diff --git a/src/main/java/doodle_jump/Game.java b/src/main/java/doodle_jump/Game.java
index 5c6807a..59be947 100644
--- a/src/main/java/doodle_jump/Game.java
+++ b/src/main/java/doodle_jump/Game.java
@@ -84,7 +84,7 @@ protected void preAction() {
      */
     protected void postAction() {
         if (this.flag) {
-            moveStageUp();
+            this.platforms.moveStageUp();
             System.out.println("Flag is true");
             this.flag = false;
         }
@@ -122,18 +122,6 @@ public void play() {
         this.showResultScreen();
     }
 
-    private void moveStageUp() {
-        int offset = UP_LIMIT;
-        for (Platform p : this.platforms.list) {
-            p.setCoordinates(p.getCoordinateX(), p.getCoordinateY() + offset);
-            if (p.getCoordinateX() > this.getHeight()) {
-                p.setCoordinates(
-                    Math.random() * (getWidth() - p.getWidth()),
-                    (Math.random() * 50) - 60);
-            }
-        }
-    }
-
     private class DoodleGameKeyListener extends KeyAdapter {
 
         @Override
diff --git a/src/main/java/doodle_jump/PlatformCollection.java b/src/main/java/doodle_jump/PlatformCollection.java
index c7f4ae3..6b5b7cc 100644
--- a/src/main/java/doodle_jump/PlatformCollection.java
+++ b/src/main/java/doodle_jump/PlatformCollection.java
@@ -35,9 +35,21 @@ public void genNewPlatforms() {
             int x = (int) (Math.random() * (getWidth() - p.getWidth()));
             int y = i * ((int) (Math.random() * 50) + 70);
             p.setCoordinates(x, y);
-            this.list.add(p);
-            this.add(p);
+            this.addObj(p);
         }
 
     }
+
+    public void moveStageUp() {
+        int offset = Game.UP_LIMIT;
+        
+        for (Platform p : this.getCollection()) {
+            p.setCoordinates(p.getCoordinateX(), p.getCoordinateY() + offset);
+            if (p.getCoordinateY() > (double) this.getHeight()) {
+                p.setCoordinates(
+                    Math.random() * (getWidth() - p.getWidth()),
+                    (Math.random() * 50) - 60);
+            }
+        }
+    }
 }
diff --git a/src/main/java/game_engine/GameObjectCollection.java b/src/main/java/game_engine/GameObjectCollection.java
index a52b6e6..289219d 100644
--- a/src/main/java/game_engine/GameObjectCollection.java
+++ b/src/main/java/game_engine/GameObjectCollection.java
@@ -1,9 +1,12 @@
 package game_engine;
 
 import java.util.ArrayList;
+
+import javax.swing.JCheckBox;
 import javax.swing.JPanel;
 import java.awt.Graphics;
 import java.awt.image.ImageObserver;
+import java.awt.Component;
 
 import game_engine.GameObject;
 
@@ -12,9 +15,7 @@
  * GameObjects have the same class.
  */
 public class GameObjectCollection<T extends GameObject> extends JPanel {
-    public ArrayList<T> list;
-    int width;
-    int height;
+    private ArrayList<T> list;
 
     /**
      * Constructor.
@@ -26,8 +27,6 @@ public GameObjectCollection(int width, int height) {
         this.setOpaque(false);
         this.setLocation(0, 0);
         this.setSize(width, height);
-        this.width = width;
-        this.height = height;
 
         this.list = new ArrayList<T>();
     }
@@ -49,12 +48,16 @@ public void addObj(T obj) {
      */
     public boolean removeObj(T obj) {
         boolean isRemoved = false;
+        Component[] componentList = this.getComponents();
 
-        if (this.list.remove(obj)) {
-            this.remove(obj);
-            isRemoved = true;
+        for (Component component: componentList) {
+            if (component == obj) {
+                this.list.remove(obj);
+                this.remove(component);
 
-            this.update();
+                isRemoved = true;
+                this.update();
+            }
         }
 
         return isRemoved;
@@ -94,9 +97,9 @@ public int getCollectionSize() {
     }
 
     /**
-     * Remove all object from list.
+     * Remove all objects from list.
      */
-    public void clearObj() {
+    public void clearCollection() {
         this.removeAll();
         this.list.clear();
         this.update();
@@ -110,11 +113,11 @@ private void update() {
         this.repaint();
     }
 
-    public int getWidth() {
-        return this.width;
-    }
-
-    public int getHeight() {
-        return this.height;
+    /**
+     * Get object collection.
+     * @return object collection.
+     */
+    public ArrayList<T> getCollection() {
+        return this.list;
     }
 }
diff --git a/src/main/java/game_engine/GamePanel.java b/src/main/java/game_engine/GamePanel.java
index 1ff75c9..b103eed 100644
--- a/src/main/java/game_engine/GamePanel.java
+++ b/src/main/java/game_engine/GamePanel.java
@@ -22,10 +22,15 @@ public GamePanel(int x, int y, Image image) {
 
         this.image = image;
         this.setSize(this.image.getWidth(null), this.image.getHeight(null));
+
         this.point = new Point<Double>();
         this.setCoordinates(x, y);
     }
 
+    /**
+     * Constructor to create with coordinates (0, 0).
+     * @param image image.
+     */
     public GamePanel(Image image) {
         this(0, 0, image);
     }
@@ -46,28 +51,56 @@ public Image getImage() {
         return this.image;
     }
 
+    /**
+     * Set coordinates in floating-point format (double).
+     * JPanel.setLocation gets coordinates in integer format after casting.
+     * @param x x-coordinate of top-left corner.
+     * @param y y-coordinate of top-left corner.
+     */
     public void setCoordinates(double x, double y) {
         this.setLocation((int) x, (int) y);
         this.point.setCoordinates(x, y);
     }
 
+    /**
+     * Set coordinates in integer format (int).
+     * JPanel.setLocation gets coordinates in integer format without casting.
+     * @param x x-coordinate of top-left corner.
+     * @param y y-coordinate of top-left corner.
+     */
     public void setCoordinates(int x, int y) {
         this.setLocation(x, y);
         this.point.setCoordinates((double) x, (double) y);
     }
 
+    /**
+     * Set x-coordinate.
+     * @param x x-coordinate of top-left corner.
+     */
     public void setCoordinateX(double x) {
         this.setCoordinates(x, this.point.getX());
     }
 
+    /**
+     * Set y-coordinates.
+     * @param y y-coordinate of top-left corner.
+     */
     public void setCoordinateY(double y) {
         this.setCoordinates(this.point.getY(), y);
     }
 
+    /**
+     * Get x-coordinate.
+     * @return x-coordinate.
+     */
     public double getCoordinateX() {
         return this.point.getX();
     }
 
+    /**
+     * Get y-coordinate.
+     * @return y-coordinate.
+     */
     public double getCoordinateY() {
         return this.point.getY();
     }