implement 2 flavors of Equals method for a class in Java

Question

public class Foo{
    private final int A;
    private final int B;
    public boolean equals(Object o){
        //type check omitted
        return A==o.A && B==o.B;
    }

}

I want to have another .equals() method like this

public boolean equals(Object o){
    return A==o.A;
}

The Foo object is first created with A,B field, then I want to send them off to a Set<E> that used the 2nd equals() method to only compares field A.

I know I can create new objects that only have A field, but overhead would be big. Any advice?

Answer 1

Using composition, you could create a FooWrapper class that provides a custom implementation of equals and add that to the set instead:

public class FooWrapper {
    public final Foo foo; //constructor omitted

    public boolean equals(Object other) {
         //type check omitted here too
         return foo.A == other.foo.A;
    }

    //hashCode omitted, but you need that too
}

Unfortunately with the Java Collections API there is no way to tell a collection to use a custom equals computation other than the method above or subclassing and overriding equals().

Edit:

It strikes me that you might instead be able to make use of a Map<Integer, Foo> instead, using foo.A as the key and foo as the value (and thus restricting it to unique values of A). I couldn't tell you whether that's suitable without more context though.