再按一次返回键退出

long exitTime=0;
	public boolean onKeyDown(int keyCode, KeyEvent event) {
		// TODO Auto-generated method stub
		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);
	}