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...

What about to add "override"?

libik
– libik

2014-09-05 09:37:34 +00:00
Commented Sep 5, 2014 at 9:37 — libik
– libik, Commented Sep 5, 2014 at 9:37
@libik override means only one value will be there finally

PSR
– PSR

2014-09-05 09:48:05 +00:00
Commented Sep 5, 2014 at 9:48 — PSR
– PSR, Commented Sep 5, 2014 at 9:48

Basil Bourque · Accepted Answer · 2018-10-04 20:25:41Z

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 4, 2018 at 20:25

Basil Bourque

347k130 gold badges950 silver badges1.3k bronze badges

answered Sep 5, 2014 at 9:37

Mena

48.6k11 gold badges90 silver badges109 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

PSR Over a year ago

Thank u.I got it now.

thkala Over a year ago

It also helps to use the IDE code completion, instead of typing manually in such cases.

PSR Over a year ago

actually i wrote this is in normal notepad.I did not use IDE.That is the problem

Mena Over a year ago

@thkala so true. And with eclipse it's just about typing "hash" and pressing Ctrl-space, then enter :)

Ankur Singhal · Accepted Answer · 2014-09-05 09:41:47Z

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 5, 2014 at 9:41

Ankur Singhal

26.1k17 gold badges92 silver badges129 bronze badges

2 Comments

lxcky Over a year ago

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.

thkala Over a year ago

@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...

Collectives™ on Stack Overflow

Hashcode , equals contract in java

2 Answers 2

4 Comments

2 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

2 Comments

Linked

Related