javaslang.option.None Maven / Gradle / Ivy

Go to download
package javaslang.option;

import java.util.NoSuchElementException;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;

/**
 * None is a singleton representation of the undefined {@link javaslang.option.Option}. The instance
 * is obtained by calling {@link #instance()}.
 *
 * @param  The type of the optional value.
 */
public final class None implements Option {

	private static final None NONE = new None<>();

	private None() {
	}

	/**
	 * Returns the singleton instance of None as {@code None} in the
	 * context of a type {@code }, e.g.
	 * 
	 * 	 * final Option<Integer> o = None.instance(); // o is of type None<Integer>
	 * 
	 * 
	 * @param  The type of the optional value.
	 * @return None
	 */
	public static  None instance() {
		@SuppressWarnings("unchecked")
		final None none = (None) NONE;
		return none;
	}

	@Override
	public T get() {
		throw new NoSuchElementException("No value present");
	}

	@Override
	public T orElse(T other) {
		return other;
	}

	@Override
	public T orElseGet(Supplier other) {
		return other.get();
	}

	@Override
	public  T orElseThrow(Supplier exceptionSupplier) throws X {
		throw exceptionSupplier.get();
	}

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

	@Override
	public void ifPresent(Consumer consumer) {
		// nothing to do
	}

	@Override
	public Option filter(Predicate predicate) {
		// semantically correct but structurally the same that return this;
		return None.instance();
	}

	@Override
	public  Option map(Function mapper) {
		return None.instance();
	}

	@Override
	public  Option flatMap(Function> mapper) {
		return None.instance();
	}

	@Override
	public void forEach(Consumer action) {
		// nothing to do
	}

	// super.equals and super.hashCode are fine because this is a singleton

	@Override
	public String toString() {
		return "None";
	}

}