An Immutable object is a kind of object whose state cannot be modified after it is created.
This is as opposed to a mutable object, which can be modified after it is created.
The basic way to create an immutable object is to make all of its data fields private and don't create any methods which can change any of the data fields once the object has been constructed.
this code helps you to understands-

package roseindia;

import java.awt.Point;

public class test {

public static void main(String[] args) {
Point point1 = new Point(0, 0);// mutable object
System.out.println(point1);
point1.setLocation(1.1, 1.0);
System.out.println(point1);
String str = new String("Gyan Singh");// immutable object
System.out.println(str);
str.replaceAll("Gyan", "Samar");
System.out.println(str);
}

}

Output of the code is as follows-
java.awt.Point[x=0,y=0]
java.awt.Point[x=1,y=1]
Gyan Singh
Gyan Singh