How can I simulate Haskell's “Either a b” in Java

Question

How can I write a typesafe Java method that returns either something of class a or something of class b? For example:

public ... either(boolean b) {
  if (b)         


        

Answer 1

My general formula for simulating algebraic data types is:

• The type is an abstract base class, and the constructors are subclasses of that
• The data for each constructor are defined in each subclass. (This allows constructors with different numbers of data to work correctly. It also removes the need to maintain invariants like only one variable is non-null or stuff like that).
• The constructors of the subclasses serve to construct the value for each constructor.
• To deconstruct it, one uses instanceof to check the constructor, and downcast to the appropriate type to get the data.

So for Either a b, it would be something like this:

abstract class Either { }
class Left extends Either {
    public A left_value;
    public Left(A a) { left_value = a; }
}
class Right extends Either {
    public B right_value;
    public Right(B b) { right_value = b; }
}

// to construct it
Either foo = new Left(some_A_value);
Either bar = new Right(some_B_value);

// to deconstruct it
if (foo instanceof Left) {
    Left foo_left = (Left)foo;
    // do stuff with foo_left.a
} else if (foo instanceof Right) {
    Right foo_right = (Right)foo;
    // do stuff with foo_right.b
}