要獲得系統時間可以用System.currentTimeMillies()，系統硬件有一個PRogramming Gems 3"中的C++代碼，主要成員變量：
thisTime：當前硬件時間，即System.currentTimeMillies()
systemTime：游戲的系統時間，即把thisTime轉換為從0遞增的時間
virtualTime：虛擬時間，從0遞增，但和真實時間不同步

代碼如下：

package game.engine.core;class Clock {    // Clock是否運行:    private boolean running;    // 當前hardware clock:    private int thisTime;    // record the last hardware clock when calling stop():    private int lastTime;    // systemTime從0開始遞增，和硬件時鐘同步：    private int systemTime;    // systemOffset就是硬件時鐘和systemTime的差:    private int systemOffset;    // 上一次停止的systemTime:    private int pauseAt;    // virtualTime starts from 0.    private int virtualTime;    // virtualOffset records how long the clock paused:    private int virtualOffset;    private int frameStart;    private int frameEnd;    private int frameCount;    public Clock() {        reset();    }    // 重置Clock：    public void reset() {        running = false;        // get the hardware clock:        thisTime = (int)System.currentTimeMillis();        lastTime = thisTime;        // and systemTime starts from 0:        systemTime = 0;        systemOffset = thisTime;        pauseAt = 0;        // init virtual time:        virtualTime = 0;        virtualOffset = 0;        // init frame time:        frameStart = 0;        frameEnd = 0;        frameCount = 0;    }    // 同步hardware clock:    private void update() {        lastTime = thisTime;        thisTime = (int)System.currentTimeMillis();        // increase the systemTime:        systemTime += (thisTime - lastTime);    }    // 啟動Clock：    public void start() {        if(!running) {            running = true;            update();            virtualOffset += (systemTime - pauseAt);            System.out.println("[start]");        }    }    // 停止Clock：    public void stop() {        if(running) {            running = false;            update();            pauseAt = systemTime;            System.out.println("[stop] at " + pauseAt);        }    }    // Clock是否運行：    pu