Question

Why does it not reach the print line “here”? In other words, why is it not using my overridden equals?
Thanks in advance

import java.util.*;
class NumberClass extends Object{
    private int number;

    public NumberClass(int n){
        number=n;

    }
    @Override
    public boolean equals(Object other){
        System.out.println("here");
        return false;
    }
    @Override
    public String toString(){

        return number+"";
    }   
}
public class HelloWorld {
    public static void main(String[] args) {

        Set <NumberClass> set= new HashSet<NumberClass>();
        set.add(new NumberClass(0));
        set.add(new NumberClass(0));
        System.out.println(set);
    }

}

Answer 1

This is because HashSet uses the hash code to test whether an object is already in the set and only uses equals if there is a collision. The default hash code for objects is an internal id, so just because the two objects are equal doesn’t mean they have the same hash code. In this case, there clearly wasn’t a collision, so there was no need for the HashSet to call equals.

This is why you should always override hashCode when you override equals. See this article for more info on this topic. You should write a hashCode method that returns equal hash values for objects that are equals. For instance:

public int hashCode() {
    return number;
}

The general requirement for hashCode is that objects that satisfy equals() should return the same hash code. Note that there’s no requirement that objects that fail an equals() test have different hash codes. If you return a constant (like Jayamohan suggests), that satisfies the hashCode contract; however, this will completely eliminate the benefits of using a HashSet and you might as well go with a simple ArrayList.