//两秒内按返回键两次退出程序
    private long exitTime = 0; 
    
    @Override 
    public boolean onKeyDown(int keyCode, KeyEvent event) { 
    if(keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN){ 
    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); 
    }