This tutorial explains the usage of Maps and HashMap with Java.
1. Using maps
1.1. Map and HashMap
The Map interface defines an object that maps keys to values.
A map cannot contain duplicate keys; each key can map to at most one value.
The HashMap class is an efficient implementation of the Map interface.
1.2. Example program
To see the following code snippets in action, put them into this main method where put test code here is stated.
package com.vogella.java.collections.map;
import java.util.HashMap;
import java.util.Map;
public class MapTester {
public static void main(String[] args) {
// put test code here
}
}1.3. Initialize a HashMap in Java
The following code demonstrates how to initialize a HashMap in Java.
1.3.1. Initialize an empty map
In the following code an empty map with keys and objects of type Strings is initialized .
Map<String, String> map = Map.of();1.3.2. Initialize a unmodifiable map with Map.of for small maps
In following example a map with several entries is initialized. This factory method supports a maximum of 10 key-value pairs and requires at least Java 9.
Map<String, String> map = Map.of("key1","value1", "key2", "value2");1.3.3. Initialize an unmodifiable map via the Map.ofEntries();
Map<String, String> map = Map.ofEntries(
Map.entry("key1","value1"),
Map.entry("key2", "value2"),
// more
Map.entry("key100", "value100")
);1.3.4. Initialize a map via the new operator
The following creates a map via the new operator and add multiple entries to it via the put operator.
Map<String, String> map = new HashMap<>();
map.put("Android", "Mobile");
map.put("Eclipse IDE", "Java");
map.put("Eclipse RCP", "Java");
map.put("Git", "Version control system");1.4. Remove an entry from a map
You can remove an entry from a map via the remove method.
Map<String, String> map = Map.of("Android","Mobile OS", "Flutter", "Development environment");
map.remove("Android");1.5. Process the map
To process every element a map you can use the forEach method, which take a lambda as parameter.
map.forEach((k, v) -> System.out.printf("%s %s%n", k, v));1.6. Convert keys in a Map to an array or a list
You can convert your keys or values to an array or list. The following code demonstrates that.
Map<String, String> map = new HashMap<>();
map.put("Android", "Mobile");
map.put("Eclipse IDE", "Java");
map.put("Eclipse RCP", "Java");
map.put("Git", "Version control system");
// convert keys to array
String[] keys = map.keySet().toArray(new String[map.keySet().size()])
for (String string : keys) {
System.out.println(string);
}
// convert keys to list
List<String> list = new ArrayList<String>(map.keySet());
for (String string : list) {
System.out.println(string);
}1.7. Getting the current value or a default for a map
You can use the getOrDefault() method to return the value for a key or a default value if the key is not present in the map.
Map<String,Integer> map = new HashMap<>();
map.put("Android", 1 + map.getOrDefault("Android", 0));
// write to command line
map.forEach((k, v) -> System.out.printf("%s %s%n", k, v));1.8. Compute if absent
The computeIfAbsent calculates and adds an entry to the map if it is not present in the map.
Map<String,Integer> map = Map.of();
Integer calculatedVaue = map.computeIfAbsent("Java", it -> 0);
System.out.println(calculatedVaue);
map.keySet().forEach(key -> System.out.println(key + " " + map.get(key)));If you need more assistance we offer Online Training and Onsite training as well as consulting