hashCode() for an array of objects for use in Hash

Question

I Have the following two classes and want to use Foo1 as keys in a HashMap. Two Foo1 objects are equal if their Foo2 objects are equal, and Foo2 objects are equal if their byte arrays satisfy Arrays.equals().

I am not quite sure what to do for the hashCode() method for Foo1. Do I just need to sum up the hashcodes from each of its Foo2 objects or is this inefficient?

public class Foo1 {

  Foo2[] foo2_array;

  @Override
  public boolean equals(Object Other) {

     for (int i = 0; i < foo2_array.length; i++) {

        if (!foo2_array[i].equals(other.foo2_array[i])
          return false;
     }

     return true;
   }

   @Override
   public int hashCode() {

      // what to here?
   }
}

public class Foo2 {

  byte[] values;

  @Override
  public boolean equals(Object other) {

      return Arrays.equals(values, other.values);
  }

  @Override
  public int hashCode() {

     return Arrays.hashCode(values);
  }
}

Answer 1

You essentially need to have some method that makes it likely that different objects will have different hash codes.

So depending on your data, you don't necessarily need to sum up the hashes of all the items in the array. You just basically need something "good enough to narrow things down".

I would put it like this: is there anything about your data that makes you suspect you couldn't just take, say, the hash code of the middle value of the array? Or maybe the combined hash codes of the first, last and middle items, for example?

(Things that would make you suspect you couldn't do that: if, say, your data had some special feature making a certain narrow subset of values occur as the middle element in the array.)

Answer 2

Your hashcode should use the same set of properties as equals for it not to break the contract.

Just use the Arrays.hashcode as done in Foo2

Also you dont have to loop through each element in your equals you can just use Arrays.equals

Foo2 equals can look like this similar to Foo1.equals

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Foo1 other = (Foo1) obj;
        if (!Arrays.equals(foo2_array, other.foo2_array))
            return false;
        return true;
    }

and hashcode similar to Foo1 hashcode

    @Override
    public int hashCode() {
        return Arrays.hashCode(foo2_array);
    }

Also while implementing equals do check for same reference and object validity for null.