Issue#550: double buffer pattern (#1024)

* Basic implementation

* implement double buffer

* add unit test

* add unit test

* Add Readme

* Change local value declaration to var

* Remove unused fields

double-buffer/src/test/java/com/iluwatar/doublebuffer/SceneTest.java

@@ -0,0 +1,74 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ * <p>
+ * 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:
+ * <p>
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ * <p>
+ * 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.doublebuffer;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.lang.reflect.Field;
+import java.util.ArrayList;
+
+/**
+ * Scene unit tests.
+ */
+public class SceneTest {
+
+  @Test
+  public void testGetBuffer() {
+    try {
+      var scene = new Scene();
+      var field1 = Scene.class.getDeclaredField("current");
+      field1.setAccessible(true);
+      field1.set(scene, 0);
+      FrameBuffer[] frameBuffers = new FrameBuffer[2];
+      FrameBuffer frameBuffer = new FrameBuffer();
+      frameBuffer.draw(0, 0);
+      frameBuffers[0] = frameBuffer;
+      var field2 = Scene.class.getDeclaredField("frameBuffers");
+      field2.setAccessible(true);
+      field2.set(scene, frameBuffers);
+      Assert.assertEquals(frameBuffer, scene.getBuffer());
+    } catch (NoSuchFieldException | IllegalAccessException e) {
+      Assert.fail("Fail to access private field.");
+    }
+  }
+
+  @Test
+  public void testDraw() {
+    try {
+      var scene = new Scene();
+      var field1 = Scene.class.getDeclaredField("current");
+      var field2 = Scene.class.getDeclaredField("next");
+      field1.setAccessible(true);
+      field1.set(scene, 0);
+      field2.setAccessible(true);
+      field2.set(scene, 1);
+      scene.draw(new ArrayList<>());
+      Assert.assertEquals(1, field1.get(scene));
+      Assert.assertEquals(0, field2.get(scene));
+    } catch (NoSuchFieldException | IllegalAccessException e) {
+      Assert.fail("Fail to access private field");
+    }
+  }
+}