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空指针异常：尝试在空对象引用上调用虚拟方法'int java.util.ArrayList.size()'

androidlistviewandroid-fragmentsmobileadapter
13
我对Fragments和自定义ListView适配器都不熟悉。能否有人帮我一下？

我有一个Fragment，里面有我的ListView。

public class RecordingListFragment extends Fragment implements OnItemClickListener {

    ListView mListView;
    TextView emptyView;
    Button options, delete,edit;
    ArrayList<String> recordings = null;
    RecordingsListAdapter mAdapter;

    public RecordingListFragment() {
    }

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

        options = (Button) rootView.findViewById(R.id.options);
        edit = (Button) rootView.findViewById(R.id.edit);
        delete = (Button) rootView.findViewById(R.id.delete);
        mListView = (ListView) rootView.findViewById(R.id.list);
        emptyView = (TextView) rootView.findViewById(R.id.empty);


        mAdapter = new RecordingsListAdapter(this, recordings);
        mListView.setAdapter(mAdapter);
        mListView.setEmptyView(emptyView);
        mListView.setOnItemClickListener(this);

        return rootView;
    }
}

以及我为其编写的XML代码

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:android1="http://schemas.android.com/apk/res/android"
    android:id="@+id/recording_list_fragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#ffffff"
    android:orientation="vertical">

    <RelativeLayout
        android:layout_height="50dp"
        android:layout_width="match_parent"
        android:orientation="horizontal"
        android:background="@drawable/title" >

        <TextView
            android1:id="@+id/title"
            android1:layout_width="wrap_content"
            android1:layout_height="50dp"
            android1:layout_alignParentTop="true"
            android1:layout_centerHorizontal="true"
            android1:layout_marginStart="80dp"
            android1:gravity="center"
            android1:text="@string/app_name"
            android1:textAlignment="center"
            android1:textColor="#000000"
            android1:textStyle="bold" />

    </RelativeLayout>

    <FrameLayout
        android:layout_height="wrap_content"
        android:layout_width="match_parent">

        <ListView
            android:id="@+id/list"
            android:animateLayoutChanges="true"
            android:layout_width="match_parent"
            android:layout_height="match_parent"/>
        <!--  android:divider="@null"--> 

        <TextView
            android:id="@+id/empty"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:gravity="center"
            android:text="@string/empty_list_text"
            android:paddingLeft="30dp"
            android:paddingRight="30dp"
            android:paddingTop="180dp" />

    </FrameLayout>
</LinearLayout>

我有自定义的列表项

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

    <CheckedTextView
        android:id="@+id/textView"
        android:layout_width="fill_parent"
        android:layout_height="?android:attr/listPreferredItemHeight"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:gravity="center_vertical"
        android:checkMark="?android:attr/listChoiceIndicatorSingle"
        android:paddingLeft="6dip"
        android:paddingRight="6dip"
        android:textColor="#000000"
        /> 

</LinearLayout>

最后，我的Adapter。

public class RecordingsListAdapter  extends BaseAdapter {
    Context ctx;
    LayoutInflater lInflater;
    ArrayList<String> recordings;

    RecordingsListAdapter(Context context, ArrayList<String> products) {
        ctx = context;
        recordings = products;
        lInflater = (LayoutInflater) ctx
        .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }

    public RecordingsListAdapter(RecordingListFragment recordingListFragment,
    ArrayList<String> recordings) {
        // TODO Auto-generated constructor stub
    }


    @Override
    public int getCount() {
        return recordings.size();
    }


    @Override
    public Object getItem(int position) {
        return recordings.get(position);
    }


    @Override
    public long getItemId(int position) {
        return position;
    }


    @Override
    public View getView(int position, View convertView, ViewGroup parent) {

        View view = convertView;
        if (view == null) {
            view = lInflater.inflate(R.layout.custom_list_item, parent, false);
        }

        return view;
    }
}

运行代码时，出现了空指针错误。

11-26 11:00:45.352: E/AndroidRuntime(10191): java.lang.RuntimeException: Unable to start activity ComponentInfo{d/recordings.RecordingsActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference
11-26 11:00:45.352: E/AndroidRuntime(10191):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at android.app.ActivityThread.access$800(ActivityThread.java:144)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at android.os.Handler.dispatchMessage(Handler.java:102)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at android.os.Looper.loop(Looper.java:135)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at android.app.ActivityThread.main(ActivityThread.java:5221)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at java.lang.reflect.Method.invoke(Native Method)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at java.lang.reflect.Method.invoke(Method.java:372)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
11-26 11:00:45.352: E/AndroidRuntime(10191): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference
11-26 11:00:45.352: E/AndroidRuntime(10191):    at recordings.RecordingsListAdapter.getCount(RecordingsListAdapter.java:36)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at android.widget.ListView.setAdapter(ListView.java:487)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at recordings.RecordingListFragment.onCreateView(RecordingListFragment.java:47)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at android.app.Fragment.performCreateView(Fragment.java:2053)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:894)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1067)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at android.app.BackStackRecord.run(BackStackRecord.java:833)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1452)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at android.app.Activity.performStart(Activity.java:5948)
11-26 11:00:45.352: E/AndroidRuntime(10191):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2261)
11-26 11:00:45.352: E/AndroidRuntime(10191):    ... 10 more

提前感谢您！！！

可能是什么是空指针异常，我该如何修复它？的重复。 - Selvin
1
ArrayList<String> recordings = null; 你应该对它进行初始化。 - Suvitruf - Andrei Apanasik
4个回答
23

您需要初始化您的录音。您正在将空值传递给适配器。

ArrayList<String> recordings = null; //You are passing this null
11

这个问题是由于ArrayList变量未被实例化所致。 需要像下面这样声明“recordings”变量，就可以解决这个问题;

ArrayList<String> recordings = new ArrayList<String>();

这将调用默认构造函数并将空字符串分配给记录变量，以便它不再为null。

2
我像你一样声明了变量，但仍然出现错误。 - s.j
5

更改

 mAdapter = new RecordingsListAdapter(this, recordings);

为了

 mAdapter = new RecordingsListAdapter(getActivity(), recordings);

同时确保在 mAdapter = new RecordingsListAdapter(this, recordings); 中 recordings!=null。

1
他没有编译时错误，所以这不是一个问题（实际上他有一个不同的代码？） - Selvin
我添加了你的更改，同时使用一些静态数据填充录音，但是我遇到了这个错误 11-26 11:21:47.125: E/AndroidRuntime(11596): java.lang.RuntimeException: Unable to start activity ComponentInfo{xxxxx/recordings.RecordingsActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.util.ArrayList.add(java.lang.Object)' on a null object reference。 - android enthusiast
@AdrianAntal 在你的 ArrayList 中添加一些记录，例如：recordings.add("ABC"); recordings.add("AXYZ"); 以此类推... - M D
0

ArrayList recordings = null; //你可以传递null

2
你好，欢迎来到 Stack Overflow！请参观一下 导览。感谢您的回答，但您能否添加一些解释说明您的代码是如何解决这个问题的呢？请查看 帮助中心 了解如何格式化代码的信息。 - Tyler2P
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