A demonstration of haskell-style pattern matching in java

Maybe::match

public abstract class Maybe<V>
{
  public abstract <T> T match(Function<V, T> some, Supplier<T> none);
  public abstract void match(Consumer<V> some, Runnable none);
  ...
  private static class Nothing<U> extends Maybe<U>
  {
    ...
    public <T> T match(Function<U, T> some, Supplier<T> none)
    {
      return none.get();
    }

    public void match(Consumer<U> some, Runnable none)
    {
      none.run();
    }
    ...
  }

  private static class Just<V> extends Maybe<V>
  {
    final V value;
...
    public <T> T match(Function<V, T> some, Supplier<T> none)
    {
      return some.apply(value);
    }

    public void match(Consumer<V> some, Runnable none)
    {
      some.accept(value);
    }
...
  }

Example usage:

Maybe<Integer> m = someOperation();
m.match(
      (Integer value) -> System.out.println("value = " + value),
      () -> System.out.println("there is no value")
);