java.lang.NullPointerException：尝试在我的代码中获取空数组的长度

Question

我有一个登录页面。如果用户成功登录，应启动一个新的 Activity，其中包含一个包含用户私人项目的 ListFragment。我遍历所有项目标题（来自 json 我得到的结果）并将其存储在一个数组中。然后用详细信息（项目标题）填充 ListFragment。只要用户单击提交，就会调用 ProjectListActivity.java。但我收到错误：

java.lang.NullPointerException: Attempt to get length of null array

我做错了什么？

private_projects_list.xml:

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" android:layout_height="match_parent"
    android:id="@+id/privateProjectsContainer">

<fragment
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    class="com.example.amangrover.finalapp1.ProjectListFragment" />

</FrameLayout>

ProjectListActivity.java:

public class ProjectListActivity extends Activity {
private final static String TAG_TITLE = "title";
private final static String TAG_ANIM_ARRAY_ID = "animation_array";
private final static String TAG_PROJECTS = "projects";
private final static String TAG_ID = "id";

JSONArray projects = null;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    new HttpGetHandler().execute();
    setContentView(R.layout.private_project_list);
    FragmentManager fm = getFragmentManager();
    FragmentTransaction ft = fm.beginTransaction();
    ft.add(R.id.privateProjectsContainer, new ProjectListFragment());
    ft.commit();
}

private class HttpGetHandler extends AsyncTask<String, Void, Void> {

    @Override
    protected Void doInBackground(String... params) {
        String jsonUrl = "some url";
        HttpGet httpGet = new HttpGet(jsonUrl);
        try {
            HttpResponse httpResponse = MainActivity.httpClient.execute(httpGet);
            HttpEntity httpEntity = httpResponse.getEntity();
            InputStream content = httpEntity.getContent();
            String result = convertToString(content);
            JSONObject jsonObject = new JSONObject(result);
            projects = jsonObject.getJSONArray(TAG_PROJECTS);
            ProjectListActivity.this.runOnUiThread(new Runnable() {
                @Override
                public void run() {
                    try{
                        ArrayList<String> projectTitle = new ArrayList<String>();
                        ArrayList<String> imageId = new ArrayList<String>();
                        ArrayList<String> playId = new ArrayList<String>();
                        for(int i=0; i<projects.length(); i++) {
                            JSONObject p = projects.getJSONObject(i);
                            String title = p.getString(TAG_TITLE);
                            String id = p.getString(TAG_ID);
                            String array_anim_id = p.getString(TAG_ANIM_ARRAY_ID);
                            String imgId = array_anim_id.substring(0, 4);
                            projectTitle.add(title);
                            imageId.add(imgId);
                            playId.add(id);
                        }
                        String[] arrProjectTitle = new String[projectTitle.size()];
                        String[] arrImageId = new String[imageId.size()];
                        final String[] pid = new String[playId.size()];
                        for(int i = 0; i<projectTitle.size(); i++)
                            arrProjectTitle[i] = projectTitle.get(i);
                        for(int j = 0; j<imageId.size(); j++)
                            arrImageId[j] = imageId.get(j);
                        for(int k = 0; k<playId.size(); k++)
                            pid[k] = playId.get(k);

                        /*Bundle b = new Bundle();
                        b.putStringArray("projectTitle", arrProjectTitle);
                        b.putStringArray("projectId", pid);
                        b.putStringArray("imageId", arrImageId);*/
                        Bundle b = new Bundle();
                        b.putStringArray("projectTitle", arrProjectTitle);
                        ProjectListFragment fragment = new ProjectListFragment();
                        fragment.setArguments(b);
                    }catch(Exception e) {

                    }
                }
            });
        }catch(Exception e) {
        }
        return null;
    }

    public String convertToString(InputStream inputStream) throws IOException {
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream));
        String line = "";
        String result = "";
        while((line = bufferedReader.readLine())!=null) {
            result += line;

        }
        inputStream.close();
        return result;
        }
    }
}

如果用户已登录，我必须对 some url 进行 GET 调用。所以我必须处理 AsyncTask。尽管我使用调试器进行了检查，但我正在使用 json url 获取数据，但错误出现在片段 class:

的代码中

ProjectListFragment.java:

public class ProjectListFragment extends ListFragment{
String[] projectTitle;
String[] projectImage;

CustomAdapter adapter;

private List<RowItem> rowItems;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

    return inflater.inflate(R.layout.list_fragment, null, false);
}

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    /*Bundle args = getActivity().getIntent().getExtras();
    projectTitle = args.getStringArray("projectTitle");
    projectImage = args.getStringArray("imageId");*/

    Bundle bundle = this.getArguments();
    if(bundle!=null){
        projectTitle = bundle.getStringArray("projectTitle");
    }

    rowItems = new ArrayList<RowItem>();

    for(int i=0; i<projectTitle.length; i++) {
        RowItem item = new RowItem(projectTitle[i], 0);
        rowItems.add(item);
    }
    adapter = new CustomAdapter(getActivity(), rowItems);
    setListAdapter(adapter);
    }
}

我在 for 循环中遇到错误。

Answer 1

    Bundle b = new Bundle();
    b.putStringArray("projectTitle", arrProjectTitle);
    ProjectListFragment fragment = new ProjectListFragment();
    fragment.setArguments(b);

    FragmentManager manager = getSupportFragmentManager();
    FragmentTransaction ft = manager.beginTransaction();
    ft.replace(R.id.privateProjectsContainer, fragment);
    ft.commit();


<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" android:layout_height="match_parent"
    android:id="@+id/privateProjectsContainer">

</FrameLayout>

最好使用代码进行片段交易。并使用您的片段替换容器布局。在您的情况下，您没有将该参数传递给新创建的 frgament

Answer 2

jsonObject.getJSONArray(TAG_PROJECTS); - returns null.
这一行有：

for(int i=0; i<projects.length(); i++) {

你得到NullPointerException
要解决此问题，请执行以下操作：

if(projects != null){
    ProjectListActivity.this.runOnUiThread(new Runnable() {/*Code here*/});
}

java.lang.NullPointerException：尝试在我的代码中获取空数组的长度

java.lang.NullPointerException: Attempt to get length of null array in my code

android

arrays

android-fragments

nullpointerexception

android-listfragment