update

src/main/java/PingPong/GamePanel.java

@@ -63,6 +63,14 @@ public class GamePanel extends JPanel implements Runnable {
             ball.setXDirection(-ball.xVelocity);
             ball.setYDirection(ball.yVelocity);
         }
+        if (paddle_1.y1 <= 0)
+            paddle_1.y1 = 0;
+        if (paddle_1.y1 >= (FRAME_HEIGHT-PADDLE_HEIGHT))
+            paddle_1.y1 = FRAME_HEIGHT-PADDLE_HEIGHT;
+        if (paddle_2.y2 <= 0)
+            paddle_2.y2 = 0;
+        if (paddle_2.y2 >= (FRAME_HEIGHT-PADDLE_HEIGHT))
+            paddle_2.y2 = FRAME_HEIGHT-PADDLE_HEIGHT;
     }