Overriding methods

[Vivienne Ryan]

Hi there,
I am currently studying Overriding methods using the "Oracle Certified Professional Java SE7 Programmer Exams 1z0-804 and 1z0-805" and I am a bit confused.
I am studying the examples given on pages 70 - 73 of the book and I was wondering if ye guys can help me to understand the information.

The original method is:

public boolean equals(Object obj)

The first piece of code:

public class Point {
        private int xPos, yPos;

        public Point(int x, int y) {
            xPos = x;
            yPos = y;
        }

        // override the equals method to perform
       // "deep" comparison of two Point objects
       public boolean equals(Point other){
            if(other == null)
                return false;
      // two points are equal only if their x and y positions are equal
            if( (xPos == other.xPos) && (yPos == other.yPos) )
              return true;
            else
              return false;
       }

       public static void main(String []args) {
           Point p1 = new Point(10, 20);
           Point p2 = new Point(50, 100);
           Point p3 = new Point(10, 20);
           System.out.println("p1 equals p2 is " + p1.equals(p2));
           System.out.println("p1 equals p3 is " + p1.equals(p3));
       }
}

Which outputs:

p1 equals p2 is false
p1 equals p3 is true

Then the main method is changed to:

public static void main(String []args) {
        Object p1 = new Point(10, 20);
        Object p2 = new Point(50, 100);
        Object p3 = new Point(10, 20);
        System.out.println("p1 equals p2 is " + p1.equals(p2));
        System.out.println("p1 equals p3 is " + p1.equals(p3));
}

Which outputs:

p1 equals p2 is false
p1 equals p3 is false

Could somebody explain to me what is going on? I don't want to be spoonfed, I just cannot wrap my head around this bit.
Why is the method not overridden?
Also why doesn't the second version of main not output the expected output(false, true)?

[Henry Wong]

Vivienne Ryan wrote:
Could somebody explain to me what is going on? I don't want to be spoonfed, I just cannot wrap my head around this bit.
Why is the method not overloaded?
Also why doesn't the second version of main not output the expected output(false, true)?

Actually, the two methods are overloaded. What they are not, and what you were expecting, were for them to be overridden. Choosing between overloaded methods is done at compile time, and the compiler does so with the reference types.

Henry

[Vivienne Ryan]

Henry Wong wrote:

Vivienne Ryan wrote:
Could somebody explain to me what is going on? I don't want to be spoonfed, I just cannot wrap my head around this bit.
Why is the method not overloaded?
Also why doesn't the second version of main not output the expected output(false, true)?

Actually, the two methods are overloaded. What they are not, and what you were expecting, were for them to be overridden. Choosing between overloaded methods is done at compile time, and the compiler does so with the reference types.

Henry

Thanks for catching my mistake. I meant to say overridden.

[Campbell Ritchie]

Whenever you think you are overriding methods, always use the @Override annotation. You can get all sorts of weird errors if you make that sort of mistake, or tiny spelling errors, as happened to somebody in this thread.