Question

我当前要创建一个自定义列表视图，以放置在我创建的片段之一中，该片段现在是我的Homefragment.java文件，如下所示：

package sg.edu.rp.c346.a3pdwork;


import android.os.Bundle;

import androidx.fragment.app.Fragment;

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;


/**
 * A simple {@link Fragment} subclass.
 */
public class HomeFragment extends Fragment {


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


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_home, container, false);
        String[] menuItems = {"Bob Contact: 99999999 ", "Alan Contact:  88888888", "Ryan Contact:   77777777"};

        ListView listView = (ListView) view.findViewById(R.id.listViewDetails);

        ArrayAdapter<String> listViewAdapter = new ArrayAdapter<String>(
                getActivity(),
                android.R.layout.simple_list_item_1,
                menuItems
        );
        listView.setAdapter(listViewAdapter);

        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                if (position == 0) {
                    Toast.makeText(getActivity(),"Hi! You CLikced the first item", Toast.LENGTH_LONG).show();
                }else if (position == 1) {
                    Toast.makeText(getActivity(),"Hi! You CLikced the Second item", Toast.LENGTH_LONG).show();
                }else if (position == 2) {
                    Toast.makeText(getActivity(),"Hi! You CLikced the Third item", Toast.LENGTH_LONG).show();

                }

            }
        });

        // Inflate the layout for this fragment
        return view;
    }

}

但是我目前对我是否正确编写它感到困惑，因为我不确定是否应该将自定义列表视图写入HomeFragment.java文件中还是应该在MainActivity.java文件中写入它？因为以前我尝试将自定义列表视图放入MainActivity.java文件中时遇到了一个错误，如下所示，我对此并不了解。

 --------- beginning of crash
2020-02-09 11:10:56.883 14328-14328/sg.edu.rp.c346.a3pdwork E/AndroidRuntime: FATAL EXCEPTION: main
    Process: sg.edu.rp.c346.a3pdwork, PID: 14328
    java.lang.RuntimeException: Unable to start activity ComponentInfo{sg.edu.rp.c346.a3pdwork/sg.edu.rp.c346.a3pdwork.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
        at sg.edu.rp.c346.a3pdwork.MainActivity.onCreate(MainActivity.java:55)
        at android.app.Activity.performCreate(Activity.java:7136)
        at android.app.Activity.performCreate(Activity.java:7127)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:193) 
        at android.app.ActivityThread.main(ActivityThread.java:6669) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

Answer 1

您通过空对象（即列表视图）调用setAdapter()。我认为您的片段布局文件listViewDetails中没有fragment_home.xml ListView。因此，请尝试将列表视图放入布局文件中。

如何在片段内创建自定义列表视图

1 个答案: