Java maybe monad

Function.java

package br.com.ngi.ginga.business.util.seq;

/**
 * Copyright (C) 2015 - NG Informática
 *
 * @author Marcelo Camargo
 * @since 08/12/2015
 */
public interface Function<Ret, Arg> {
    Ret call(Arg arg);
}

IMaybe.java

package br.com.ngi.ginga.lib.monad;

import br.com.ngi.ginga.business.util.seq.Function;

/**
 * Copyright (C) 2015 - NG Informática
 *
 * @author Marcelo Camargo
 * @since 10/02/16
 */
public interface IMaybe<A> {
  <B> IMaybe<B> bind(Function<B, A> fn);
  A fromJust();
  A fromMaybe(A def);
  boolean isJust();
  boolean isNothing();
  <B> B maybe(B def, Function<B, A> fn);
}

Just.java

package br.com.ngi.ginga.lib.monad;

import br.com.ngi.ginga.business.util.seq.Function;

/**
 * Copyright (C) 2015 - NG Informática
 *
 * @author Marcelo Camargo
 * @since 10/02/16
 */
public class Just<A> implements IMaybe<A> {
  private A value;

  public Just(A value) {
    this.value = value;
  }

  @Override
  public <B> IMaybe bind(Function<B, A> fn) {
    return Maybe.maybe(fn.call(this.value));
  }

  @Override
  public A fromJust() {
    return this.value;
  }

  @Override
  public A fromMaybe(A def) {
    return this.value;
  }

  @Override
  public boolean isJust() {
    return true;
  }

  @Override
  public boolean isNothing() {
    return false;
  }

  @Override
  public <B> B maybe(B def, Function<B, A> fn) {
    return fn.call(null);
  }
}

Maybe.java

package br.com.ngi.ginga.lib.monad;

/**
 * Copyright (C) 2015 - NG Informática
 *
 * @author Marcelo Camargo
 * @since 10/02/16
 */
public final class Maybe {
  public static <T> IMaybe maybe(T value) {
    if (value instanceof IMaybe) {
      return (IMaybe) value;
    }

    return value == null ? new Nothing() : new Just<T>(value);
  }
}

Nothing.java

package br.com.ngi.ginga.lib.monad;

import br.com.ngi.ginga.business.util.seq.Function;

/**
 * Copyright (C) 2015 - NG Informática
 *
 * @author Marcelo Camargo
 * @since 10/02/16
 */
public class Nothing<A> implements IMaybe<A> {
  @Override
  public IMaybe bind(Function fn) {
    return this;
  }

  @Override
  public A fromJust() {
    throw new RuntimeException("Cannot call fromJust() on Nothing");
  }

  @Override
  public A fromMaybe(A def) {
    return def;
  }

  @Override
  public boolean isJust() {
    return false;
  }

  @Override
  public boolean isNothing() {
    return true;
  }

  @Override
  public <B> B maybe(B def, Function<B, A> fn) {
    return def;
  }
}