Android Studio：列表视图的 ArrayAdapter 问题

Question

我正在尝试为列表视图使用自定义布局，以及从 res/strings 中提取的字符串数组。起初我尝试使用

adapter = ArrayAdapter.createFromResource(this,R.array.android_versions,R.layout.list_view_custom)

但是收到一条错误消息，您必须为文本视图提供资源 ID，否则模拟器将关闭。所以我尝试使用 newArrayAdapter，如下面的代码所示，使用第 4 行获取我的字符串数组。由于在我尝试使用自定义布局之前这一切都有效，我认为问题出在第 4、9 和 10 行的某个地方。

public class ListViewActivity extends AppCompatActivity {
    ListView listview;
    ArrayAdapter<String> adapter;
    List<String> android_versions = Arrays.asList(getResources().getStringArray(R.array.android_versions));
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_list_view);
        listview = (ListView)findViewById(R.id.listview);
        adapter = new ArrayAdapter<>(this,R.layout.list_view_custom,R.id.listview_custom,android_versions);
        listview.setAdapter(adapter);
        listview.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                Toast.makeText(getBaseContext(),parent.getItemAtPosition(position) + " is selected",Toast.LENGTH_LONG).show();
            }
        });

    }

}

这里是ListViewActivity的布局，content_list_view.，是通过activity_list_view.

得到的

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    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"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context="com.example.andrewtakao.testapp.ListViewActivity"
    tools:showIn="@layout/activity_list_view">
    <ListView
        android:id="@+id/listview"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content">

    </ListView>

</RelativeLayout>

这是我尝试在 ListView 中实现的布局

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">
    <TextView
        android:id="@+id/listview_custom"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textColor="@color/colorPrimary"
        android:textAppearance="?android:textAppearanceLarge"
        android:text="@string/apple"
        android:gravity="center_horizontal"/>

</LinearLayout>

最后是我最新的错误消息：

-18 18:51:17.682 12567-12567/com.example.andrewtakao.testapp I/Choreographer: Skipped 112 frames! The application may be doing too much work on its main thread. 10-18 18:55:11.808 12567-12567/com.example.andrewtakao.testapp D/AndroidRuntime: Shutting down VM 10-18 18:55:11.809 12567-12567/com.example.andrewtakao.testapp E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.andrewtakao.testapp, PID: 12567 java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.andrewtakao.testapp/com.example.andrewtakao.testapp.ListViewActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2327) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) at android.app.ActivityThread.-wrap11(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5417) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference at android.content.ContextWrapper.getResources(ContextWrapper.java:87) at android.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:81) at android.support.v7.app.AppCompatActivity.getResources(AppCompatActivity.java:551) at com.example.andrewtakao.testapp.ListViewActivity.(ListViewActivity.java:20) at java.lang.Class.newInstance(Native Method) at android.app.Instrumentation.newActivity(Instrumentation.java:1067) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2317) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) at android.app.ActivityThread.-wrap11(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5417) at java.lang.reflect.Method.invoke(Native Method)

谁能解释一下我做错了什么？？非常感谢。

Answer 1

您将空上下文传递给的异常详细信息，我怀疑它在这里：

List<String> android_versions = Arrays.asList(getResources().getStringArray(R.array.android_versions));

将您的列表实例移到 onCreate 中。

List<String> android_versions;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_list_view);
    android_versions = Arrays.asList(getResources().getStringArray(R.array.android_versions))
    listview = (ListView)findViewById(R.id.listview);

解释： 在 Activity 中使用 getResources() 隐含地引用了 Activity 本身，它是 Context 在其自愿。由于在调用 onCreate 之前 Context 尚未实例化，因此 Context 将为空。只有在 onCreate 被调用后， Context 才可用。

Android Studio：列表视图的 ArrayAdapter 问题

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