如何將鍵擊Z添加到keyPressed（KeyEvent e）以將球跳轉/重新繪制到新的隨機位置？ （使用KeyListener演示。）

Question

我的Java經驗有限，特別是當它與圖形有關時。 到目前為止，我對這個問題有很大的幫助。 它的入門代碼（在底部）在我在Eclipse中測試時工作得很好。 我在課堂上用它來教一個高中生。 按照下面評論中的說明，有人知道一個簡單的方法來擴展已經很簡單的程序，使球在按下Z按鈕后跳轉/移動/重新繪制到一個隨機的新位置？ 我正在考慮將以下代碼添加到keyPressed（KeyEvent e）方法：

else if(keyCode == KeyEvent.VK_Z)    
{     
   //Not sure what to add here. Just want to change X and Y coordinates to 
   //anything, so long as the program works and stays fairly simple  
   //for now.
   //----> g.fillOval(x + radius, y + radius, 2 * radius, 2 * radius);   
   // this line above says "g can not be resolved". 
} 

。

任何提示程序簡單的提示或想法都將非常感激。 謝謝！

。

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

嘗試做這樣的事情：

public void keyPressed(KeyEvent e){
if(e.getKeyCode == KeyEvent.VK_Z){

moveRand();
//Assuming that is the method for moving the ball randomly.
    }
}

然后

public void moveRand(Graphics2D g){

g.drawOval(Random.nextInt() / window.getWidth() + 1, Random.nextInt() / window.getHight() + 1, ballWidth, ballHight);


}

希望這有幫助。

Answer 2

無法解析g的原因是它在keyPressed()方法中不存在。 g是方法paint()所謂參數。 這意味着您只能在paint()方法中引用它。 您可以在方法paint()之外聲明另一個名為g變量，但它不會是相同的g 。 它甚至不必具有相同的類型。

也就是說，您不需要直接引用g來解決您的問題。 paint()方法已經處理繪制圓形。 您只需要更改圓圈的位置並告訴應用程序重新繪制它。 如果為x和y分配新值，然后調用repaint() ，則圓圈將移動。 請注意，已在keyPressed()方法的末尾調用repaint() 。

這是一個例子。 將等於右側的代碼替換為可能適合您的問題的代碼。

else if(keyCode == KeyEvent.VK_Z)    
{     
   x = x + radius;
   y = y + radius;
}

如果你看看其他if / else子句，你會發現這也是他們正在做的事情。