Equals and HashCode

[Juan Manuel Alberto de los Santos]

i was reading the K&B book, when i came to this



this code i made, says the contrary

class test {
 
    public static void main(String[] args) {
        test x = new test();
        test y = new test();
        System.out.println(x.equals(y)); //true
        System.out.println(x.hashCode() == y.hashCode()); //false
    }
 
    public boolean equals(Object o) {
        return true;
    }
}

thanks !
[ October 10, 2008: Message edited by: Juan Manuel Alberto de los Santos ]

[Varun Chopra]

Your book must also say whenever you override equals(), you have to override hashcode() as well.

[Thomas Thevis]

this code i made, says the contrary

Well, you can always create code breaking so called general contracts. The book specifies the conditions which have to be fulfilled to satisfy the general contract for java.lang.Object.hashCode() (have a look at the javadocs there).
It is necessary to satisfy the general contract to ensure that other classes relying on that contract work properly. For example, hash based collections like java.util.HashSet may not work properly if you break that contract. Do you know why?

Regards,
Thomas