使用API​​提取数据后，数组列表返回空列表

Question

我实际上是使用Android的api获取数据的。我试图在回收者视图中显示获取的数据。我写了一个函数来获取数据，它返回一个通知列表。已获取数据，但仍返回空列表。

这是home_activity，其中从上一个活动传递了一些数据，这些数据用于检查用户是教职员工还是学生。 如果用户是学生，则使用学生适配器填充回收者视图；如果用户是教师，则使用教师适配器。

public class home_activity extends AppCompatActivity {

    private String user_desig;

    private RecyclerView recyclerView_student;
    private RecyclerView recyclerView_teacher;
    private StudentNoticeRecyclerViewAdapter studentNoticeRecyclerViewAdapter;
    private TeacherNoticeRecyclerViewAdapter teacherNoticeRecyclerViewAdapter;
    private List<Notice> notice_list;
    private RequestQueue queue;



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.home_main);

        Intent i =getIntent();

        User recieved_user = (User)i.getSerializableExtra("userObj");
        user_desig = recieved_user.getUser_designation();

        //Log.d("Designation",user_desig);

        queue = Volley.newRequestQueue(home_activity.this);




        if (user_desig.contentEquals("student")){
                   //Log.d("Designation",user_desig);
            final LinearLayoutManager layoutManager = new LinearLayoutManager(this);
            layoutManager.setOrientation(LinearLayoutManager.VERTICAL);
              recyclerView_student = (RecyclerView)findViewById(R.id.recycler_user);
              recyclerView_student.setHasFixedSize(true);
              recyclerView_student.setLayoutManager(layoutManager);
              notice_list = new ArrayList<>();
              notice_list = getStudentNotices();
            Log.d("Response:",notice_list.toString());


            studentNoticeRecyclerViewAdapter = new StudentNoticeRecyclerViewAdapter(this, notice_list);
              recyclerView_student.setAdapter(studentNoticeRecyclerViewAdapter);
              studentNoticeRecyclerViewAdapter.notifyDataSetChanged();
        }

        else{
            final LinearLayoutManager layoutManager = new LinearLayoutManager(this);
            layoutManager.setOrientation(LinearLayoutManager.VERTICAL);
            recyclerView_teacher = (RecyclerView)findViewById(R.id.recycler_user);
            recyclerView_teacher.setHasFixedSize(true);
            recyclerView_teacher.setLayoutManager(layoutManager);
             notice_list = new ArrayList<>();

            notice_list = getTeacherNotices();

            teacherNoticeRecyclerViewAdapter = new TeacherNoticeRecyclerViewAdapter(this,notice_list);
            recyclerView_teacher.setAdapter(teacherNoticeRecyclerViewAdapter);
            teacherNoticeRecyclerViewAdapter.notifyDataSetChanged();
        }


    }

    public List<Notice> getStudentNotices(){
        notice_list.clear();

        JsonObjectRequest noticeStudent = new JsonObjectRequest(Request.Method.GET,
                constants.STUDENT_NOTICE_API_URL, new Response.Listener<JSONObject>() {
            @Override
            public void onResponse(JSONObject response) {
                try {
                    JSONArray listNotices = response.getJSONArray("studentNoticeList");
                    //Log.d("Response", listNotices.toString());
                        for (int i=0;i<listNotices.length();i++){
                            JSONObject jsonObject = listNotices.getJSONObject(i);
                            Notice noticeObjs =  new Notice();
                            noticeObjs.setNotice_title(jsonObject.getString("s_notice_title"));
                            noticeObjs.setNotice_id(Integer.parseInt(jsonObject.getString("s_notice_id")));
                            noticeObjs.setNotice_content(jsonObject.getString("s_notice_content"));
                            noticeObjs.setUser_id('s');

                            //Log.d("Response Obj",noticeObjs.toString());
                            notice_list.add(noticeObjs);
                            //Log.d("Response Object",notice_list.toString());
                    }

                } catch (JSONException e) {
                    e.printStackTrace();
                }
            }
        }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
                Log.d("Error : " ,error.getMessage());
            }
        }
        );

        queue.add(noticeStudent);
        //Log.d("Response return:",notice_list.toString());
        return notice_list;
    }

    public List<Notice> getTeacherNotices(){
        notice_list.clear();

        JsonObjectRequest noticeTeacher = new JsonObjectRequest(Request.Method.GET,
                constants.TEACHER_NOTICE_API_URL, new Response.Listener<JSONObject>() {
            @Override
            public void onResponse(JSONObject response) {
                try {
                    JSONArray listNotices = response.getJSONArray("teacherNoticeList");

                    if(listNotices.length() > 0){
                        for(int i=0;i<listNotices.length();i++){
                            JSONObject jsonObject = listNotices.getJSONObject(i);
                            Notice noticeObjt = new Notice();

                            noticeObjt.setUser_id('t');
                            noticeObjt.setNotice_id(Integer.parseInt(jsonObject.getString("t_notice_id")));
                            noticeObjt.setNotice_title(jsonObject.getString("t_notice_title"));
                            noticeObjt.setNotice_content(jsonObject.getString("t_notice_content"));

                            notice_list.add(noticeObjt);
                        }
                    }
                } catch (JSONException e) {
                    e.printStackTrace();
                }

            }
        }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
                Log.d("Error : " ,error.getMessage());
            }
        }

        );

        queue.add(noticeTeacher);

        return notice_list;

    }
}

Answer 1

渲染onResponse时，您不必等待RecyclerView回调。 将以下代码放在OnResponse()

中

teacherNoticeRecyclerViewAdapter = new TeacherNoticeRecyclerViewAdapter(this,notice_list);
recyclerView_teacher.setAdapter(teacherNoticeRecyclerViewAdapter);
teacherNoticeRecyclerViewAdapter.notifyDataSetChanged();

Answer 2

通常在后台线程而不是主线程中获取数据，当您返回notice_list并立即返回notice_list并且其为空时，您可以将接口发送到获取数据的方法来处理响应，或者像这样调用onResponse方法：

public void getTeacherNotices(){
        notice_list.clear();

        JsonObjectRequest noticeTeacher = new JsonObjectRequest(Request.Method.GET,
                constants.TEACHER_NOTICE_API_URL, new Response.Listener<JSONObject>() {
            @Override
            public void onResponse(JSONObject response) {
                try {
                    JSONArray listNotices = response.getJSONArray("teacherNoticeList");

                    if(listNotices.length() > 0){
                        for(int i=0;i<listNotices.length();i++){
                            JSONObject jsonObject = listNotices.getJSONObject(i);
                            Notice noticeObjt = new Notice();

                            noticeObjt.setUser_id('t');
                            noticeObjt.setNotice_id(Integer.parseInt(jsonObject.getString("t_notice_id")));
                            noticeObjt.setNotice_title(jsonObject.getString("t_notice_title"));
                            noticeObjt.setNotice_content(jsonObject.getString("t_notice_content"));

                            notice_list.add(noticeObjt);
                        }
                        handleTeacherNotices(notice_list)
                    }
                } catch (JSONException e) {
                    e.printStackTrace();
                }

            }
        }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
                Log.d("Error : " ,error.getMessage());
            }
        }

        );

        queue.add(noticeTeacher);

    }

private void handleTeacherNoticesResponse( List<Notice> notice_list){
       final LinearLayoutManager layoutManager = new LinearLayoutManager(this);
            layoutManager.setOrientation(LinearLayoutManager.VERTICAL);
            recyclerView_teacher = (RecyclerView)findViewById(R.id.recycler_user);
            recyclerView_teacher.setHasFixedSize(true);
            recyclerView_teacher.setLayoutManager(layoutManager);
            teacherNoticeRecyclerViewAdapter = new      TeacherNoticeRecyclerViewAdapter(this,notice_list);
            recyclerView_teacher.setAdapter(teacherNoticeRecyclerViewAdapter);
            teacherNoticeRecyclerViewAdapter.notifyDataSetChanged();
}

Answer 3

尝试

public class home_activity extends AppCompatActivity {

private String user_desig;

private RecyclerView recyclerView_student;
private RecyclerView recyclerView_teacher;
private StudentNoticeRecyclerViewAdapter studentNoticeRecyclerViewAdapter;
private TeacherNoticeRecyclerViewAdapter teacherNoticeRecyclerViewAdapter;
private List<Notice> notice_list;
private RequestQueue queue;



@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.home_main);

    Intent i =getIntent();

    User recieved_user = (User)i.getSerializableExtra("userObj");
    user_desig = recieved_user.getUser_designation();

    //Log.d("Designation",user_desig);

    queue = Volley.newRequestQueue(home_activity.this);




    if (user_desig.contentEquals("student")){
               //Log.d("Designation",user_desig);
        final LinearLayoutManager layoutManager = new LinearLayoutManager(this);
        layoutManager.setOrientation(LinearLayoutManager.VERTICAL);
          recyclerView_student = (RecyclerView)findViewById(R.id.recycler_user);
          recyclerView_student.setHasFixedSize(true);
          recyclerView_student.setLayoutManager(layoutManager);
          notice_list = new ArrayList<>();


        studentNoticeRecyclerViewAdapter = new StudentNoticeRecyclerViewAdapter(this, notice_list);
          recyclerView_student.setAdapter(studentNoticeRecyclerViewAdapter);
          getStudentNotices();
    }

    else{
        final LinearLayoutManager layoutManager = new LinearLayoutManager(this);
        layoutManager.setOrientation(LinearLayoutManager.VERTICAL);
        recyclerView_teacher = (RecyclerView)findViewById(R.id.recycler_user);
        recyclerView_teacher.setHasFixedSize(true);
        recyclerView_teacher.setLayoutManager(layoutManager);
         notice_list = new ArrayList<>();

        teacherNoticeRecyclerViewAdapter = new TeacherNoticeRecyclerViewAdapter(this,notice_list);
        recyclerView_teacher.setAdapter(teacherNoticeRecyclerViewAdapter);
        getTeacherNotices();
    }


}

public void getStudentNotices(){
    notice_list.clear();

    JsonObjectRequest noticeStudent = new JsonObjectRequest(Request.Method.GET,
            constants.STUDENT_NOTICE_API_URL, new Response.Listener<JSONObject>() {
        @Override
        public void onResponse(JSONObject response) {
            try {
                JSONArray listNotices = response.getJSONArray("studentNoticeList");
                //Log.d("Response", listNotices.toString());
                    for (int i=0;i<listNotices.length();i++){
                        JSONObject jsonObject = listNotices.getJSONObject(i);
                        Notice noticeObjs =  new Notice();
                        noticeObjs.setNotice_title(jsonObject.getString("s_notice_title"));
                        noticeObjs.setNotice_id(Integer.parseInt(jsonObject.getString("s_notice_id")));
                        noticeObjs.setNotice_content(jsonObject.getString("s_notice_content"));
                        noticeObjs.setUser_id('s');

                        //Log.d("Response Obj",noticeObjs.toString());
                        notice_list.add(noticeObjs);
                        //Log.d("Response Object",notice_list.toString());
                }
                studentNoticeRecyclerViewAdapter.notifyDataSetChanged();

            } catch (JSONException e) {
                e.printStackTrace();
            }
        }
    }, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {
            Log.d("Error : " ,error.getMessage());
        }
    }
    );

    queue.add(noticeStudent);
    //Log.d("Response return:",notice_list.toString());
}

public void getTeacherNotices(){
    notice_list.clear();

    JsonObjectRequest noticeTeacher = new JsonObjectRequest(Request.Method.GET,
            constants.TEACHER_NOTICE_API_URL, new Response.Listener<JSONObject>() {
        @Override
        public void onResponse(JSONObject response) {
            try {
                JSONArray listNotices = response.getJSONArray("teacherNoticeList");

                if(listNotices.length() > 0){
                    for(int i=0;i<listNotices.length();i++){
                        JSONObject jsonObject = listNotices.getJSONObject(i);
                        Notice noticeObjt = new Notice();

                        noticeObjt.setUser_id('t');
                        noticeObjt.setNotice_id(Integer.parseInt(jsonObject.getString("t_notice_id")));
                        noticeObjt.setNotice_title(jsonObject.getString("t_notice_title"));
                        noticeObjt.setNotice_content(jsonObject.getString("t_notice_content"));

                        notice_list.add(noticeObjt);
                    }
                }
                teacherNoticeRecyclerViewAdapter.notifyDataSetChanged();
            } catch (JSONException e) {
                e.printStackTrace();
            }

        }
    }, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {
            Log.d("Error : " ,error.getMessage());
        }
    }

    );

    queue.add(noticeTeacher);

    return notice_list;

}

}