物理返回键

boolean isExit;
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK) {
        if (!isExit) {
            isExit = true;
            Toast.makeText(getApplicationContext(), "再按一次退出程序", Toast.LENGTH_SHORT).show();
            mHandler.sendEmptyMessageDelayed(0, 2000);
        } else {
            Intent intent = new Intent(Intent.ACTION_MAIN);
            intent.addCategory(Intent.CATEGORY_HOME);
            startActivity(intent);
            System.exit(0);
        }
        return false;
    } else {
        return super.onKeyDown(keyCode, event);
    }
}

Handler mHandler = new Handler() {

    @Override
    public void handleMessage(Message msg) {
        // TODO Auto-generated method stub
        super.handleMessage(msg);
        isExit = false;
    }

};