对象等于方法和哈希码方法

Question

I'm revising for my upcoming exam and I've came across a question from a past paper that I'm unsure on.

The question is the following : Describe the equivalence relation that the Object equals method implements. What relationship must hold between the Object equals method and hasCode methods?

If this came up in the exam then I wouldn't be too sure how about answering this. I'll try and give it my best shot. Since the Object equals method is checking whether two objects are equal, the hashcode gives objects ascii values. If you have two objects that are the same it is possible that they can be given the same hascode since they're equal. The object equals method you are told whether they're equal whereas the hashcode method gives you a value which you can then use to compare with other hashcodes and find out whether they're the same or not. Also could have I of mentioned something about overriding methods ? I'm not if that could come into play in the answer.

My answer is probably completely wrong but the best I could think off :P

Answer 1

To the Java API Documentation! especially Object.hashCode

If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.

This means that if the equal method returns true, hashCode must return the same value for both objects.

x.equals(y) == true    =>   x.hashCode() == y.hashCode()

It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.

But! If the objects are not the same they may have the same hashCode()! (But it is not advised)

The requirements for the equals() method are well described, so i won't repeat them here.

Answer 2

The question is clearly answered in the descriptions of both the equals() method and the hashCode() method. Have a look at equals method description and hashCode method description .

Here is the relevant answer you might be looking for:

The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true).

Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.

This the hashCode method description:

If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.

Also, the hashCode method does not give the ASCII value of an object, merely an integer value that uniquely defines the object.