NullPointerException：尝试在空对象引用上调用虚拟方法“int java.util.ArrayList.size()”

Question

我是 Fragments 和自定义 ListView 适配器的新手。谁能帮帮我？

我有我的Fragment，我有我的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;
    }
}

还有我的 xml

<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>

我有我的自定义列表项

<?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;
    }
} 

当我运行它时，我得到了这个空指针

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

提前谢谢你！！！

Answer 1

您应该初始化您的录音。您正在传递给适配器 null

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

Answer 2

这个问题是由于 ArrayList 变量没有被实例化。 需要像下面这样声明“录音”变量，这应该可以解决问题；

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

这会调用默认构造函数并将空字符串分配给录制变量，使其不再为空。

Answer 3

改变

 mAdapter = new RecordingsListAdapter(this, recordings);

到

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

同时确保recordings!=nullmAdapter = new RecordingsListAdapter(this, recordings);

Answer 4

ArrayList 记录 = null; //可以传null