重写返回键并是否保留原键功能

	@Override
	public boolean onKeyDown(int keyCode, KeyEvent event) {
		if (keyCode == KeyEvent.KEYCODE_BACK) {
			if (relative.getVisibility() == 0) {
				adapter.showAllNO();
				relative.setVisibility(View.GONE);
			} else {
				finish();
			}

		} 
		return super.onKeyDown(keyCode, event);
	}

return super.onKeyDown(keyCode, event);是保留原返回键功能

改为return false;只执行重写的代码，不再结束当前Activity