java - 渲染具有多个类的矩形

Question

我正在制作一个简单的游戏项目，在尝试创建在屏幕上移动的矩形时遇到问题。这是主类:

`公共(public)类 Main 扩展 Canvas 实现 Runnable {

private static final long serialVersionUID = 1L;

private JFrame frame;
boolean running = false;

Graphics g;

static int HEIGHT = 500;
static int WIDTH = HEIGHT * 16 / 9;

SoundHandler sh = new SoundHandler();

//Game state manager
private GameStateManager gsm;

public Main()
{
    //window
    frame = new JFrame("Game");

    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setLayout(new BorderLayout());

    frame.add(this, BorderLayout.CENTER);
    frame.pack();

    frame.setSize(WIDTH, HEIGHT);
    frame.setResizable(false);
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);

    init();
}

void init()
{
    gsm = new GameStateManager();
    sh.playMusic("Undertale.wav", 1);
}

public synchronized void start(){
    running = true;
    new Thread(this).start();
}

public synchronized void stop(){
    running = false;
}

//game loop
public void run() 
{
    //init time loop variables
    long lastLoopTime = System.nanoTime();
    final int TARGET_FPS = 60;
    final long OPTIMAL_TIME = 1000000000 / TARGET_FPS;
    double lastFpsTime = 0;
    int fps = 0;

    while(running)
    {
        //work out how long its been since last update
        //will be used to calculate how entities should
        //move this loop
        long now = System.nanoTime();
        long updateLength = now - lastLoopTime;
        lastLoopTime = now;
        double delta = updateLength / ((double)OPTIMAL_TIME);

        //update frame counter
        lastFpsTime += updateLength;
        fps++;

        //update FPS counter
        if(lastFpsTime >= 1000000000)
        {
            System.out.println("FPS " + fps);
            lastFpsTime = 0;
            fps = 0;
        }

        //game updates
        update(delta);

        //draw
        draw(g);

        try{
            Thread.sleep((lastLoopTime - System.nanoTime() + OPTIMAL_TIME)/1000000 );
        }catch(InterruptedException e){
            System.out.println("Error in sleep");
        }
    }
}

private void update(double delta)
{
    //updates game state code
    gsm.update(delta);
}

public void draw(Graphics g)
{
    gsm.draw(g);
}`

这是我想要用来绘制矩形的类

package me.mangodragon.gamestate;

导入java.awt.Graphics;

公共(public)类 MainState 扩展 GameState{

int x;

public MainState(GameStateManager gsm){
    this.gsm = gsm;
}

public void init() {

}

public void update(double delta) {
    x += 2 * delta;
}

public void draw(Graphics g) {
    g.drawRect(x, 0, 50, 50);
    g.dispose();
}

public void keyPressed(int k) {

}

public void keyReleased(int k) {

}

}我不断收到此错误:

Exception in thread "Thread-4" java.lang.NullPointerException
at me.mangodragon.gamestate.MainState.draw(MainState.java:22)
at me.mangodragon.gamestate.GameStateManager.draw(GameStateManager.java:37)
at me.mangodragon.main.Main.draw(Main.java:118)
at me.mangodragon.main.Main.run(Main.java:100)
at java.lang.Thread.run(Unknown Source)

我尝试修复它，但找不到问题所在。

谢谢!

Answer 1

您永远不会将任何内容分配给g(Graphics)。现在，在你跑开并尝试弄清楚如何做到这一点之前，我强烈建议你摆脱这个变量，它会给你带来太多问题。

通常，当系统想要绘制您的组件时，它会调用您的 paint 方法并向您传递它希望您绘制的 Graphics 上下文。这种方法称为被动绘画，因为绘画请求是随机出现的，这并不是您真正想要的。另一个问题是 java.awt.Canvas 不是双缓冲的，这会导致组件更新时发生闪烁。

您可能想看看Painting in AWT and Swing和 Performing Custom Painting了解更多详情

您可以使用 JPanel 来代替，它是双缓冲的，但使用 java.awt.Canvas 的主要原因是您可以使用 BufferStrategy API。这不仅提供了双缓冲，还为您提供了一种直接控制绘画过程(或 Activity 绘画)的方法。

参见BufferStrategy和 BufferStrategy and BufferCapabilities了解更多详情