Android 按两次返回键退出应用

      

	long exitSytemTime = 0;
	@Override
	public boolean onKeyDown(int keyCode, KeyEvent event) {
	
		if(keyCode == KeyEvent.KEYCODE_BACK){
			if(System.currentTimeMillis() - exitSytemTime > 2000){
				Toast.makeText(getApplicationContext(), R.string.exitSystem, Toast.LENGTH_SHORT).show();
				exitSytemTime = System.currentTimeMillis();
				return true;
			}else{
				finish();
			}
			
		}
		return super.onKeyDown(keyCode, event);
	}