Java简单JFrame为null

Question

我已经制作了简单的 JFrame，它工作正常，除非当我尝试从其他类访问它时它返回 null。所以我为它做了 getter (public static JFrame getWindow()) 它返回 null。如果我将 JFrame 设置为 public 并尝试以这种方式访问​​它，它将返回 null。当我创建时，它在游戏引擎启动后立即变为空。

主要:

public class Main {

private static String title = "2D SquareWorld 0.";
private static String version = "";

private static JFrame window;
private static Container container;

public static void main(String[] args) {

    GameEngine game = new GameEngine();

    window = new JFrame();

    window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    window.setTitle(title + Version.newVersion());
    window.setResizable(false);
    window.add(game);
    window.setSize(1000, 720);
    window.setLocationRelativeTo(null);
    window.setVisible(true);

    game.start();

}

public static Container getContainer() {

    return container;

}

public static JFrame getWindow() {

    return window;

}

}

游戏引擎:

package GameEngine;

public class GameEngine extends Canvas implements Runnable, KeyListener,                                                       MouseListener, MouseMotionListener {

private static final long serialVersionUID = 1L;

private Thread thread;
private boolean running;

private GameStateManager gsm;

public GameEngine() {

    gsm = new GameStateManager();

}

public void start() {

    thread = new Thread(this, "Game");
    thread.start();

    running = true;

    init();

}

public void init() {

    Data.setValue("ScreenWidth", getWidth());
    Data.setValue("ScreenHeight", getHeight());

    addKeyListener(this);

}

public void update(double delta) {

}

public void render() {

    BufferStrategy bs = getBufferStrategy();

    if(bs == null) {

        createBufferStrategy(2);

        return;

    }

    Graphics g = bs.getDrawGraphics();
    Graphics2D g2 = (Graphics2D) g;

    g2.setColor(Color.white);
    g2.fillRect(0, 0, getWidth(), getHeight());

    gsm.draw(g2);

    g2.dispose();
    bs.show();

}

public void run() {

    long lastLoopTime = System.nanoTime();
    long lastFpsTime = 0;

    final int TARGET_FPS = Fps.TargetFPS;
    final long OPTIMAL_TIME = 1000000000 / TARGET_FPS;   

    int CurrentFps = 0;

    while(running) {

        long now = System.nanoTime();
        long updateLength = now - lastLoopTime;

        lastLoopTime = now;

        double delta = updateLength / ((double) OPTIMAL_TIME);

        lastFpsTime += updateLength;

        CurrentFps++;

        if (lastFpsTime >= 1000000000) {

            Fps.FPS = CurrentFps;

            lastFpsTime = 0;
            CurrentFps = 0;

        }

        update(delta);

        render();

        try {

            if(((lastLoopTime-System.nanoTime() + OPTIMAL_TIME) / 1000000) >= 1) {

                Thread.sleep((lastLoopTime-System.nanoTime() + OPTIMAL_TIME) / 1000000);

            }

        }catch(InterruptedException IE) {

        }

    }

}

public void mouseDragged(MouseEvent me) {

}

public void mouseMoved(MouseEvent me) {

}

public void mouseClicked(MouseEvent me) {

}

public void mouseEntered(MouseEvent me) {

}

public void mouseExited(MouseEvent me) {

}

public void mousePressed(MouseEvent me) {

}

public void mouseReleased(MouseEvent me) {

}

public void keyPressed(KeyEvent ke) {

    gsm.keyPressed(ke.getKeyCode());

}

public void keyReleased(KeyEvent ke) {

}

public void keyTyped(KeyEvent ke) {

}

}

我从这个类中调用了 getWindow:

public class LoginState extends GameState {

private GameStateManager gsm;

public LoginState(GameStateManager gsm) {

    this.gsm = gsm;

}

private JTextField username;
private JTextField password;

public void init() {

    username = new JTextField(25);

    username.setVisible(true);
    username.setBounds(20, 20, 50, 50);

    Main.getWindow().add(username);

}

public void draw(Graphics2D g) {

    g.setColor(Color.gray);
    //g.fillRect(0, 0, Data.getIntegerValue("ScreenWidth"), Data.getIntegerValue("ScreenHeight"));

}

游戏状态管理器:

public class GameStateManager {

private ArrayList<GameState> gameStates;

public static final int LOGINSTATE = 0;
public static final int MENUSTATE = 1;
public static final int PLAYSTATE = 2;

private static int currentState;

public GameStateManager() {

    gameStates = new ArrayList<GameState>();

    currentState = LOGINSTATE;

    gameStates.add(new LoginState(this));
    gameStates.add(new MenuState(this));
    gameStates.add(new PlayState(this));

    gameStates.get(currentState).init();

}

请帮忙。

Answer 1

感谢您的更新，但是...您仍然没有显示 LoginWindow 的初始化位置。

猜测 - 您的程序是从与您显示的不同的 main 方法开始的，因此创建 JFrame 并将其分配给静态窗口字段的 main 方法永远不会被调用。我建议您避免以这种方式使用静态方法和字段，它们依赖于 main 方法来初始化。 Java 是一种 OOP 语言 - 因此，请使您的代码符合 OOP 要求，并在保证被调用的代码中创建所需的对象，然后将它们分配给非静态字段。

编辑:只需交换两行即可:

GameEngine game = new GameEngine();
window = new JFrame();

至

window = new JFrame();
GameEngine game = new GameEngine();