Java how does HashMap identify the key object if hashCode are the same and equals is true?

Question

I always thought hat HashMap is identifying its keys by using hasCode and equals. But this code still does not give me the value back. What am I missing?

import java.util.HashMap;
import java.util.Map;

/**
 * Created by kic on 25.04.15.
 */
public class TypePair {
    private final Class a;
    private final Class b;

    public TypePair(Class a, Class b) {
        this.a = a;
        this.b = b;
    }

    public TypePair(Object a, Object b) {
        this.a = a.getClass();
        this.b = b.getClass();
    }

    public boolean isPair (Object a, Object b) {
        return this.a.isAssignableFrom(a.getClass()) && this.b.isAssignableFrom(b.getClass());
    }

    @Override
    public boolean equals(Object obj) {
        if (obj instanceof TypePair) {
            return a.isAssignableFrom(((TypePair) obj).a) && b.isAssignableFrom(((TypePair) obj).b);
        } else {
            return false;
        }
    }

    @Override
    public int hashCode() {
        return 1;
    }

    public static void main(String[] args) {
        TypePair a = new TypePair(Number.class, String.class);
        TypePair b = new TypePair(12, "hello");

        Map<TypePair, Boolean> test = new HashMap<>();
        test.put(a, true);

        System.out.println(a.hashCode() == b.hashCode());
        System.out.println(a.equals(b));
        System.out.println("und? " + test.get(a));
        System.out.println("und? " + test.get(b));
    }
}

This code prints:

true
true
und? true
und? null

Check what b.equals(a) returns.

resueman
– resueman

2015-04-25 13:41:11 +00:00
Commented Apr 25, 2015 at 13:41 — resueman
– resueman, Commented Apr 25, 2015 at 13:41
@resueman damn it you are right!

KIC
– KIC

2015-04-25 13:42:15 +00:00
Commented Apr 25, 2015 at 13:42 — KIC
– KIC, Commented Apr 25, 2015 at 13:42

Mark Rotteveel · Accepted Answer · 2022-07-12 06:01:04Z

5

Your implementation of equals() is violating the contract (it is not symmetric), that's why it does not work correctly.

You need to use something like:

@Override
public boolean equals(Object obj) {
    if (obj instanceof TypePair) {
        TypePair objTypePair = (TypePair) obj;
        return a            .isAssignableFrom(objTypePair.a) && b            .isAssignableFrom(objTypePair.b) ||
               objTypePair.a.isAssignableFrom(a)             && objTypePair.b.isAssignableFrom(b);
    } else {
        return false;
    }
}

edited Jul 12, 2022 at 6:01

Mark Rotteveel

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answered Apr 25, 2015 at 13:41

Crazyjavahacking

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6 Comments

KIC Over a year ago

you are right: return (a.isAssignableFrom(((TypePair) obj).a) && b.isAssignableFrom(((TypePair) obj).b)) || obj.equals(this); and it works.

Radiodef Over a year ago

Or just this.a == that.a && this.b == that.b.

KIC Over a year ago

@Radiodef I think you did not get the point that I want to match super classes so this.a is never that.a

Radiodef Over a year ago

Oops, I didn't see the ||. Then this answer is actually still a wrong implementation of equals because it's not transitive. You can't actually do what you're trying to do.

Radiodef Over a year ago

See e.g. ideone.com/c1hAjy It still doesn't work. equals: "It is transitive: if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true."

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