In Java, why must equals() and hashCode() be consistent?

Question

If I override either method on a class, it must make sure that if A.equals(B) == true then A.hashCode() == B.hashCode must also be true.

Can

Answer 1

Here's a little example:

Set myFoos = new HashSet(); 
Foo firstFoo = new Foo(123,"Alpha");
myFoos.add(firstFoo);

// later in the processing you get another Foo from somewhere
Foo someFoo = //use imagination here...; 
// maybe you get it from a database... and it's equal to Foo(123,"Alpha)

if (myFoos.contains(someFoo)) {
   // maybe you win a million bucks.
}

So, imagine that the hashCode that gets created for firstFoo is 99999 and it winds up at a specific spot in the myFoos HashSet. Later when you get the someFoo and you look for it in the myFoos HashSet, it needs to generate the same hashCode so you can find it.