overriding equals and hashCode - java

Question

I want to override the equals and hashCode method in my Point class. I want to use the Point1 class object in my list and I want to check if on list there is a point with the same coordinates and it needn't to be the same object. only have the same values in fields.

here is my code and I have no idea why it doesn't work

package prac1; import java.util.ArrayList; import java.util.List; class Point{ private Integer x; private Integer y; Point(int x,int y) { this.x = x; this.y = y; } @Override public int hashCode() { int hash = 5; hash = hash +(this.x != null ? this.x.hashCode() : 0); hash = hash/12 + (this.y != null ? this.y.hashCode() : 0); //to have unique hashCode for different objects return hash; } @Override public boolean equals(Object other) { if(this == other) return true; if(other == null) return false; if(getClass() != other.getClass()) return false; Point1 test = (Point1)other; if(this.x == test.getX() && this.y == test.getY()) return true; return false; } int getX(){ return this.x; } int getY() {return this.y; } } public class NewClass{ public static void main(String args[]) { List<Point1> lista = new ArrayList<Point1>(); Point1 t = new Point1(1,1); lista.add(t); System.out.println(lista.contains(t)); // true System.out.println(lista.contains(new Point1(1,1))); // false ? } }

it returns:

 true false

Can anyone tell me what am I doing wrong?

what is Point1 ? is that supposed to be Point? also: important to notice: when I run your code, assuming that Point and Point1 are supposed to be the same class, I get 'true' 'true' as result — Stultuske
– Stultuske, Commented May 29, 2014 at 9:55

cyon · Accepted Answer · 2014-05-29 10:16:26Z

If I rename your Point class to Point1 then it produces true true on my machine, so your code works.

There is a bug in your code however. You are using == on Integer objects. This will only work with integer values between -128 and 127 (reference) because these are cached by the JVM. But it won't work for larger/smaller values. Instead use .equals

Point1 test = (Point1)other; if(this.x.equals(test.getX()) && this.y.equals(test.getY())) return true; return false;

Here I left out an if for when this.x == null or this.y == null which is impossible in your code at the moment since the constructor accepts primitives only.

@user3131037 You're welcome. Can I ask why you are using integer division in your hash function? This seems like it might cause you to lose set bits and cause more hash collisions.

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