在按一次退出程序

private long waitTime = 2000; 
private long touchTime = 0;
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
if (event.getAction() == KeyEvent.ACTION_DOWN && KeyEvent.KEYCODE_BACK == keyCode) {
long currentTime = System.currentTimeMillis();
if ((currentTime - touchTime) >= waitTime) {
Toast.makeText(context, "再按一次退出程序", Toast.LENGTH_SHORT).show();
touchTime = currentTime;
} else {
finish();
System.exit(0);
}
return true;
}
   
return super.onKeyDown(keyCode, event);
}