请提供详细代码

好的，以下是具体的代码实现：

MainActivity.java：

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private Button btnHome, btnUserList, btnSettings, btnAbout;
    private Fragment homeFragment, userListFragment, settingsFragment, aboutFragment;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // 初始化按钮和 Fragment
        btnHome = findViewById(R.id.btn_home);
        btnUserList = findViewById(R.id.btn_user_list);
        btnSettings = findViewById(R.id.btn_settings);
        btnAbout = findViewById(R.id.btn_about);

        btnHome.setOnClickListener(this);
        btnUserList.setOnClickListener(this);
        btnSettings.setOnClickListener(this);
        btnAbout.setOnClickListener(this);

        homeFragment = new HomeFragment();
        userListFragment = new UserListFragment();
        settingsFragment = new SettingsFragment();
        aboutFragment = new AboutFragment();

        // 默认显示 HomeFragment
        FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
        transaction.replace(R.id.fragment_container, homeFragment);
        transaction.commit();
    }

    @Override
    public void onClick(View v) {
        FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
        switch (v.getId()) {
            case R.id.btn_home:
                transaction.replace(R.id.fragment_container, homeFragment);
                break;
            case R.id.btn_user_list:
                transaction.replace(R.id.fragment_container, userListFragment);
                break;
            case R.id.btn_settings:
                transaction.replace(R.id.fragment_container, settingsFragment);
                break;
            case R.id.btn_about:
                transaction.replace(R.id.fragment_container, aboutFragment);
                break;
        }
        transaction.commit();
    }
}

其中，MainActivity 中包含了四个按钮和四个 Fragment，当点击按钮时，使用 FragmentTransaction.replace() 方法切换对应的 Fragment。

fragment_user_list.xml：

<ListView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/user_list"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

UserListFragment.java：

public class UserListFragment extends Fragment {

    private ListView userList;
    private UserAdapter adapter;

    public UserListFragment() {
        // Required empty public constructor
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_user_list, container, false);
        userList = view.findViewById(R.id.user_list);
        adapter = new UserAdapter(getContext(), getUserList());
        userList.setAdapter(adapter);
        return view;
    }

    private List<User> getUserList() {
        // TODO: 获取用户列表数据
        return new ArrayList<>();
    }
}

其中，UserListFragment 中包含一个 ListView 控件，用于显示用户列表。在 onCreateView() 方法中，加载 fragment_user_list 布局文件，并为 ListView 设置适配器。

UserAdapter.java：

public class UserAdapter extends BaseAdapter {

    private Context context;
    private List<User> userList;

    public UserAdapter(Context context, List<User> userList) {
        this.context = context;
        this.userList = userList;
    }

    @Override
    public int getCount() {
        return userList.size();
    }

    @Override
    public Object getItem(int position) {
        return userList.get(position);
    }

    @Override
    public long getItemId(int position) {
        return position;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        if (convertView == null) {
            convertView = LayoutInflater.from(context).inflate(R.layout.item_user, parent, false);
        }

        User user = userList.get(position);

        TextView tvName = convertView.findViewById(R.id.tv_name);
        TextView tvAge = convertView.findViewById(R.id.tv_age);
        tvName.setText(user.getName());
        tvAge.setText(String.valueOf(user.getAge()));

        return convertView;
    }
}

其中，UserAdapter 继承自 BaseAdapter，用于将用户列表数据显示在 ListView 中。

以上就是该功能的详细代码实现。需要注意的是，getUserList() 方法和 User 类的实现需要根据具体需求自行实现。