Is it possible in java make something like Comparator but for implementing custom equals() and hashCode()

Question

I have an array of objects and I want to concatenate it with another array of objects, except that objects that have same id\'s. That objects are used in many places in the

Answer 1

HashingStrategy is the concept you're looking for. It's a strategy interface that allows you to define custom implementations of equals and hashcode.

public interface HashingStrategy
{
    int computeHashCode(E object);
    boolean equals(E object1, E object2);
}

As others have pointed out, you can't use a HashingStrategy with the built in HashSet or HashMap. Eclipse Collections includes a set called UnifiedSetWithHashingStrategy and a map called UnifiedMapWithHashingStrategy.

Let's look at an example. Here's a simple Data class we can use in a UnifiedSetWithHashingStrategy.

public class Data
{
    private final int id;

    public Data(int id)
    {
        this.id = id;
    }

    public int getId()
    {
        return id;
    }

    // No equals or hashcode
}

Here's how you might set up a UnifiedSetWithHashingStrategy and use it.

java.util.Set set =
  new UnifiedSetWithHashingStrategy<>(HashingStrategies.fromFunction(Data::getId));
Assert.assertTrue(set.add(new Data(1)));

// contains returns true even without hashcode and equals
Assert.assertTrue(set.contains(new Data(1)));

// Second call to add() doesn't do anything and returns false
Assert.assertFalse(set.add(new Data(1)));

Why not just use a Map? UnifiedSetWithHashingStrategy uses half the memory of a UnifiedMap, and one quarter the memory of a HashMap. And sometimes you don't have a convenient key and have to create a synthetic one, like a tuple. That can waste more memory.

How do we perform lookups? Remember that Sets have contains(), but not get(). UnifiedSetWithHashingStrategy implements Pool in addition to MutableSet, so it also implements a form of get().

Note: I am a committer for Eclipse Collections.