如何为子类编写hashCode（）

Question

I have a question about hashCode. For example:

class A{
   String name;
   int age;

   @Override
   public int hashCode(){
       int hash = 0;
       hash = age;
       hash = 31*hash + name.hashCode();
   }
}
class B extends A{
}
class C extends A{
}    

My question is, if I instantiate one B object and one C object with the same name and age. Then the hashCode() for A and B are the same as well. Is it correct for hashCode()? if not, what should I do to modify my code?

Answer 1

Sure, that's ok for hashCode() to do that.

Of course, if B or C adds any new fields (which would be common), then they should override hashCode() , call super.hashCode() and "extend" the value as well.

Also, as @bradimus hinted in a comment to question, when you override hashCode() , you should very likely also override equals() .