Immutable Strings

class MyPoint
{
  public int x=3, y=5;

  public void print()
  {
    System.out.println("x = " + x + "; y = " + y );
  }
}

class PointTester
{
  public static void main ( String[] args )
  {
    MyPoint pt = new MyPoint();

    pt.print();

    pt.x = 45;  pt.y = 83;

    pt.print();
  }
}

It is good that String objects are immutable (they can't be changed) because the main() method can be sure that the message is completely under its control. Although the print() method gets a copy of the reference, it can't change the original object.

Not all objects are immutable.

Important: Public instance variables of objects can be changed by any method that has a reference to the object, even if the reference is in a formal parameter.

(If an instance variable is neither public nor private it can be changed by a method that is in the same package. For now, all of our code is in the same package, so the effect is the same as if it were public.) The programs has a user-defined class and a testing program.

The main() method uses the default constructor of class MyPoint. This is the constructor you get automatically if you do not define one yourself.