocates memory for 10 integers
                array = new int[10];

                // initialize values
                array[0] = 10;
                // initialize second element
                array[1] = 20;
                array[2] = 30;
                array[3] = 40;
                array[4] = 50;
                array[5] = 60;
                array[6] = 70;
                array[7] = 80;
                array[8] = 90;
                array[9] = 100;

        }
}

17.2. Enhanced for loop for Arrays and Collections

Arrays and collections can be processed with a simpler for loop.

for(declaration : expression)
{
        // body of code to be executed
}

The following code demonstrates its usage.

package com.vogella.javaintro.array;

public class TestMain {
        public static void main(String[] args) {
                // declares an array of integers
                int[] array;

                // allocates memory for 10 integers
                array = new int[10];

                // initialize values
                array[0] = 10;
                // initialize second element
                array[1] = 20;
                array[2] = 30;
                array[3] = 40;
                array[4] = 50;
                array[5] = 60;
                array[6] = 70;
                array[7] = 80;
                array[8] = 90;
                array[9] = 100;

                int idx = 0;
                for (int i : array) {
                    System.out.println("Element at index " + idx + " :"  + i);
                    idx++;
                }
        }
}

18. Strings

18.1. Strings in Java

The String class represents character strings. All string literals, for example, "hello", are implemented as instances of this class. An instance of this class is an object. Strings are immutable, e.g., an assignment of a new value to a String object creates a new object.

18.2. String pool in Java

For memory efficiency Java uses a String pool. The string pool allows string literals to be reused. This is possible because strings in Java are immutable.

If the same string literal is used in several places in the Java code, only one copy of that string is created.

Whenever a String object is created and gets a string literal assigned, e.g., as in String s = "constant", the string pool is used. However, the new operator forces a new String copy to be allocated, for example, in String s = new String("constant");.

18.3. Compare Strings in Java

To compare the String objects s1 and s2, use the s1.equals(s2) method.

A String comparison with == is incorrect, as == checks for object reference equality. == sometimes gives the correct result, as Java uses a String pool. The following example would work with ==.

This would work as expected.

String a = "Hello";
String b = "Hello";
if (a==b) {
        // if statement is true
        // because String pool is used and
        // a and b point to the same constant
}

This comparison would fail.

String a = "Hello";
String b = new String("Hello");
if (a==b) {

} else {
        // if statement is false
        // because String pool is used and
        // a and b point to the same constant
}

WARNING:Therefore, you should always use the equals() method when you compare strings.

18.4. Working with Strings

The following lists the most common string operations.

Table 5. Table Title
Command Description

"Testing".equals(text1);

Return true if text1 is equal to "Testing". The check is case-sensitive.

"Testing".equalsIgnoreCase(text1);

Return true if text1 is equal to "Testing". The check is not case-sensitive. For example, it would also be true for "testing".

StringBuffer str1 = new StringBuffer();

Define a new String with a variable length.

str.charat(1);

Return the character at position 1. (Note: strings are arrays of chars starting with 0)

str.substring(1);

Removes the first characters.

str.substring(1, 5);

Gets the substring from the second to the fifth character.

str.indexOf("Test")

Look for the String "Test" in String str. Returns the index of the first occurrence of the specified string.

str.lastIndexOf("ing")

Returns the index of the last occurrence of the specified String "ing" in the String str. StringBuffer does not support this method. Hence first convert the StringBuffer to String via method toString.

str.endsWith("ing")

Returns true if str ends with String "ing"

str.startsWith("Test")

Returns true if String str starts with String "Test".

str.trim()

Removes leading and trailing spaces.

str.replace(str1, str2)

Replaces all occurrences of str1 by str2

str2.concat(str1);

Concatenates str1 at the end of str2.

str.toLowerCase() / str.toUpperCase()

Converts the string to lower- or uppercase

str1 + str2

Concatenate str1 and str2

String[] array = myString.split("-"); String[] array2 = myString.split("\\.");

Splits the character separated myString into an array of strings. Attention: the split string is a regular expression, so if you using special characters which have a meaning in regular expressions, you need to quote them. In the second example the . is used and must be quoted by two backslashes.

19. Lambdas

19.1. What are lambdas?

The Java programming language supports lambdas as of Java 8. A lambda expression is a block of code with parameters. Lambdas allows to specify a block of code which should be executed later. If a method expects a functional interface as parameter it is possible to pass in the lambda expression instead.

The type of a lambda expression in Java is a functional interface.

19.2. Purpose of lambda expressions

Using lambdas allows to use a condensed syntax compared to other Java programming constructs. For example the Collection interfaces has forEach method which accepts a lambda expression.

List<String> list = Arrays.asList("vogella.com","google.com","heise.de" )
list.forEach(s-> System.out.println(s));

19.3. Using method references

You can use method references in a lambda expression. Method reference define the method to be called via CalledFrom::method. CalledFrom can be * instance::instanceMethod * SomeClass::staticMethod * SomeClass::instanceMethod

List<String> list = new ArrayList<>();
list.add("vogella.com");
list.add("google.com");
list.add("heise.de");
list.forEach(System.out::println);

19.4. Difference between a lambda expression and a closure

The Java programming language supports lambdas but not closures. A lambda is an anonymous function, e.g., it can be defined as parameter. Closures are code fragments or code blocks which can be used without being a method or a class. This means that a closure can access variables not defined in its parameter list and that it can also be assigned to a variable.

20. Streams

20.1. What are streams?

A stream from the java.util.stream package is a sequence of elements from a source that supports aggregate operations.

20.2. IntStream

Allow to create a stream of sequence of primitive int-valued elements supporting sequential and parallel aggregate operations.

package com.vogella.java.streams;

import java.util.ArrayList;
import java.util.List;
import java.util.stream.IntStream;

public class IntStreamExample {

        public static void main(String[] args) {
                // printout the numbers from 1 to 100
                IntStream.range(1, 101).forEach(s -> System.out.println(s));

                // create a list of integers for 1 to 100
                List<Integer> list = new ArrayList<>();
                IntStream.range(1, 101).forEach(it -> list.add(it));
                System.out.println("Size " + list.size());
        }

}

20.3. Reduction operations with streams and lambdas

Allow to create a stream of sequence of primitive int-valued elements supporting sequential and parallel aggregate operations.

package com.vogella.java.streams;

public class Task {
        private String summary;
        private int duration;

        public Task(String summary, int duration) {
                this.summary = summary;
                this.duration = duration;
        }
        public String getSummary() {
                return summary;
        }
        public void setSummary(String summary) {
                this.summary = summary;
        }
        public int getDuration() {
                return duration;
        }
        public void setDuration(int duration) {
                this.duration = duration;
        }

}
package com.vogella.java.streams;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class StreamTester {

        public static void main(String[] args) {
                Random random = new Random();
                // Generate a list of random task
                List<Task> values = new ArrayList<>();
                IntStream.range(1, 20).forEach(i -> values.add(new Task("Task" + random.nextInt(10), random.nextInt(10))));


                // get a list of the distinct task summary field
                List<String> resultList =
                                        values.stream().filter(
                                                        t -> t.getDuration() > 5).map(
                                                                        t -> t.getSummary()).distinct().collect(Collectors.toList());
                System.out.println(resultList);


                // get a concatenated string of Task with a duration longer than 5 hours
                String collect =
                                values.stream().filter(
                                                t -> t.getDuration() > 5).map(
                                                                t -> t.getSummary()).distinct().collect(Collectors.joining("-"));
                System.out.println(collect);
        }

}

20.4. Example for filtering and mapping the content of a list

The following example demonstrates how to use streams to filter a list, perform a mapping operation and to create one final result string from it with the reduce method.

import java.util.Arrays;
import java.util.List;

public class JavaStreamExample {
        public static void main(String[] args) {
                List<String> list = Arrays.asList("Hello", "Streams", "Not");

                String result = list.stream().
                filter(s->s.contains("e")).
                map(s-> s.toUpperCase() ).
                reduce("", (a,b)-> a+ " " + b);

                System.out.println(result + "!");

        }
}

20.5. Streams and lambda examples

The following is a larger example for the usage of streams. The code is based on the slides from the Goldmann Sachs Collection presentation at EclipseCOn 2015

Lets assume the following data model.

package com.vogella.java.streams;

public enum PetType {
        DOG,
        HAMSTER,
        CAT,
        BIRD
}
package com.vogella.java.streams;

public class Pet {
        private PetType type;
        private String name;
        private int age;

        public Pet(PetType cat, String name, int age) {
                this.type = cat;
                this.name = name;
                this.age = age;

        }

        public PetType getType() {
                return type;
        }

        public String getName() {
                return name;
        }

        public int getAge() {
                return age;
        }

}
package com.vogella.java.streams;

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.stream.Collectors;

public class Person {
        public Person(String firstName, String lastName) {
                this.lastName = lastName ;
                this.firstName = firstName;
        }
        private String firstName;
        private String lastName;
        private List<Pet> pets = new ArrayList<>();

        public Person addPet(PetType cat, String name, int age) {
                pets.add(new Pet(cat,name, age ));
                return this;
        }

        public boolean hasPet(PetType petType) {
                return pets.stream().anyMatch(p-> p.getType().equals(petType));
        }

        public boolean isNamed(String string) {
                return (firstName + " " + lastName).equals(string);
        }

        public List<Pet> getPets() {
                return pets;
        }

        public Collection<PetType> getPetTypes() {
                return pets.stream().map(pet-> pet.getType()).collect(Collectors.toSet());
        }

        public String getFirstName() {
                return firstName;
        }

        public String getLastName() {
                return lastName;
        }




}

With this data model you can use streams and lambdas to filter and search the data as demonstrated in the following example.

package com.vogella.java.streams;

import java.util.Arrays;
import java.util.Collections;
import java.util.IntSummaryStatistics;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;

public class Example {
        // examples based on
        // https://www.eclipsecon.org/europe2015/sites/default/files/slides/2015-11-05%20EclipseCon-%20EclipseCollectionsByExample_0.pdf
        private List<Person> people;

        public static void main(String[] args) {
                List<Person> persons = createData();

                // do any of the persons have a cat?
                boolean result = persons.stream().anyMatch(person -> person.hasPet(PetType.CAT));
                // how many people have cats?

                long catCount = persons.stream().filter(person -> person.hasPet(PetType.CAT)).count();
                System.out.println(catCount);

                // who has a cat?
                List<Person> peopleWithCats = persons.stream().filter(person -> person.hasPet(PetType.CAT))
                                .collect(Collectors.toList());
                // who does not have a cat
                List<Person> peopleWithoutCats = persons.stream().filter(person -> !person.hasPet(PetType.CAT))
                                .collect(Collectors.toList());

                // partition people with/without cats
                Map<Boolean, List<Person>> catsAndNoCats = persons.stream()
                                .collect(Collectors.partitioningBy(person -> person.hasPet(PetType.CAT)));

                // get the name of Tims cats
                Person person = persons.stream().filter(each -> each.isNamed("Tim Taller")).findFirst().get();
                boolean test = ("Dolly & Spot")
                                .equals(person.getPets().stream().map(Pet::getName).collect(Collectors.joining(" & ")));
                System.out.println("Tim has Dolly & Spot as cats " + test);

                // get the set of all pet types
                Set<PetType> allPetTypes = persons.stream().flatMap(p -> p.getPetTypes().stream()).collect(Collectors.toSet());
                System.out.println(allPetTypes);
                // group people by their last name

                Map<String, List<Person>> byLastName = persons.stream().collect(Collectors.groupingBy(Person::getLastName));
                Map<String, List<Person>> byLastNameTargetBag = persons.stream()
                                .collect(Collectors.groupingBy(Person::getLastName));
                System.out.println(byLastName);

                // get the age statistics of pets
                List<Integer> agesList = persons.stream().flatMap(p -> p.getPets().stream()).map(Pet::getAge)
                                .collect(Collectors.toList());
                IntSummaryStatistics stats = agesList.stream().collect(Collectors.summarizingInt(i -> i));
                System.out.println("Min " + stats.getMin() + " Max " + stats.getMax() + " Average " + stats.getAverage());

                // counts by pet age
                Map<Integer, Long> counts = Collections.unmodifiableMap(persons.stream().flatMap(p -> p.getPets().stream())
                                .collect(Collectors.groupingBy(Pet::getAge, Collectors.counting())));
        }

        private static List<Person> createData() {

                return Arrays.asList(
                                new Person("Mary", "Smith").addPet(PetType.CAT, "Tabby", 2),
                                new Person("Tim", "Taller").addPet(PetType.CAT, "Dolly", 3).addPet(PetType.DOG, "Spot", 2),
                                new Person("Ted", "Smith").addPet(PetType.DOG, "Spike", 4),
                                new Person("Jake", "Snake").addPet(PetType.DOG, "Serpy", 1),
                                new Person("Lars", "Vogel").addPet(PetType.BIRD, "Twirly", 1),
                                new Person("Harry", "Hamster").addPet(PetType.HAMSTER, "Fuzzy", 1).addPet(PetType.HAMSTER, "Wuzzy", 1)
                                );
        }

}

21. Optional

If you call a method or access a field on an object which is not initialized (null) you receive a NullPointerException (NPE). The java.util.Optional class can be used to avoid these NPEs.

java.util.Optional is a good tool to indicate that a return value may be absent, which can be seen directly in the method signature rather than just mentioning that null may be returned in the JavaDoc.

If you want to call a method on an Optional object and check some property you can use the filter method. The filter method takes a predicate as an argument. If a value is present in the Optional object and it matches the predicate, the filter method returns that value; otherwise, it returns an empty Optional object.

You can create an Optional in different ways:

// use this if the object is not null
opt = Optional.of(o);
// creates an empty Optional, if o is null
opt = Optional.ofNullable(o);
// create an empty Optional
opt = Optional.empty();

The ifPresent method can be used to execute some code on an object if it is present. Assume you have a Todo object and want to call the getId() method on it. You can do this via the following code.

Todo todo = new Todo(-1);
Optional<Todo> optTodo = Optional.of(todo);
// get the id of the todo or a default value
optTodo.ifPresent(t-> System.out.println(t.getId()));

Via the map method you can transform the object if it is present and via the filter method you can filter for certain values.

Todo todo = new Todo(-1);
Optional<Todo> optTodo = Optional.of(todo);

// get the summary (trimmed) of todo if the id is higher than 0
Optional<String> map = optTodo.filter(o -> o.getId() > 0).map(o -> o.getSummary().trim());

// same as above but print it out
optTodo.filter(o -> o.getId() > 0).map(o -> o.getSummary().trim()).ifPresent(System.out::println);

To get the real value of an Optional the get() method can be used. But in case the Optional is empty this will throw a NoSuchElementException. To avoid this NoSuchElementException the orElse or the orElseGet can be used to provide a default in case of absence.

// using a String
String s = "Hello";
Optional<String> maybeS = Optional.of(s);
// get length of the String or -1 as default
int len = maybeS.map(String::length).orElse(-1);

// orElseGet allows to construct an object / value with a Supplier
int calStringlen = maybeS.map(String::length).orElseGet(()-> "Hello".length());

22. Type Conversion

If you use variables of different types Java requires for certain types an explicit conversion. The following gives examples for this conversion.

22.1. Conversion to String

Use the following to convert from other types to Strings

// Convert from int to String
String s1 = String.valueOf ( 10 ); // "10"
// Convert from double to String
String s2 = String.valueOf ( Math.PI ); // "3.141592653589793"
// Convert from boolean to String
String s3 = String.valueOf ( 1 < 2 ); // "true"
// Convert from date to String
String s4 = String.valueOf ( new Date() ); // "Tue Jun 03 14:40:38 CEST 2003"

22.2. Conversion from String to Number

// Conversion from String to int
int i = Integer.parseInt(String);
// Conversion from float to int
float f = Float.parseFloat(String);
// Conversion from double to int
double d = Double.parseDouble(String);

The conversion from string to number is independent from the locale settings, e.g., it is always using the English notification for number. In this notification a correct number format is "8.20". Thed German number "8,20" would result in an error.

To convert from a German number, you have to use the NumberFormat class. The challenge is that when the value is, for example, 98.00 then the NumberFormat class would create a Long which cannot be casted to Double. Hence the following complex conversion class.

private Double convertStringToDouble(String s) {

                Locale l = new Locale("de", "DE");
                Locale.setDefault(l);
                NumberFormat nf = NumberFormat.getInstance();
                Double result = 0.0;
                try {
                        if (Class.forName("java.lang.Long").isInstance(nf.parse(s))) {
                                result = Double.parseDouble(String.valueOf(nf.parse(s)));
                        } else {
                                result = (Double) nf.parse(new String(s));
                        }
                } catch (ClassNotFoundException e1) {
                        e1.printStackTrace();
                } catch (ParseException e1) {
                        e1.printStackTrace();
                }
                return result;
        }

22.3. Double to int

int i = (int) double;

22.4. SQL Date conversions

Use the following to convert a Date to a SQL date

package test;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;

public class ConvertDateToSQLDate {

private void convertDateToSQL(){
         SimpleDateFormat template =
                        new SimpleDateFormat("yyyy-MM-dd");
                  java.util.Date enddate =
                        new java.util.Date("10/31/99");
                  java.sql.Date sqlDate =
                        java.sql.Date.valueOf(
                                        template.format(enddate));

}
        public static void main(String[] args) {
                ConvertDateToSQLDate date = new ConvertDateToSQLDate();
                date.convertDateToSQL();
        }

}

23. Schedule tasks

Java allows you to schedule tasks. A scheduled tasks can perform once or several times.

java.util.Timer and java.util.TimerTask can be used to schedule tasks. The object which implements TimeTask will then be performed by the Timer based on the given interval.

package schedule;

import java.util.TimerTask;

public class MyTask extends TimerTask {
        private final String string;
        private int count = 0;

        public MyTask(String string) {
                this.string = string;
        }

        @Override
        public void run() {
                count++;
                System.out.println(string + " called " + count);
        }

}
package schedule;

import java.util.Timer;

public class ScheduleTest {

        public static void main(String[] args) {
                Timer timer = new Timer();

                // wait 2 seconds (2000 milli-secs) and then start
                timer.schedule(new MyTask("Task1"), 2000);

                for (int i = 0; i < 100; i++) {
                        // wait 1 seconds and then again every 5 seconds
                        timer.schedule(new MyTask("Task " + i), 1000, 5000);
                }
        }
}

TIP: Improved job scheduling is available via the open source framework quartz. See http://www.onjava.com/lpt/a/4637 http://www.onjava.com/lpt/a/4637 or http://www.quartz-scheduler.org/ http://www.quartz-scheduler.org/ for an explanation.

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