Update Method pattern (#1106)

* Add update method pattern

* Add unit tests

* Add README.md

* Resolve conflict

update-method/src/main/java/com/iluwatar/updatemethod/World.java

@@ -0,0 +1,114 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.updatemethod;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Random;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * The game world class. Maintain all the objects existed in the game frames.
+ */
+public class World {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(World.class);
+
+  protected List<Entity> entities;
+
+  protected volatile boolean isRunning;
+
+  public World() {
+    entities = new ArrayList<>();
+    isRunning = false;
+  }
+
+  /**
+   * Main game loop. This loop will always run until the game is over. For
+   * each loop it will process user input, update internal status, and render
+   * the next frames. For more detail please refer to the game-loop pattern.
+   */
+  private void gameLoop() {
+    while (isRunning) {
+      processInput();
+      update();
+      render();
+    }
+  }
+
+  /**
+   * Handle any user input that has happened since the last call. In order to
+   * simulate the situation in real-life game, here we add a random time lag.
+   * The time lag ranges from 50 ms to 250 ms.
+   */
+  private void processInput() {
+    try {
+      int lag = new Random().nextInt(200) + 50;
+      Thread.sleep(lag);
+    } catch (InterruptedException e) {
+      LOGGER.error(e.getMessage());
+    }
+  }
+
+  /**
+   * Update internal status. The update method pattern invoke udpate method for
+   * each entity in the game.
+   */
+  private void update() {
+    for (var entity : entities) {
+      entity.update();
+    }
+  }
+
+  /**
+   * Render the next frame. Here we do nothing since it is not related to the
+   * pattern.
+   */
+  private void render() {}
+
+  /**
+   * Run game loop.
+   */
+  public void run() {
+    LOGGER.info("Start game.");
+    isRunning = true;
+    var thread = new Thread(this::gameLoop);
+    thread.start();
+  }
+
+  /**
+   * Stop game loop.
+   */
+  public void stop() {
+    LOGGER.info("Stop game.");
+    isRunning = false;
+  }
+
+  public void addEntity(Entity entity) {
+    entities.add(entity);
+  }
+
+}