Hashcode , equals contract in java

Question

public class VO {

    public int hashcode()
    {
        return 0;
    }
    public boolean equals(Object obj)
    {
        return true;
    }

    public static void main(String args[])
    {
        VO vo1 = new VO();
        VO vo2 = new VO();

        Map<VO,Integer> map = new HashMap<VO, Integer>();

        map.put(vo1, 1);
        map.put(vo2, 1);

        System.out.println(map.size());
    }
}

I am getting the output is :2

But as per my knowledge the output is 1.

When i am placing an element in map it will check the hashcode for the key,if that hashcode is same then it will go to check equals.If both the methods returns same it will override the previous value.

In my case both the methods are(hashcode and equals) returns 0 and true.So finally there must be one element in the map.But here i am getting size as 2.

What might be the reason.Thanks in dvance...

@libik override means only one value will be there finally – PSR Sep 05 '14 at 09:48 — PSR, Sep 05 '14 at 09:48

score 11 · Accepted Answer · edited Oct 04 '18 at 20:25

11

You are not overriding Object.hashCode, you're implementing your own hashcode() method (mind the capitalized C).

A good practice is to always use @Override annotations when overriding. See: When do you use Java's @Override annotation and why?

edited Oct 04 '18 at 20:25

Basil Bourque

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answered Sep 05 '14 at 09:37

Mena

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Thank u.I got it now. – PSR Sep 05 '14 at 09:38
It also helps to use the IDE code completion, instead of typing manually in such cases. – thkala Sep 05 '14 at 09:38
actually i wrote this is in normal notepad.I did not use IDE.That is the problem – PSR Sep 05 '14 at 09:39
@thkala so true. And with eclipse it's just about typing "hash" and pressing Ctrl-space, then enter :) – Mena Sep 05 '14 at 09:39

score 3 · Answer 2 · answered Sep 05 '14 at 09:41

3

hashcode() (with small "c") was just acting as a normal method, and instead Object class method hashCode() was called.

public class VO {

    @Override
    public int hashCode() {
        return 0;
    }

    @Override
    public boolean equals(Object obj) {
        return true;
    }
}

answered Sep 05 '14 at 09:41

Ankur Singhal

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1

Even if you don't put the `@Override` annotation, it will still override the same method in the superclass. In this case, the problem was the letter `c` in his `hashcode` method. It's suppose to be `C`. – lxcky Sep 05 '14 at 09:44
1

@J.Lucky: true, but if you *do* put the annotation, modern Java compilers will let you know if you are not actually overridding or implementing anything... – thkala Sep 05 '14 at 09:49

Hashcode , equals contract in java

2 Answers2