overriding equals and hashCode in Collection

Question

I am new in Collection, as per my knowledge I have already override the hashCode() and Equals() method in Data class, but when I trying to search the element it is giving "not found". Why? Please look at the code below aand help me to find the bug.

import java.util.*;

public class WordCounter {
    public static void main(String args[]) {
        HashSet<Data> set=new HashSet<Data>();  
        set.add(new Data("this",2));
        set.add(new Data("that",3));
        set.add(new Data("which",6));
        set.add(new Data("how",7));

        System.out.println(new Data("how",7).hashCode());

        set.add(new Data("hey",3));
        set.add(new Data("me",5));

        Iterator<Data> itr=set.iterator();
        while(itr.hasNext()) {
            Data d=itr.next();
            d.display();
        }
        Data e=new Data("how",7);
        System.out.println(e.hashCode()+"\t");
        if(set.contains(e))
            System.out.println("found");
        else
            System.out.println("not found");

    }
}

Data Class:

class Data {
    String word;
    int fre;

    public Data(String w,int f) {
        word=w;
        fre=f;
    }

    public void display() {
        System.out.println(word+"\t"+fre);
    }

    public boolean equals(Data e) {
        return this.word.equals(e.word) && this.fre == e.fre;   
    }

    public  int hashCode() {
        return this.word.hashCode() + this.fre;     
    }
}

Niels Bech Nielsen · Answer 1 · 2014-07-02T11:24:53.443

6

Perhaps your equals method is not used?

The signature is

public boolean equals(Object obj);

And it appears you have

public boolean equals(Data e);

Consider adding @Overrideannotations when you want to override a parent method (in this case java.lang.Object)

Update

Changing your method to this solves the case.

public boolean equals(Object d) {
    Data e = (Data) d;
    return this.word.equals(e.word) && this.fre == e.fre;
}

edited Jul 02 '14 at 11:24

answered Jul 02 '14 at 11:13

Niels Bech Nielsen

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"Perhaps"...? Those are **overloaded** methods, of course it's not using the methods provided. +1 for adding annotations, most IDEs have an option to add missing ones. – Ordous Jul 02 '14 at 11:17
"But I have use generics it is ging error when I am using public boolean equals(Object obj) " – Dikshant Jul 02 '14 at 11:20
Yes, there is no perhaps, sorry. I was merely trying to present him the question he should have asked himself. – Niels Bech Nielsen Jul 02 '14 at 11:20
equals(Data) and equals(Object) are not the same. I see no use of generics in class Data. I have tested your code locally, and I can augment it correctly. – Niels Bech Nielsen Jul 02 '14 at 11:23
"sorry ,I don't understand ,will u please explain again or write a correct code for me." – Dikshant Jul 02 '14 at 11:29
@NielsBechNielsen, you are not checking if `d` is in fact `Data` before casting it. – Bartek Maraszek Jul 02 '14 at 11:30
A type check is required `if (!(d instanceof Data)) return false` – Ambrish Jul 02 '14 at 11:31
@NielsBechNielsen,thanks Neils ,your help is very useful,now I got it. – Dikshant Jul 02 '14 at 11:38

score 1 · Answer 2 · edited May 23 '17 at 12:22

You equals() method is not used, because it has wrong signature. The correct signature is public boolean equals(Object o){ /* ... */ }. Because the signatures don't match, you are effectively overloading the method instead of overriding. The working equals() example:

@Override
public boolean equals(Object e)
{
    if(!(e instanceof Data)){
        return false;
    }
    Data d = (Data)e;
    return this.word.equals(d.word) && this.fre == d.fre;
}

When overriding a method, use the @Override annotation - then the code fails to compile if the signatures don't match, thus:

@Override
public boolean equals(Data d){ ... }

Will give you an error and save some debugging time. Also check out this question.

overriding equals and hashCode in Collection

2 Answers2