安卓返回键监听：退出程序

private long exitTime=0;

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {

if (keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN) {
//System.currentTimeMillis()无论何时调用，肯定大于2000
if ((System.currentTimeMillis() - exitTime) > 2000)
{
Toast.makeText(getApplicationContext(), "再按一次退出程序", Toast.LENGTH_SHORT).show();
exitTime = System.currentTimeMillis();
} else {
finish();
System.exit(0);
}

return true;
}
return super.onKeyDown(keyCode, event);
}

// 添加Activity到栈
public void addActivity(Activity activity) {
if (activityStack == null) {
activityStack = new Stack<Activity>();
}
activityStack.add(activity);
}

// 获取当前Activity（栈中最后一个添加的）
public Activity getCurrentActivity() {
Activity activity = activityStack.lastElement();
return activity;
}

//结束栈中最后一个压入的Activity
public void finishActivity() {
Activity activity = activityStack.lastElement();
finishActivity(activity);
}

// 结束指定类名的Activity
public void finishActivity(Class<?> cls) {
for (Activity activity : activityStack) {
if (activity.getClass().equals(cls)) {
finishActivity(activity);
}
}
}

// 结束栈中指定的Activity
public void finishActivity(Activity activity) {
if (activity != null) {
activityStack.remove(activity);
activity.finish();
activity = null;
}
}

// 结束所有Activity
public void finishAllActivity() {
for (int i = 0, size = activityStack.size(); i < size; i++) {
if (null != activityStack.get(i)) {
activityStack.get(i).finish();
}
}
activityStack.clear();
System.exit(0);
}

// 退出应用程序
public void AppExit() {
try {
finishAllActivity();
} catch (Exception e) {
}
}

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//添加Activity到堆栈
Utils.addActivity(this);

}

@Override
protected void onDestroy() {
super.onDestroy();
//结束Activity&从栈中移除该Activity
Utils.finishActivity(this);
}