private boolean canExit = false;
    
    private Handler handler = new Handler(){
        public void handleMessage(android.os.Message msg) {
            if (msg.what == 0) {
                canExit = false;
            }
        };
    };
    
    @Override
    public boolean onKeyUp(int keyCode, KeyEvent event) {
        // TODO Auto-generated method stub
        
        if (keyCode == KeyEvent.KEYCODE_BACK) {
            System.out.println("haha");
            
            handler.sendEmptyMessageDelayed(0, 2000);
            Toast.makeText(MainActivity.this, "再按一次就退出", 0).show();
            
            if(canExit){
                
                
                return super.onKeyUp(keyCode, event);
            }
            canExit = true;
            return true;
        }
        
        return super.onKeyUp(keyCode, event);
    }