Android 按两次back键退出 -- 效率最高版

private long exitTime = 0;  
  
@Override  
public boolean onKeyDown(int keyCode, KeyEvent event) {  
    if(keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN){  
                  
    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);  
}