finish Q4a

src/Q4.java

@@ -27,6 +27,28 @@ public class Q4 extends AbstractGame {
     private final Font font = new Font("res/conformable.otf", 24);
     private static final Point FONT_POSITION = new Point(32, 32);
 
+    // Now, we are revising the catch the Ball game so that the player moves toward the ball step by step. In
+    //this question, you will learn how to set the player's moving direction and how to let the player move
+    //towards this direction by one step. Let the movement of the player controlled by the Enter key, not
+    //the arrow keys:
+    // (0) If the Enter key is pressed (use Input.wasPressed method), do the following:
+
+    // (a) Calculate the direction (xd; yd) pointing from the player to the ball: for (x1; y1) and (x2; y2) being
+    // the positions of the player and the ball respectively, set:
+    //      xd = (x2-x1) / Math.sqrt(Math.pow(x1-x2, 2) + Math.pow(y1-y2, 2))
+    //      yd = (y2-y1) / Math.sqrt(Math.pow(x1-x2, 2) + Math.pow(y1-y2, 2))
+    private double playerDirectionX = 0,
+                   playerDirectionY = 0;
+
+    /**
+     * calculate the direction for the player based on the input point w.r.t. player's location
+     * @param Dest destination for the player to move to
+     */
+    public void setPlayerDirectionTo(Point Dest){
+        double Len = new Point(playerX, playerY).distanceTo(Dest);
+        playerDirectionX = (Dest.x - playerX)/Len;
+        playerDirectionY = (Dest.y - playerY)/Len;
+    }
 
     public static void main(String[] args) {
         new Q4().run();
@@ -37,12 +59,19 @@ public class Q4 extends AbstractGame {
         if (input.wasPressed(Keys.ESCAPE)) {
             System.out.println("closed");
             Window.close();
+        } else {
+            // (0)
+            if (input.wasPressed(Keys.ENTER)) {
+                // (a)
+                this.setPlayerDirectionTo(BALL_POSITION);
             }
 
             if (new Point(playerX, playerY).distanceTo(BALL_POSITION) <= SCORE_DISTANCE) {
                 // System.out.println("Great job!");
                 font.drawString("Great job!", FONT_POSITION.x, FONT_POSITION.y);
             }
+        }
+
 
         playerImage.draw(playerX, playerY);
         ballImage.draw(BALL_POSITION.x, BALL_POSITION.y);