listView.setAdapter() 方法上的 NullPointerException

Question

所以我遇到了这个问题。我知道为什么。但是首先编写代码，这样您也会知道为什么:)。

LoveMailsActivity.java:

package com.example.ex;

import android.app.DialogFragment;
import android.app.FragmentManager;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;
import android.widget.AdapterView.OnItemClickListener;

public class LoveMailsActivity extends AppCompatActivity
{
FragmentManager fragmentManager;
CustomDialogFragment CustomDialogFragment;

//CustomArrayAdapter customArrayAdapter;
ArrayAdapter<String> arrayAdapter;
ListView dialogListView;

TextView loveEmailOpeningWordsHeaderTextView;

String[] items = {"Hello","Hello there","Hi","Hi there"};

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

    viewsInitialization();

    fragmentManager = getFragmentManager();
    CustomDialogFragment = new CustomDialogFragment();

    //customArrayAdapter = new CustomArrayAdapter(LoveMailsActivity.this, items);
    arrayAdapter = new ArrayAdapter<String>(LoveMailsActivity.this, R.layout.list_view_row,R.id.row_text_view_id,items);
    dialogListView.setAdapter(arrayAdapter); // NullPointerExeption

    loveEmailOpeningWordsHeaderTextView.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick(View v) 
        {
            CustomDialogFragment.show(fragmentManager, "Opening words");
        }
    });
} // End of onCreate() method

@Override
public boolean onCreateOptionsMenu(Menu menu) 
{
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.love_mails, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) 
{
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) 
    {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

private void viewsInitialization()
{
    loveEmailOpeningWordsHeaderTextView = (TextView) findViewById(R.id.love_email_opening_words_header_text_view_id);
    dialogListView = (ListView) findViewById(R.id.dialog_list_view_id);
}

public class CustomDialogFragment extends DialogFragment
{
    TextView listViewItemTextView;
    ArrayAdapter<String> arrayAdapter;
    ListView dialogListView;
    String[] items = {"Hello","Hello there","Hi","Hi there"};

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,Bundle savedInstanceState)
    {
        View rootView = inflater.inflate(R.layout.dialog, container,false);

        getDialog().setTitle("Choose an option"); // Set dialog title

        listViewItemTextView = (TextView) rootView.findViewById(R.id.list_view_item_text_view_id);
        dialogListView = (ListView) rootView.findViewById(R.id.dialog_list_view_id);

        getDialog().setTitle("Opening Words"); // Setting dialog title

        /*
        CustomArrayAdapter customArrayAdapter = new CustomArrayAdapter(getActivity(), items);
        dialogListView.setAdapter(customArrayAdapter);
        */
        dialogListView.setOnItemClickListener(new OnItemClickListener() 
        {
            @Override
            public void onItemClick(AdapterView<?> parent, View view,int position, long id)
            {
                Toast.makeText(getActivity(), items[position], Toast.LENGTH_SHORT).show();
            }
        });

        return rootView;
    }
}

}

activity_love_mails.xml:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.predesignedmails.LoveMailsActivity" >

<TextView
    android:id="@+id/love_email_emai_to_send_to_text_view_id"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="@string/emai_to_send_to_text_view_text"
    android:textSize="16sp"
    android:textStyle="bold"
    android:textColor="@color/opening_words_list_view_header_color"
    />

<EditText
    android:id="@+id/love_email_email_to_send_to_edit_text_id"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:hint="@string/emai_to_send_to_edit_text_hint"
    android:inputType="textEmailAddress"
    android:textSize="18sp"
    android:textColor="@color/selection_text_color"
    android:layout_below="@id/love_email_emai_to_send_to_text_view_id"
    />

<TextView
    android:id="@+id/love_email_opening_words_header_text_view_id"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="@string/love_email_opening_words_text_view_text"
    android:textSize="16sp"
    android:textStyle="bold"
    android:textColor="@color/opening_words_list_view_header_color"
    android:layout_below="@id/love_email_email_to_send_to_edit_text_id"
    />

<TextView
    android:id="@+id/love_email_opening_words_text_view_id"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="@string/initial_text"
    android:textSize="18sp"
    android:textColor="@color/selection_text_color"
    android:layout_below="@id/love_email_opening_words_header_text_view_id"
    />


</RelativeLayout>

现在我在 dialogListView.setAdapter(arrayAdapter); // NullPointerExeption 行上得到明显的 NullPointerException，因为我在 activity_love_mails.xml 中没有任何名为 dialog_list_view_id 的 ListView。我在另一个 XML 文件中有那种 ListView：

dialog.xml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent" >

<ListView 
    android:id="@+id/dialog_list_view_id"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    />


</RelativeLayout>

现在我做错了什么？我的实现方式显然是错误的，无法弄清楚是什么？ CustomDialogFragment class？ ArrayAdapter?

编辑

LogCat:

09-02 19:47:08.208: E/AndroidRuntime(21574): FATAL EXCEPTION: main
09-02 19:47:08.208: E/AndroidRuntime(21574): Process: com.example.predesignedmails, PID: 21574
09-02 19:47:08.208: E/AndroidRuntime(21574): java.lang.RuntimeException: Unable to start activity   ComponentInfo{com.example.predesignedmails/com.example.predesignedmails.LoveM    ailsActivity}: java.lang.NullPointerException: Attempt to invoke virtual     method 'void  android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null  object reference
09-02 19:47:08.208: E/AndroidRuntime(21574):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2305)
09-02 19:47:08.208: E/AndroidRuntime(21574):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2367)
09-02 19:47:08.208: E/AndroidRuntime(21574):    at android.app.ActivityThread.access0(ActivityThread.java:148)
09-02 19:47:08.208: E/AndroidRuntime(21574):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1283)
09-02 19:47:08.208: E/AndroidRuntime(21574):    at android.os.Handler.dispatchMessage(Handler.java:102)
09-02 19:47:08.208: E/AndroidRuntime(21574):    at android.os.Looper.loop(Looper.java:135)
09-02 19:47:08.208: E/AndroidRuntime(21574):    at android.app.ActivityThread.main(ActivityThread.java:5274)
09-02 19:47:08.208: E/AndroidRuntime(21574):    at java.lang.reflect.Method.invoke(Native Method)
09-02 19:47:08.208: E/AndroidRuntime(21574):    at java.lang.reflect.Method.invoke(Method.java:372)
09-02 19:47:08.208: E/AndroidRuntime(21574):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:90    09-02 19:47:08.208: E/AndroidRuntime(21574):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:704)
09-02 19:47:08.208: E/AndroidRuntime(21574): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
09-02 19:47:08.208: E/AndroidRuntime(21574):    at com.example.predesignedmails.LoveMailsActivity.onCreate(LoveMailsActivity.java:45)
09-02 19:47:08.208: E/AndroidRuntime(21574):    at android.app.Activity.performCreate(Activity.java:5977)
09-02 19:47:08.208: E/AndroidRuntime(21574):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
09-02 19:47:08.208: E/AndroidRuntime(21574):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2258)
09-02 19:47:08.208: E/AndroidRuntime(21574):    ... 10 more

Answer 1

您的列表视图在对话框片段中，您正试图在 activity 中调用 findViewById，因此无法找到它，因此它为空。

在 onCreateView 方法的对话框片段中将您的适配器设置为列表视图。

干杯:)

Answer 2

仅仅创建片段对象并不意味着它是 onCreateView() 或调用任何其他生命周期方法。您可以在 activity 中处理您的片段，或者在 activity XML 中声明您的列表视图。然后只有你可以获得列表视图。对于交易片段，你搜索 Google 你会得到很多例子。

现在您的情况是 dialogfragment，因此可以使用 show() 来完成。

此外，当片段不是一种 dialogfragment 时，activity 中的 findviewbyId () 是有效的，因为片段的视图成为 activity 的视图层次结构的一部分。

listView.setAdapter() 方法上的 NullPointerException

NullPointerException on a listView.setAdapter() method

android

android-listview

android-arrayadapter

android-xml

android-dialogfragment