Violating rule of reflexivity when overriding equals method

Question

I have a doubt from the book Efective Java. The doubt is regarding equals method
reflexive rule violation. The book says the following:

"If you were to violate it and then add an instance of your class to a collection, the collection's contains method would almost certainly say that the collection did not contain the instance that you just added."

To test it I wrote an example class, but the contains method doesn't return false It returns true. Can anybody tell what is the problem?

Answer 1

I agree that the result of this program is indeed puzzling:

import java.util.*;

class Item {
    @Override
    public boolean equals(Object obj) {
        return false;   // not even equal to itself.
    }
}

class Test {
    public static void main(String[] args) {
        Collection<Item> items = new HashSet<Item>();
        Item i = new Item();
        items.add(i);
        System.out.println(items.contains(i));   // Prints true!
    }
}

The answer is that the contains implementation checks argument == object before doing argument.equals(object). The result from contains is true since item == item holds, even though item.equals(item) returns false.

Assuming equals follows its contract (is reflexive), this way of implementing contains is correct.

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If you read the quote you posted carefully, the author includes the word "almost" :) It seems you stumbled across one of the few exceptions to the rule.

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Other collections (ArrayList for instance) uses equals directly, and if you change from new HashSet<Item>() to new ArrayList<Item>() in the above program it prints false as expected.