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Java Simplified. Extensions and Useful Constructs for the Java Platform. Codeprimate Elements (a.k.a. cp-elements) is a Java library and micro-framework used to simplify the development of software applications written in Java. Elements packages several APIs into one library in order to address various application concerns and aspects of software design and development collectively and conveniently. Elements is a highly simple, yet robust and proven library built on solid OO principles, software design patterns and best practices to effectively solve common and reoccurring problems in software development.

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/*
 * Copyright 2016 Author or Authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.cp.elements.lang.support;

import org.cp.elements.lang.Assert;
import org.cp.elements.lang.Composite;

/**
 * The ComposableRunnable class is an implementation of the Runnable interface and the Composite Design Pattern to
 * compose multiple Runnable objects and treat it as a single instance of Runnable.
 *
 * @author John J. Blum
 * @see java.lang.Runnable
 * @since 1.0.0
 */
@SuppressWarnings("unused")
public class ComposableRunnable implements Composite, Runnable {

  protected static final ComposableRunnable INSTANCE = new ComposableRunnable();

  private final Runnable leftRunnable;
  private final Runnable rightRunnable;

  /**
   * Returns the single instance of {@link ComposableRunnable} used to compose 2 or more indiviudal {@link Runnable}
   * objects into a {@link Composite} {@link Runnable} object.
   *
   * @return the single instance of {@link ComposableRunnable}.
   * @see org.cp.elements.lang.support.ComposableRunnable
   */
  public static ComposableRunnable getInstance() {
    return INSTANCE;
  }

  /**
   * Default, private constructor used to construct a Singleton instance of the {@link ComposableRunnable} used to
   * compose 2 or more individual {@link Runnable} objects into a Composite {@link Runnable} object.
   */
  private ComposableRunnable() {
    this.leftRunnable = () -> {};
    this.rightRunnable = () -> {};
  }

  /**
   * Constructs an instance of the ComposableRunnable class composed an initialized with 2 Runnable objects.
   *
   * @param leftRunnable the left Runnable node in the composed graph.
   * @param rightRunnable the right Runnable node in the composed graph.
   * @throws NullPointerException if either the left or right Runnable object is null.
   */
  private ComposableRunnable(Runnable leftRunnable, Runnable rightRunnable) {
    Assert.notNull(leftRunnable, "The left Runnable object cannot be null");
    Assert.notNull(rightRunnable, "The right Runnable object cannot be null");

    this.leftRunnable = leftRunnable;
    this.rightRunnable = rightRunnable;
  }

  /**
   * Composes two {@link Runnable} objects into a single, compound {@link Runnable} object.
   *
   * Returns the {@code leftRunnable} if {@code rightRunnable} is {@literal null}
   * and {@code rightRunnable} if the {@code leftRunnable} is {@literal null}.
   *
   * @param leftRunnable {@link Runnable} object in the composition.
   * @param rightRunnable {@link Runnable} object in the composition.
   * @return a {@link Runnable} object composed with the given left and right {@link Runnable} objects.
   * @see java.lang.Runnable
   */
  public Runnable compose(Runnable leftRunnable, Runnable rightRunnable) {
    return (leftRunnable == null ? rightRunnable : (rightRunnable == null ? leftRunnable
      : new ComposableRunnable(leftRunnable, rightRunnable)));
  }

  /**
   * Runs the left and right Runnable objects/nodes in the graph.
   *
   * @see java.lang.Runnable#run()
   */
  @Override
  public void run() {
    leftRunnable.run();
    rightRunnable.run();
  }
}