Question

我不知道该怎么做。应用程序一直在崩溃，经过几个小时的搜索后，我什么都没得到。有谁知道问题在哪里？

以下是我声明的所有变量：

SectionsPagerAdapter mSectionsPagerAdapter;

ViewPager mViewPager;
public ArrayList<String> numberList = new ArrayList<String>();
public ListView listOfItems;

这是 onCreate 功能：

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    listOfItems = (ListView) findViewById(R.id.listView);
    setContentView(R.layout.activity_main);

    numberList.add("Hello");
    numberList.add("World");

    StableArrayAdapter listAdapter = new StableArrayAdapter(this, android.R.layout.simple_list_item_1, numberList);

    listOfItems.setAdapter(listAdapter);

    // Create the adapter that will return a fragment for each of the three
    // primary sections of the activity.
    mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

    // Set up the ViewPager with the sections adapter.
    mViewPager = (ViewPager) findViewById(R.id.pager);
    mViewPager.setAdapter(mSectionsPagerAdapter);

}

[ EDIT ]这是 fragment_main.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:paddingLeft="@dimen/activity_horizontal_margin" android:paddingRight="@dimen/activity_horizontal_margin" android:paddingTop="@dimen/activity_vertical_margin" android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity$PlaceholderFragment">

    <ListView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/listView"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_below="@+id/addButton" />

</RelativeLayout>

它还提供以下错误日志：

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.adnan.figura/com.adnan.figura.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:2693)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2758)
        at android.app.ActivityThread.access$900(ActivityThread.java:177)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1448)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:145)
        at android.app.ActivityThread.main(ActivityThread.java:5942)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1388)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1183)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
        at com.adnan.figura.MainActivity.onCreate(MainActivity.java:63)
        at android.app.Activity.performCreate(Activity.java:6289)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2758)
        at android.app.ActivityThread.access$900(ActivityThread.java:177)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1448)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:145)
        at android.app.ActivityThread.main(ActivityThread.java:5942)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1388)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1183)

Answer 1

您应该初始化setContentView。下面的listOfItems。

应该是这样的：

的setContentView（R.layout.activity_main）;

listOfItems =（ListView）findViewById（R.id.listView）;

并检查R.layout.activity_main

上是否存在R.id.listView

Answer 2

你必须说

setContentView(R.layout.activity_main);

首先是

，然后是ListView的findViewById。

Answer 3

你有一个非常基本的错误，可以在几秒钟内解决。

请将两行的顺序更改为此。

setContentView(R.layout.activity_main);
listOfItems = (ListView) findViewById(R.id.listView);

首先需要设置内容，然后创建视图，然后使用findViewById（）。

您实际上是在创建视图之前尝试查找视图。希望我能解决你的问题并解释你的错误。

setAdapter（）函数的NullPointerException

3 个答案: