NullPointerException 与 ArrayAdapter

Question

我正在使用 Eclipse，我有一个使用默认“RelativeLayout”布局的活动。我在布局中包含了一个微调器，这是当前的 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="mahana.timestat.InputMenu" >

    <Spinner
        android:id="@+id/activitiesSpinner"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true" />

</RelativeLayout>

在我的 .java 中，我找到微调器，创建一个 arrayList，手动填充它，创建一个适配器并将其设置为微调器的适配器。代码如下：

//find the spinner:
Spinner activitiesSpinner = (Spinner) findViewById(R.id.activitiesSpinner);
//create an arrayList:
ArrayList<String> activities = new ArrayList<String>();
//clear it and insert two strings:
activities.clear();
activities.add("First");
activities.add("Second");
//create an adapter (I expect the error to be somewhere in here)
ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_spinner_item, R.id.activitiesSpinner, activities);
//and finally set the adapter for the spinner
activitiesSpinner.setAdapter(adapter);

当我使用 adapter.getCount() 方法时，它会显示正确的数字 (2)。 当我评论设置适配器的最后一行时，它运行良好并在我的设备上显示一个空的微调器。 当我留下最后一行并运行它时，它会在 nullPointerException 上崩溃。这是跟踪：

10-02 19:50:05.645: E/AndroidRuntime(21432): FATAL EXCEPTION: main
10-02 19:50:05.645: E/AndroidRuntime(21432): Process: mahana.timestat, PID: 21432
10-02 19:50:05.645: E/AndroidRuntime(21432): java.lang.NullPointerException
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:392)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.widget.ArrayAdapter.getView(ArrayAdapter.java:362)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.widget.AbsSpinner.onMeasure(AbsSpinner.java:193)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.widget.Spinner.onMeasure(Spinner.java:504)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.view.View.measure(View.java:16538)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:719)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:455)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.view.View.measure(View.java:16538)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5140)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.view.View.measure(View.java:16538)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5140)
10-02 19:50:05.645: E/AndroidRuntime(21432): at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:327)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.view.View.measure(View.java:16538)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5140)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
10-02 19:50:05.645: E/AndroidRuntime(21432): at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2373)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.view.View.measure(View.java:16538)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1964)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1160)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1342)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1047)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5896)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.view.Choreographer.doCallbacks(Choreographer.java:574)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.view.Choreographer.doFrame(Choreographer.java:544)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.os.Handler.handleCallback(Handler.java:733)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.os.Handler.dispatchMessage(Handler.java:95)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.os.Looper.loop(Looper.java:136)
10-02 19:50:05.645: E/AndroidRuntime(21432): at android.app.ActivityThread.main(ActivityThread.java:5103)
10-02 19:50:05.645: E/AndroidRuntime(21432): at java.lang.reflect.Method.invokeNative(Native Method)
10-02 19:50:05.645: E/AndroidRuntime(21432): at java.lang.reflect.Method.invoke(Method.java:515)
10-02 19:50:05.645: E/AndroidRuntime(21432): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790)
10-02 19:50:05.645: E/AndroidRuntime(21432): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:606)
10-02 19:50:05.645: E/AndroidRuntime(21432): at dalvik.system.NativeStart.main(Native Method)

当我尝试查找这个问题时，我主要发现人们不小心在他们的 arrayList 中的某个地方留下了一个空值，但这绝对不是我的问题，因为我手动填充它。 任何关于问题可能出在哪里的想法都将不胜感激。

Answer 1

因为您使用的是默认的 simple_spinner_item 布局，所以它包含 TextView 和 text1 id 而不是 Spinner。所以不要在 ArrayAdapter 构造函数中传递 R.id.activitiesSpinner 作为第三个参数传递 android.R.id.text1 ：

ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
                                android.R.layout.simple_spinner_item, 
                                android.R.id.text1, activities);

或

  ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
                                android.R.layout.simple_spinner_item,activities);