Android - RecyclerView NullPointerException getItemCount？

我有一个recyclerView，它让我崩溃：Android - RecyclerView NullPointerException getItemCount？

这里是我的StartActivity：

public class StartActivity extends AppCompatActivity { 

    TextView txtTest; 
    private ProgressDialog pDialog; 
    // These tags will be used to cancel the requests 
    private String tag_json_obj = "jobj_req", tag_json_arry = "jarray_req"; 

    private RecyclerView.Adapter mAdapter; 
    RecyclerView UserCode_Recycler; 
    private LinearLayoutManager mLayoutManager; 


    List<Marketing_Code> userCodeList; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_start); 
     txtTest = (TextView) findViewById(R.id.txtTest); 
     UserCode_Recycler = (RecyclerView) findViewById(R.id.UserCode_Recycler); 
     pDialog = new ProgressDialog(this); 
     pDialog.setMessage("Loading..."); 
     pDialog.setCancelable(false); 

     Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
     setSupportActionBar(toolbar); 

     FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab); 
     if (fab != null) { 
      fab.setOnClickListener(new View.OnClickListener() { 
       @Override 
       public void onClick(View view) { 
        Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG) 
          .setAction("Action", null).show(); 
       } 
      }); 
     } 

     makeJsonArryReq(); 

     userCodeList = new ArrayList<>(); 
     // create an Object for Adapter 
     mAdapter = new UserCodeList_Adapter(userCodeList, StartActivity.this); 
     // set the adapter object to the Recyclerview 
     UserCode_Recycler.setAdapter(mAdapter); 

     mAdapter.notifyDataSetChanged(); 


     UserCode_Recycler.setHasFixedSize(true); 

     mLayoutManager = new LinearLayoutManager(this); 
     // use a linear layout manager 
     UserCode_Recycler.setLayoutManager(mLayoutManager); 

    } 

    private void showProgressDialog() { 
     if (!pDialog.isShowing()) 
      pDialog.show(); 
    } 

    private void hideProgressDialog() { 
     if (pDialog.isShowing()) 
      pDialog.hide(); 
    } 


    /** 
    * Making json array request 
    */ 
    private void makeJsonArryReq() { 
     showProgressDialog(); 
     JsonArrayRequest req = new JsonArrayRequest(Const.Marketing_List, 
       new Response.Listener<JSONArray>() { 
        @Override 
        public void onResponse(JSONArray response) { 
         Log.d("MYData", response.toString()); 
         userCodeList = MarketingCode_JSONParser.parseFeed(response.toString()); 
         /* // create an Object for Adapter 
         mAdapter = new UserCodeList_Adapter(userCodeList, StartActivity.this); 
         // set the adapter object to the Recyclerview 
         Search_Recycler.setAdapter(mAdapter); 
         mAdapter.notifyDataSetChanged(); 
         //txtTest.setText(response.toString());*/ 
         mAdapter.notifyDataSetChanged(); 
         hideProgressDialog(); 
        } 
       }, new Response.ErrorListener() { 
      @Override 
      public void onErrorResponse(VolleyError error) { 
       VolleyLog.d("Custom Log Error", "Error: " + error.getMessage()); 
       hideProgressDialog(); 
      } 
     }); 

     // Adding request to request queue 
     AppController.getInstance().addToRequestQueue(req, tag_json_arry); 
     // Cancelling request 
     // ApplicationController.getInstance().getRequestQueue().cancelAll(tag_json_arry); 
    } 


    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.menu_main, menu); 
     return true; 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // Handle action bar item clicks here. The action bar will 
     // automatically handle clicks on the Home/Up button, so long 
     // as you specify a parent activity in AndroidManifest.xml. 
     int id = item.getItemId(); 

     //noinspection SimplifiableIfStatement 
     if (id == R.id.action_settings) { 
      return true; 
     } 

     return super.onOptionsItemSelected(item); 
    } 
}

而且我的适配器：

public class UserCodeList_Adapter extends RecyclerView.Adapter<UserCodeList_Adapter.ViewHolder> { 

    private List<Marketing_Code> ucList; 
    public static Activity activity; 
    public UserCodeList_Adapter(List<Marketing_Code> userCodeList, Activity activity) { 
     this.ucList = userCodeList; 
     this.activity = activity; 
    } 
    @Override 
    public UserCodeList_Adapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) { 
     // create a new view 
     View itemLayoutView = LayoutInflater.from(parent.getContext()).inflate(R.layout.listmarketing_cardview, null); 

     // create ViewHolder 
     ViewHolder viewHolder = new ViewHolder(itemLayoutView); 
     return viewHolder; 
    } 

    @Override 
    public void onBindViewHolder(UserCodeList_Adapter.ViewHolder viewHolder, int position) { 
     String userCode =String.valueOf(ucList.get(position).getMarketCode()); 
     viewHolder.txtUserID.setText(userCode); 
    } 

    @Override 
    public int getItemCount() { 
     return ucList.size(); 
     //return ucList == null ? 0 : ucList.size(); 
    } 

    public static class ViewHolder extends RecyclerView.ViewHolder { 

     public TextView txtUserID; 

     public Marketing_Code items; 

     public ViewHolder(View itemLayoutView) { 
      super(itemLayoutView); 

      txtUserID = (TextView) itemLayoutView.findViewById(R.id.txtUserID); 
      // Onclick event for the row to show the data in toast 
      itemLayoutView.setOnClickListener(new View.OnClickListener() { 

       @Override 
       public void onClick(View v) { 
       } 
      }); 

     } 

    } 
}

来源

2016-08-02 Jo Jo Roid

您尚未初始化userCodeList。

看到您的代码，您将适配器基于该列表，但尚未初始化。因此，当适配器试图了解您的列表中有多少项目时，会抛出NullPointerException。

您的userCodeList声明更改为一个象下面这样：

List<Marketing_Code> userCodeList = new ArrayList<>();

看到你更新的问题，你似乎从一个JSON响应基础数据。如果是这样的话，你现在正在做的是差不多是正确的。

观察这个稍加修改你的代码片段：

private void makeJsonArryReq() { 
    showProgressDialog(); 
    JsonArrayRequest req = new JsonArrayRequest(Const.Marketing_List, 
      new Response.Listener<JSONArray>() { 
       @Override 
       public void onResponse(JSONArray response) { 
        Log.d("MYData", response.toString()); 
        /* YOUR OLD CODE -> */ // userCodeList = MarketingCode_JSONParser.parseFeed(response.toString()); 
        /* HOW IT SHOULD'VE BEEN */ userCodeList.addAll(MarketingCode_JSONParser.parseFeed(response.toString())); 
        mAdapter.notifyDataSetChanged(); 
        hideProgressDialog(); 
       } 
      }, new Response.ErrorListener() { 
     @Override 
     public void onErrorResponse(VolleyError error) { 
      VolleyLog.d("Custom Log Error", "Error: " + error.getMessage()); 
      hideProgressDialog(); 
     } 
    }); 

// ...

看看上面的代码中的注释部分。这样做应该可以解决您的问题并使数据在屏幕上可见。

来源

2016-08-02 07:50:11 ridsatrio

不要误会我的任何崩溃了，但我的列表是空的？ –

这是真的吗？ String userCode = String.valueOf（ucList.get（position）.getMarketCode（））; –

恩，这是..你应该在后面的过程中填写这个'List'。只要调用'adapter.notifyDataSetChaged（）'，你应该看到你的项目显示在屏幕上。 – ridsatrio

两种方式来处理这个问题：

您需要检查，如果该列表为空，要求检查其大小

public int getItemCount() { 
    if(ucList == null) 
     return 0; 
    return ucList.size(); 
}

另外，之前，这是推荐的方法来这个问题，是在开始时将它初始化为空列表，或者确保它在被关联的适配器访问之前始终被初始化。

List<Marketing_Code> userCodeList = new ArrayList<>();

希望这有助于。

来源

2016-08-02 07:52:38 akdsouza

第一种方式可能会起作用。但是不能保证它能够阻止'null' List导致的随后的崩溃。适配器严重依赖于它们的'List'，所以它不应该首先是'null'。 – ridsatrio

同意。更新回答点2以在访问之前通过初始化陈述推荐的方法 – akdsouza

我想你需要使用replaceAll函数作为列表，而不是直接等同。这里

userCodeList = MarketingCode_JSONParser.parseFeed(response.toString());

尝试检查，如果userCodeList包含initialzing适配器之前的任何数据。

来源

2016-08-02 07:53:12

请执行下列操作，并尝试

// create an Object for Adapter   
    userCodeList = new ArrayList<>(); 
    mAdapter = new UserCodeList_Adapter(userCodeList, StartActivity.this);

来源

2016-08-02 07:53:19 LvN

Android - RecyclerView NullPointerException getItemCount？

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