java - 只有在 while true 循环中有 System.out.println() 时线程才能正确运行

Question

基本上，我正在制作一个更新玩家位置的游戏，它使用这个线程:

@Override
public void run() {
    while(true) {
        System.out.println();
        updatePos(x, y);
    }

}

它工作正常，但如果我删除 System.out.println()，它就会停止运行。我不知道这是为什么，全类如下:

public class Player extends Block implements KeyListener, Runnable {

int x;
int y;
int speed;

boolean upPressed; 
boolean downPressed;
boolean rightPressed;
boolean leftPressed;

static Sprite sprite = new Sprite("grass.png");

public Player(int x, int y, int speed) {
    super(x, y, sprite);
    this.x = x;
    this.y = y;
    this.speed = speed;
    Thread playerThread = new Thread(this, "Player Thread");
    playerThread.start();
}

@Override
public void keyPressed(KeyEvent e) {
    if (e.getKeyCode() == KeyEvent.VK_UP) {
        y -= speed;
    }

    if (e.getKeyCode() == KeyEvent.VK_DOWN)
        downPressed = true;

    if (e.getKeyCode() == KeyEvent.VK_RIGHT)
        rightPressed = true;

    if (e.getKeyCode() == KeyEvent.VK_LEFT)
        leftPressed = true;

}

@Override
public void keyReleased(KeyEvent e) {
    if (e.getKeyCode() == KeyEvent.VK_UP)
        upPressed = false;

    if (e.getKeyCode() == KeyEvent.VK_DOWN)
        downPressed = false;

    if (e.getKeyCode() == KeyEvent.VK_RIGHT)
        rightPressed = false;

    if (e.getKeyCode() == KeyEvent.VK_LEFT)
        leftPressed = false;

}

@Override
public void keyTyped(KeyEvent e) {

}

@Override
public void run() {
    while(true) {
        System.out.println();
        updatePos(x, y);
    }

}}

最后说明:忽略 boolean 值 upPresses 和 downPressed，我唯一关注的是:

if (e.getKeyCode() == KeyEvent.VK_UP) {
    y -= speed;
}

Answer 1

像一个循环

while(true) {
    updatePos(x, y);
}

会完全占用 CPU。使用 println 它开始表现得更好的原因可能是因为每次 I/O 迭代产生几百个周期。

我建议你根据所需的帧率添加一个小的 sleep 方法:

while (true) {
    try {
        Thread.sleep(10); // for 100 FPS
    } catch (InterruptedException ignore) {
    }
    updatePos(x, y);
}

或者，更好的是，采用事件驱动的方法，例如 java.util.Timer . (这会更符合 Java 的习惯。)