ConsumerFunction (Java 8)

package ch.claude_martin.lambda;

import java.lang.reflect.Constructor;
import java.util.Objects;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.function.UnaryOperator;

/**
 * A {@link Function} that returns a <code>null</code> of type Void.
 *
 * <p>
 * This is to demonstrate why a {@link Consumer} isn't also a {@link Function}.
 * <p>
 * This interface could come handy if you need a collection of functional methods that are all of
 * type {@link Function} but you want to include some {@link Consumer}s.
 * <p>
 * {@link UnaryOperator} is a Function as it has a return value, but a {@link Consumer} does not
 * return anything. There exists a member {@link #VOID} that references an instance of {@link Void}
 * or null and can be used as the returned value.
 * <p>
 * The same is true for similar Types such as Supplier. There you'd have to pass something to
 * {@link Supplier#get() get}.
 */
@FunctionalInterface
public interface ConsumerFunction<T> extends Function<T, Void> {

  /**
   * {@inheritDoc}
   *
   * @returns instance of {@link Void} or null
   */
  public Void apply(T t);

  /** @see Consumer#accept(Object) */
  default public void accept(final T t) {
    apply(t);
  }

  /** Convert this object to a regular {@link Consumer}. */
  default public Consumer<T> toConsumer() {
    return ConsumerFunction.this::apply;
  }

  /**
   * This instance of {@link Void} can be used as the returned value. It is null if no instance
   * could be created.
   */
  public static final Void VOID = ((Supplier<Void>) () -> {
    try {
      final Constructor<Void> constructor = Void.class.getDeclaredConstructor();
      constructor.setAccessible(true);
      return constructor.newInstance();
    } catch (Throwable t) {
      return null;
    }
  }).get();

  /** Convert some {@link Consumer} to a {@link Function}. */
  public static <T> ConsumerFunction<T> toFunction(final Consumer<T> consumer) {
    return t -> {
      consumer.accept(t);
      return VOID;
    };
  }

  /** @see Consumer#andThen(Consumer) */
  default ConsumerFunction<T> andThen(final ConsumerFunction<? super T> after) {
    Objects.requireNonNull(after);
    return (T t) -> {
      apply(t);
      after.apply(t);
      return VOID;
    };
  }

  /** @see Consumer#andThen(Consumer) */
  default Consumer<T> andThen(final Consumer<? super T> after) {
    Objects.requireNonNull(after);
    return (T t) -> {
      apply(t);
      after.accept(t);
    };
  }
}