安卓中点击返回键提醒用户是不是真的返回

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);	}	

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