CODEWITHSUNDEEP
javadictionarycollectionshashmaphashcode
Koray Tugay
Koray Tugay

Reputation: 23770

Value found from Map not consistent with identical hashcodes and equals

Maybe not the best Title, please feel free to edit it.

This is the code I have..

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

public class App {
    public static void main(String[] args) {
        final Foo foo = new Foo();
        final Foo bar = new Foo();

        System.out.println("foo equals foo: " + foo.equals(foo));
        System.out.println("foo equals bar: " + foo.equals(bar));

        System.out.println("foo hashcode: " + foo.hashCode());
        System.out.println("bar hashcode: " + bar.hashCode());

        final Map<Foo, Integer> foos = new HashMap<Foo, Integer>();
        foos.put(foo, -99);

        System.out.println("foos.getfoo: " + foos.get(foo));
        System.out.println("foos.getbar: " + foos.get(bar));
    }    
}

class Foo {
    @Override
    public boolean equals(Object o) {
        return false;
    }

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

So before reading further can you guess what the output for the following 2 statement will be?

System.out.println("foos.getfoo: " + foos.get(foo));
System.out.println("foos.getbar: " + foos.get(bar));

I would expect to see:

null
null

since even though the hashCodes do match, equals for any instance of Foo will always return false, so using an instance of Foo as a key in a Map should not be useful at all..

However the output is:

:~ $ javac App.java
:~ $ java App
foo equals foo: false
foo equals bar: false
foo hashcode: -1
bar hashcode: -1
foos.getfoo: -99
foos.getbar: null

What am I missing? How is -99 retrieved when I use an object that has an hashcode -1 and is NOT equal to itself, but then get null later with a same type of instance that is also NOT equal to what I have in the Map and also has hashcode -1?

Upvotes: 4

Views: 86

Answers (1)

davidxxx
davidxxx

Reputation: 131326

Because the get() method of HashMap is optimized to check first the object reference equality before looking equals() :

Look at the Node<K,V> getNode(int hash, Object key) method that is invoked by the V get(Object key) method :

final Node<K,V> getNode(int hash, Object key) {
    Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (first = tab[(n - 1) & hash]) != null) {
        if (first.hash == hash && // always check first node
            ((k = first.key) == key || (key != null && key.equals(k))))
            return first;
        if ((e = first.next) != null) {
            if (first instanceof TreeNode)
                return ((TreeNode<K,V>)first).getTreeNode(hash, key);
            do {
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    return e;
            } while ((e = e.next) != null);
        }
    }
    return null;
}

This :

(k = first.key) == key

and 

(k = e.key) == key

refer to this optimization.

Besides, here, you violate the rule of the equals() contract that says that is has to be reflexive :

class Foo {
    @Override
    public boolean equals(Object o) {
        return false;
    }    
   ...
}

for any non-null reference value x, x.equals(x) should return true.

From the moment where a class violates the equals() contract, you cannot have any guarantee that the classes that manipulate instances of the flawed class will have the expected behavior.

Upvotes: 7

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