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

The suggestions already provided, although feasible, are not complete as they rely on some null references and effectively make "Either" masquerade as a tuple of values. A disjoint sum is obviously one type or the other.

I'd suggest having a look at the implementation of FunctionalJava's Either as an example.

Answer 2

The closest I can think of is a wrapper around both values that lets you check which value is set and retrieve it:

class Either<TLeft, TRight> {
    boolean isLeft;

    TLeft left;
    TRight right;

    Either(boolean isLeft, TLeft left1, TRight right) {
        isLeft = isLeft;
        left = left;
        this.right = right;
    }

    public boolean isLeft() {
        return isLeft;
    }

    public TLeft getLeft() {
        if (isLeft()) {
            return left;
        } else {
            throw new RuntimeException();
        }
    }

    public TRight getRight() {
        if (!isLeft()) {
            return right;
        } else {
            throw new RuntimeException();
        }
    }

    public static <L, R> Either<L, R> newLeft(L left, Class<R> rightType) {
        return new Either<L, R>(true, left, null);
    }

    public static <L, R> Either<L, R> newRight(Class<L> leftType, R right) {
        return new Either<L, R>(false, null, right);
    }
}

class Main {
    public static void main(String[] args) {
        Either<String,Integer> foo;
        foo = getString();
        foo = getInteger();
    }

    private static Either<String, Integer> getInteger() {
        return Either.newRight(String.class, 123);
    }

    private static Either<String, Integer> getString() {
        return Either.newLeft("abc", Integer.class);
    }   
}