Immutable objects

Objects of type String are immutable objects because they have no way (no methods) to change their state, i.e., the string they represent.

Objects that cannot change their state are called immutable. They represent exactly the same information for all their lifetime.

Example:

public class UpperLowerCase {
  public static void main(String[] args) {
    String s, upper, lower;
    s = new String("Hello");
    upper = s.toUpperCase();
    lower = s.toLowerCase();
    System.out.println(s);
    System.out.print("upper = ");
    System.out.println(upper);
    System.out.print("lower = ");
    System.out.println(lower);
  }
}

Notice that this program constructs 3 different strings (that are not modified anymore):

• the string "Hello", by calling the constructor,
• the string "HELLO", denoted by the variable upper, and
• the string "hello", denoted by the variable lower.