HashSet adds duplicate entries despite implementing both hashCode() and equals()

Question

I have the following class:

class Point
{
     double x, y;

     // .... constructor and other functions here

     public boolean equals(Point p)
     {
         if(p==null) return(false);
         return(x==p.x && y==p.y);
     }

    public int hashCode()
    {
        int result=17;

        long c1=Double.doubleToLongBits(x);
        long c2=Double.doubleToLongBits(y);

        int ci1=(int)(c1 ^ (c1 >>> 32));
        int ci2=(int)(c2 ^ (c2 >>> 32));

        result = 31 * result + ci1;
        result = 31 * result + ci2;

        return result;
    }
}

Now, if I write the following code:

    Point x=new Point(11,7);
    Point y=new Point(11,7);

    System.out.println("hash-code of x=" + x.hashCode());
    System.out.println("hash-code of y=" + y.hashCode());
    System.out.println("x.equals(y) = " + x.equals(y));
    System.out.println("x==y = " + (x==y));

    java.util.HashSet<Point> s=new java.util.HashSet<Point>();
    s.add(x);
    System.out.println("Contains "+y.toString()+" = "+s.contains(y));
    s.add(y);
    System.out.println("Set size: "+s.size());
    java.util.Iterator<Point> itr=s.iterator();
    while(itr.hasNext()) System.out.println(itr.next().toString());

I am getting the following output:

hash-code of x=79052753
hash-code of y=79052753
x.equals(y) = true
x==y = false
Contains (11.0,7.0) = false
Set size: 2
(11.0,7.0)
(11.0,7.0)

Please help me in understanding why contains() returns false (even after equals() and hashCode() return the same value) and how can I rectify this (i.e. preventing Java from adding duplicate elements). Thanks in advance.

Answer 1

You have changed the method signature from Object.equals(Object), so you aren't correctly overriding equals. I suggest you use the @Override annotation to catch this class of bugs. Your method should look something like,

@Override
public boolean equals(Object o)
{
     if (this == o) {
         return true;
     }
     if (o instanceof Point) {
         Point p = (Point) o;
         return(x==p.x && y==p.y);
     }
     return false;
}

Answer 2

You are not overriding the equals method in Object.

The signature of equals method is:

public boolean equals(Object obj)

and not

public boolean equals(Point obj)

And please do not compare double values using ==. You should use Double.equals instead.