如果类具有引用类型成员java，如何实现equality和hashCode方法？

Question

I am trying to implement the following example to override the equality and hashCode method if the class has reference type member but no luck. Any help would be highly appreciated. Thanking you all in advance..

     class Point{
    private int x, y;
    Point (int x, int y)
    {
        this.x =x;
        this.y = y;
    }

} 


class Circle 
{
    int radius; 
    Point point ;

    Circle(int x, int y, int radius)
    {
        point = new Point (x ,y);
        this.radius = radius;
    }



    @Override
    public boolean equals(Object arg) {

        if(arg == null) return false;
        if(arg == this) return true;
        if(arg instanceof Circle) 
        {
            if(this.point ==((Circle) arg).point && this.radius == ((Circle)           arg).radius)
            {
                return true;
            }

        } 


        return false;
    }

    @Override
    public int hashCode() {

        return point.hashCode() ^ this.radius;
    }

}


public class TestClass{

    public static void main(String args[])
    {
        Set<Circle> circle = new HashSet<> ();
        circle.add(new Circle(10,20,40));

        System.out.println(circle.contains(new Circle(10,20,40))); //
    }

}

**************************Edited Version, suggested by Alnitak*****************

Now i do get the expected result "true" for equal and "false" for non-equal objects. But the print statement in the Circle's equal method is not executed when the objects values are not equal. I don't know what i am missing, though i get the equality result "false" as expected for non equal objects.

 class Point{
    private int x, y;
    Point (int x, int y)
    {
        this.x =x;
        this.y = y;
    }
    @Override
    public boolean equals(Object arg) {
        if(arg == null ) return false;
        if(arg== this) return true;
        if(arg instanceof Point)
        {
            Point p = (Point) arg;
            if(p.x == x && p.y == y )
            {
                return true;
            }
        }
        return false;

    }
    @Override
    public int hashCode() {

        return (this.x*1124739) ^ (this.y*95);
    }

} 


class Circle 
{
    int radius; 
    Point point ;

    Circle(int x, int y, int radius)
    {

        System.out.println("Circle object created x= " + x);
        point = new Point (x ,y);
        this.radius = radius;
    }



    @Override
    public boolean equals(Object arg) {

        if(arg == null) return false;
        if(arg == this) return true;
        if(arg instanceof Circle)   
        {
            System.out.println("checking circles objects for equality "); 
            // Doesn't get printed when circle objects values are not equal

            Circle c = (Circle) arg;
            return (point.equals(c.point) && radius == c.radius);

        } 


        return false;
    }

    @Override
    public int hashCode() {

        return point.hashCode() ^ this.radius *37;
    }

}


public class TestClass{

    public static void main(String args[])
    {
        Set<Circle> circle = new HashSet<> ();
        circle.add(new Circle(10,20,40));

        System.out.println(circle.contains(new Circle(11,20,40))); //
    }

}

Answer 1

Your existing Circle.equals() method only checks for referential equality, not value equality. It will always fail because each Circle contains a newly constructed Point object that is unique to that instance.

You should create a proper hashCode and equals method for the Point class.

Within the Circle class you can then use Point.equals() to check for value equality of the held reference, for example:

public boolean equals(Object arg) {
    if (arg == null) return false; 
    if (arg == this) return true;
    if (arg instanceof Circle) {
        Circle c = (Circle)arg;
        return radius == c.radius && point.equals(c.point);
    }
    return false;
}

For the Circle hashcode, a simple option is to generate a local hashcode and xor it with the Point's hashcode.