java - 如何在 JPanel 对象上调用 getWidth

Question

我需要调用getWidth()在我的 JPanel 对象“gameboard”上并将其存储在变量“width”中。这看起来很简单

int width = gameboard.getWidth();

但是我收到错误“在定义字段之前无法引用字段”。这让我相信引用了错误的对象，所以我将其更改为 int width = this.gameboard.getWidth();现在 Eclipse 显示 gameboard 指的是 GameBoard Game.gameboard (这是我想要的 JPanel 对象)，但我得到 Exception in thread "AWT-EventQueue-0"这是一些代码，我希望它足以解释我的情况(所有代码都有 1000 多行，因为这是我的教授写的，他让我们修改)。

class Game extends JFrame {
    /* Type your variable declarations here */

    // Score of game
    int score = 0;

    // Get board size
    // int width = this.gameboard.getWidth();
    // int height = this.gameboard.getHeight();

// Game objects -- Was going to pass width and height variable to these
Sprite cat = new Sprite(new ImageIcon("cat.gif").getImage(), 267, 167);
Sprite bananas1 = new Sprite(new ImageIcon("bananas.png").getImage(), randomNumber(0, gameboard.getWidth()), randomNumber(0, 480));
Sprite bananas2 = new Sprite(new ImageIcon("bananas.png").getImage(), randomNumber(gameboard.getWidth(),640), randomNumber(0, 480));

...

 /** The panel where all of the game's images are rendered */
    GameBoard gameboard = null;

...

/**
 * Constructor for the game.
 * <p>
 * <pre>
 * The following tasks are performed:
 * a frame is displayed on the screen
 * a clock is created; the clock invoked repaint 15 times per second
 * a keyboard listener listens for presses of the arrow keys & space bar
 * </pre>
 * <p>
 */
public Game() {

    /* how large is the window? */
    setSize(640,480);

    /* if the end-user clicks on the x in the upper right corner, */
    /* close this app                                             */
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    /* allow the window to receive the focus; */
    /* we want this window to see key presses */
    setFocusable(true);

    /* this window can receive keyboard events */
    addKeyListener(new MyKeyListener());

    /* make the window visible */
    setVisible(true);

    /* add MyPanel to the window */
    setLayout(new GridLayout(1,1));
    gameboard = new GameBoard();
    add(gameboard);

    validate();

}//Game




/**
 * Panel that displays all the graphics in the game.
 * <p>
 * Why do I need a panel--why can't I display the graphics in the frame?
 * I want to the top of the graphics area to be at y=0.
 * <p>
 * Offscreen buffers are used to create a rock-solid animation that does not blink.
 * <p>
 * Focus is given to the panel so that the panel can listen for key presses.
 * <p>
 * The clock invokes repaint 15 times per second.
 */
public class GameBoard extends JPanel {

    /** offscreen buffers are used to create a rock-solid animation that does not blink */
    protected Image offscreenImage = null;
    /** offscreen buffers are used to create a rock-solid animation that does not blink */
    protected Graphics offscreenGraphics = null;

    /**
     * Constructor for the main panel in this game;
     * all of the graphics are displayed on this panel.
     * <p>
     * <pre>
     * Focus is given to the panel so that the panel can listen for key pressed.
     * NOTE:  Focus determines who receives the characters that are typed on the keyboard--
     * the entity that has the focus receives the characters.
     * A keyboard listener is created to listen for key pressed.
     * A clock is created; the clock invokes repaint 15 times per second.
     * </pre>
     * <p>
     */
    public GameBoard() {
        /* allow this panel to get the focus */
        setFocusable(true);
        /* give this panel the focus */
        requestFocus();
        /* Now that this panel has the focus, this panel can receive keyboard events */
        addKeyListener(new MyKeyListener());
        /* this window can receive mouse motion events */
        addMouseMotionListener(new MyMouseMotionListener());
        /* this window can receive mouse events */
        addMouseListener(new MyMouseListener());
        /* start a clock that invokes repaint 15 times per second */
        new ThreadClock().start();
    }//MyPanel

...

    /**
     * Play the game
     * @param args Command line arguments
     */
    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(new Runnable(){
            public void run() {
                new Game();
            }
            });
    }//main

}//Game

Answer 1

class Game extends JFrame {
    /* Type your variable declarations here */

    // Score of game
    int score = 0;

    // Get board size
    int width = this.gameboard.getWidth();
    int height = this.gameboard.getHeight();

    /** The panel where all of the game's images are rendered */
    GameBoard gameboard = null;

基于此你会遇到很多问题......

1. 当你初始化width时和height , GameBoard将是null 。这将导致NullPointerException ，会发生这种情况，因为实例字段在构造函数执行之前就已初始化
2. 如果您要初始化 GameBoard当它被声明时( GameBoard gameboard = new GameBoard() )，那么当 width 时和height被读取，它们将是 0 ，如gameBoard不会被添加到父容器中，也不会被布局。

这就提出了一个问题，为什么？如果您需要知道 gameboard 的宽度或高度，你应该在需要知道的时候问它。

你应该知道，随着框架大小的调整，以这种方式存储值是没有用的，如果用户调整框架大小，这些值就会变得无效......

旁注...

• setFocusable(true);实在是没啥用，有一个JRootPane键盘和框架本身之间的内容 Pane 及其内容(可能还有玻璃 Pane )，其中任何一个都可能会窃取框架的焦点，从而使其无用......
• addKeyListener(new MyKeyListener());由于上述原因，这是没有用的，你应该使用 key bindings相反，它克服了所有焦点问题......
• setVisible(true);应该在初始化 UI 并添加所有初始组件之后最后完成