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

Based on the answer by Riccardo, following code snippet worked for me:

public class Either {
        private L left_value;
        private R right_value;
        private boolean right;

        public L getLeft() {
            if(!right) {
                return left_value;
            } else {
                throw new IllegalArgumentException("Left is not initialized");
            }
        }

        public R getRight() {
            if(right) {
                return right_value;
            } else {
                throw new IllegalArgumentException("Right is not initialized");
            }
        }

        public boolean isRight() {
            return right;
        }

        public Either(L left_v, Void right_v) {
           this.left_value = left_v;
           this.right = false;
        }

        public Either(Void left_v, R right_v) {
          this.right_value = right_v;
          right = true;
        }

    }

Usage:

Either onlyString = new Either("string", null);
Either onlyInt = new Either(null, new Integer(1));

if(!onlyString.isRight()) {
  String s = onlyString.getLeft();
}