The class java.util.Calendar is an abstract encapsulation of the object Date. The concrete implementation is java.util.GregorianCalendar.

Calendar provides getter and setter for the date fields.

public final int get(int field)
public final void set(int field, int value) 

Create a new Java Project called JavaIntroCalendar. Create the following class for testing.

package test;

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.GregorianCalendar;

public class CalendarTest {
  public static void main(String[] args) {
    // Constructor allows to set year, month and date
    Calendar cal1 = new GregorianCalendar(2008, 01, 01);
    // Constructor could also be empty
    // Calendar cal2 = new GregorianCalendar();
    // Change the month
    cal1.set(Calendar.MONTH, Calendar.MAY);

    System.out.println("Year: " + cal1.get(Calendar.YEAR));
    System.out.println("Month: " + (cal1.get(Calendar.MONTH) + 1));
    System.out.println("Days: " + cal1.get(Calendar.DAY_OF_MONTH));

    // Format the output with leading zeros for days and month
    SimpleDateFormat date_format = new SimpleDateFormat("yyyyMMdd");
    System.out.println(date_format.format(cal1.getTime()));

  }
} 

Use the following commands to convert to a Date from various formats.

package conversion;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;

public class ConversionExamplesDate {

  // Convert from String to date
  private void stringToDate() {
    
    try {
      Date date1;
      date1 = new SimpleDateFormat("MM/dd/yy").parse("05/18/05");
      System.out.println(date1);
      Date date2 = new SimpleDateFormat("MM/dd/yyyy").parse("05/18/2007");
      System.out.println(date2);
    } catch (ParseException e) {
      e.printStackTrace();
    }
  }

  // Convert from millisecs to a String with a defined format
  private void calcDate(long millisecs) {
    SimpleDateFormat date_format = new SimpleDateFormat("MMM dd,yyyy HH:mm");
    Date resultdate = new Date(millisecs);
    System.out.println(date_format.format(resultdate));
  }
  
  private void writeActualDate(){
    Calendar cal = new GregorianCalendar();
    Date creationDate = cal.getTime();
    SimpleDateFormat date_format = new SimpleDateFormat("MMM dd,yyyy HH:mm");
    System.out.println(date_format.format(creationDate));
  }
  

  public static void main(String[] args) {
    ConversionExamplesDate convert = new ConversionExamplesDate();
    convert.stringToDate();
    convert.calcDate(System.currentTimeMillis());
    convert.writeActualDate();
  }
}