安卓中点击返回键提醒用户是否真的返回

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);
	}