如何在keyPressed(KeyEvent e)方法中添加键击来更改球的颜色?简单的KeyListener演示

Question

我的Java经验有限，尤其是在涉及图形时。到目前为止，我在这个问题上得到了很好的帮助。这是我在Eclipse中测试时运行良好的起始代码（如下）。我在课堂上用它来教一个高中生。根据下面评论中的说明，有人知道一种简单的方法吗？在按下按钮C后，可以扩展已经很简单的程序来更改球的颜色？我正在考虑将以下代码添加到keyPressed（KeyEvent e）方法中：

else if(keyCode == KeyEvent.VK_C)    
{     
   //Not sure what code to add here 
   //g.setColor(Color.green);   ----> this line says "g can not be resolved". 
}

任何保持程序简单的技巧或想法都将不胜感激。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

private Color currentColor;
...
public void paint(Graphics g)
{
    g.setColor(Color.white);
    g.fillRect(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT);
    g.setColor(currrentColor);
    g.fillOval(x - radius, y - radius, 2 * radius, 2 * radius);
}
...
public void keyPressed(KeyEvent e)                  
{
    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;
    }else currentColor = Color.BLUE;
    repaint();
}

你可能应该这样做。

Answer 2

您在按键事件中没有'g'图形变量，因此您会从compilor中得到一个未解决的变量错误。需要将要绘制的颜色存储在变量中，并在绘制方法中使用它。

有一个作为颜色的类实例变量。最初它将是

colorBall = Color.white

然后点击C，如果它的白色变为绿色，反之亦然。

在油漆中，只需使用变量为上色

  g.setColor(colorBall );