Question

请原谅这个问题的可能性。我正在重新访问Java，以便将它传授给年幼的孩子，而且过去从未有过很多关于Graphics的经验。我目前还没有可用于运行此代码或调试的IDE，所以我想知道是否有人可以帮助我从概念上理解我应该编写什么样的代码以扩展程序。

下面有一个示例程序可以正常运行。我只需要通过添加一些按键功能（如程序说明/注释中所述）来扩展它，并确保球到达屏幕边缘但仍然完全可见。

如果我正确添加了额外的击键功能或者我有多近或多远，请告诉我。我在考虑在 keyPressed（KeyEvent e）方法中添加以下代码行。

...

else if(keyCode == KeyEvent.VK_Z)
{
    g.fillOval(x + radius, y + radius, 2 * radius, 2 * radius);
}
else if(keyCode == KeyEvent.VK_S)
{
    g.fillOval(x - radius, y - radius, radius, radius);
}
else if(keyCode == KeyEvent.VK_B)
{
    g.fillOval(x - radius, y - radius, 4 * radius, 4 * radius);    
}
else if(keyCode == KeyEvent.VK_C)    
{     
   g.setColor(Color.green);    
}

我不完全确定if-else块（上方和下方）中的代码应该如何更新球的特征。部分原因是因为我可能对重绘（）和绘制（图形g）缺乏了解。任何见解或提示都非常感谢。

import java.awt.*;
import java.awt.event.*;                            // #1
import javax.swing.*;   

/******************************************************************************
 * 
 * KeyListenerDemo.java
 * Demonstrates getting keyboard input using the KeyListener interface.
 * 
 * Program 18: Extend this program by adding a few more keystroke commands:
 *      z     (VK_Z)    - Cause the ball to jump to a random new location.
 *      s     (VK_S)    - Make the ball smaller - multiply its diameter 1/2.
 *      b     (VK_B)    - Make the ball bigger - multiply its diameter by 2.
 *      c     (VK_C)    - Change the color (in any way you'd like).
 *
 *  In addition, modify the program to ensure the following:
 *  - The ball goes all the way to the edge of the screen but stays
 *          completely on the screen. 
 *  - If a doubled diameter doesn't fit, make it as large as possible.
 *  - Be sure the ball never completely disappears.
 * 
 *****************************************************************************/
public class KeyListenerDemo extends JFrame
                        implements KeyListener      // #2
{
    // Class Scope Finals
    private static final int SCREEN_WIDTH = 1000;
    private static final int SCREEN_HEIGHT = 800;
    private static final int START_RADIUS = 25;
    private static final int START_X = 100;
    private static final int START_Y = 100;
    private static final int STEP_SIZE = 10;

    // Class Scope Variables
    private static int x = START_X;             // x at center of the ball
    private static int y = START_Y;             // y at center of the ball
    private static int radius = START_RADIUS;   // radius of the ball

    // Methods
    /**
     * Create the window and register this as a KeyListener
     * 
     * @param args
     */
    public static void main (String[] args)
    {
        // Set up the JFrame window.
        KeyListenerDemo gp = new KeyListenerDemo();
        gp.setSize(SCREEN_WIDTH, SCREEN_HEIGHT);
        gp.setVisible(true);

        gp.addKeyListener(gp);                          // #3
        // If this class had a constructor and you moved this line into
        //   that constructor it could not refer to gp since that variable
        //   is local to this method.  Instead you would write::
        // addKeyListener(this);
    }

    /**
     * Called when a key is first pressed
     * Required for any KeyListener
     * 
     * @param e     Contains info about the key pressed
     */
    public void keyPressed(KeyEvent e)                  // #4A
    {
        int keyCode = e.getKeyCode();
        if (keyCode == KeyEvent.VK_LEFT)
        {
            x = x - STEP_SIZE;
        }
        else if (keyCode == KeyEvent.VK_RIGHT)
        {
            x = x + STEP_SIZE;
        }
        else if (keyCode == KeyEvent.VK_UP)
        {
            y = y - STEP_SIZE;
        }
        else if (keyCode == KeyEvent.VK_DOWN)
        {
            y = y + STEP_SIZE;
        }
        repaint();
    }

    /**
     * Called when typing of a key is completed
     * Required for any KeyListener
     * 
     * @param e     Contains info about the key typed
     */
    public void keyTyped(KeyEvent e)                    // #4B
    {
    }

    /**
     * Called when a key is released
     * Required for any KeyListener
     * 
     * @param e     Contains info about the key released
     */
    public void keyReleased(KeyEvent e)                 // #4C
    {
    }

    /**
     * paint - draw the figure
     * 
     * @param g     Graphics object to draw in
     */
    public void paint(Graphics g)
    {
        g.setColor(Color.white);
        g.fillRect(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT);

        g.setColor(Color.blue);
        g.fillOval(x - radius, y - radius, 2 * radius, 2 * radius);
    }
}

Answer 1

这看起来一目了然（虽然我也不经常使用图形）。

请注意，如果您要释放该键，您的椭圆可能会在下次重绘时从屏幕上消失。

您可以创建一个简单的Oval包装类来存储x，y，width和height等详细信息，并将这些Oval放入一个列表中，该列表是{{{ 1}}。

你的if-blocks将类似于：

KeyListenerDemo

在else if(keyCode == KeyEvent.VK_Z) { Oval o = new Oval(x + radius, y + radius, 2 * radius, 2 * radius); ovals.add(o); // where ovals is an ArrayList<Oval> or LinkedList<Oval> g.fillOval(o.x, o.y, o.width, o.height); }方法中，您循环浏览列表并重新绘制这些椭圆。

修改：我实现了椭圆级，如：

public void paint(Graphics g)

该列表简单地声明为public class Oval { int x; int y; int width; int height; public Oval(int x, int y, int width, int height) { this.x = x; this.y = y; this.width = width; this.height = height; } }，位于主方法的正上方。要循环它（在您的绘图方法中，protected List<Oval> ovals = new LinkedList<>();可用，您可以使用：

希望这有帮助！

simple KeyListenerDemo.java示例：添加更多击键问题

1 个答案: