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

Thanks to Derive4J algebraic data types are now very easy in Java. All you have to do is create the following class:

import java.util.function.Function;

@Data
public abstract class Either {

  Either(){}

  /**
   * The catamorphism for either. Folds over this either breaking into left or right.
   *
   * @param left  The function to call if this is left.
   * @param right The function to call if this is right.
   * @return The reduced value.
   */
  public abstract  X either(Function left, Function right);
}

And Derive4J will take care of creating constructors for the left and rights cases, as well as a pattern matching syntax alla Haskell, mapper methods for each sides, and more.