course2 定义子弹类，绘制出子弹

src/com/demo/tank/coruse2/Bullet.java

@@ -0,0 +1,65 @@
+package com.demo.tank.coruse2;
+
+import java.awt.*;
+
+public class Bullet {
+    private int x, y;
+    private Direction direction;
+    private static final int SPEED = 5;
+    private static final int WIDTH = 20;
+    private static final int HEIGHT = 20;
+
+    public Bullet(int x, int y, Direction direction) {
+        this.x = x;
+        this.y = y;
+        this.direction = direction;
+    }
+
+    public void paint(Graphics g){
+        Color color = g.getColor();
+        g.setColor(Color.BLACK);
+        g.fillOval(x, y, WIDTH, HEIGHT);
+        g.setColor(color);
+        move();
+    }
+
+    private void move() {
+        switch (direction){
+            case UP: y -= SPEED;
+                break;
+            case DOWN: y += SPEED;
+                break;
+            case LEFT: x -= SPEED;
+                break;
+            case RIGHT: x += SPEED;
+                break;
+            default:
+                break;
+        }
+    }
+
+
+    public int getX() {
+        return x;
+    }
+
+    public void setX(int x) {
+        this.x = x;
+    }
+
+    public int getY() {
+        return y;
+    }
+
+    public void setY(int y) {
+        this.y = y;
+    }
+
+    public Direction getDirection() {
+        return direction;
+    }
+
+    public void setDirection(Direction direction) {
+        this.direction = direction;
+    }
+}

src/com/demo/tank/coruse2/TankFrame.java

@@ -8,6 +8,7 @@ import java.awt.event.WindowEvent;
 
 public class TankFrame extends Frame {
     Tank tank = new Tank(500, 500, Direction.UP);
+    Bullet bullet = new Bullet(520, 440, Direction.UP);
 
     public TankFrame(){
         setVisible(true);
@@ -26,6 +27,7 @@ public class TankFrame extends Frame {
     @Override
     public void paint(Graphics g){
         tank.paint(g);
+        bullet.paint(g);
     }
 
     class MyKeyListener extends KeyAdapter{