Android Fragment Recycler anzeigen bekommen java.lang.NullPointerException

Question

Ich möchte eine RecyclerView in meinem Fragment haben. Ich benutze RecyclerViewFragment.java von https://developer.android.com/samples/RecyclerView/src/com.example.android.recyclerview/RecyclerViewFragment.html#l19, aber ich bekomme weiterhin die Ausnahme.

Hier ist der Code.

RecyclerViewFragment.java

package com.example.narehh.todolist.Fragments; 

import android.os.Bundle; 
import android.support.v4.app.Fragment; 
import android.support.v7.widget.GridLayoutManager; 
import android.support.v7.widget.LinearLayoutManager; 
import android.support.v7.widget.RecyclerView; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.RadioButton; 

import com.example.narehh.todolist.Adapters.TaskListAdapter; 
import com.example.narehh.todolist.JavaClasses.Task; 
import com.example.narehh.todolist.R; 

import java.util.ArrayList; 



/** 
* Demonstrates the use of {@link RecyclerView} with a {@link LinearLayoutManager} and a 
* {@link GridLayoutManager}. 
*/ 
public class TaskListFragment extends Fragment { 

    private static final String TAG = "TaskListFragment"; 
    private static final String KEY_LAYOUT_MANAGER = "layoutManager"; 
    private static final int SPAN_COUNT = 2; 
    private static final int DATASET_COUNT = 60; 

    private enum LayoutManagerType { 
     GRID_LAYOUT_MANAGER, 
     LINEAR_LAYOUT_MANAGER 
    } 

    protected LayoutManagerType mCurrentLayoutManagerType; 

    protected RadioButton mLinearLayoutRadioButton; 
    protected RadioButton mGridLayoutRadioButton; 

    protected RecyclerView mRecyclerView; 
    protected TaskListAdapter mAdapter; 
    protected RecyclerView.LayoutManager mLayoutManager; 
    // protected String[] mDataset; 
    protected ArrayList<Task> mDataset; 

    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 

     // Initialize dataset, this data would usually come from a local content provider or 
     // remote server. 
     initDataset(); 
    } 

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

     mRecyclerView = (RecyclerView) rootView.findViewById(R.id.task_list_rv); 

     // LinearLayoutManager is used here, this will layout the elements in a similar fashion 
     // to the way ListView would layout elements. The RecyclerView.LayoutManager defines how 
     // elements are laid out. 
     mLayoutManager = new LinearLayoutManager(getActivity()); 

     mCurrentLayoutManagerType = LayoutManagerType.LINEAR_LAYOUT_MANAGER; 

     if (savedInstanceState != null) { 
      // Restore saved layout manager type. 
      mCurrentLayoutManagerType = (LayoutManagerType) savedInstanceState 
        .getSerializable(KEY_LAYOUT_MANAGER); 
     } 
     setRecyclerViewLayoutManager(mCurrentLayoutManagerType); 

     mAdapter = new TaskListAdapter(mDataset); 
     // Set CustomAdapter as the adapter for RecyclerView. 
     mRecyclerView.setAdapter(mAdapter); 

//  mLinearLayoutRadioButton = (RadioButton) rootView.findViewById(R.id.linear_layout_rb); 
//  mLinearLayoutRadioButton.setOnClickListener(new View.OnClickListener() { 
//   @Override 
//   public void onClick(View v) { 
//    setRecyclerViewLayoutManager(LayoutManagerType.LINEAR_LAYOUT_MANAGER); 
//   } 
//  }); 
// 
//  mGridLayoutRadioButton = (RadioButton) rootView.findViewById(R.id.grid_layout_rb); 
//  mGridLayoutRadioButton.setOnClickListener(new View.OnClickListener() { 
//   @Override 
//   public void onClick(View v) { 
//    setRecyclerViewLayoutManager(LayoutManagerType.GRID_LAYOUT_MANAGER); 
//   } 
//  }); 

     return rootView; 
    } 

    /** 
    * Set RecyclerView's LayoutManager to the one given. 
    * 
    * @param layoutManagerType Type of layout manager to switch to. 
    */ 
    public void setRecyclerViewLayoutManager(LayoutManagerType layoutManagerType) { 
     int scrollPosition = 0; 

     // If a layout manager has already been set, get current scroll position. 
     if (mRecyclerView.getLayoutManager() != null) { 
      scrollPosition = ((LinearLayoutManager) mRecyclerView.getLayoutManager()) 
        .findFirstCompletelyVisibleItemPosition(); 
     } 

     switch (layoutManagerType) { 
      case GRID_LAYOUT_MANAGER: 
       mLayoutManager = new GridLayoutManager(getActivity(), SPAN_COUNT); 
       mCurrentLayoutManagerType = LayoutManagerType.GRID_LAYOUT_MANAGER; 
       break; 
      case LINEAR_LAYOUT_MANAGER: 
       mLayoutManager = new LinearLayoutManager(getActivity()); 
       mCurrentLayoutManagerType = LayoutManagerType.LINEAR_LAYOUT_MANAGER; 
       break; 
      default: 
       mLayoutManager = new LinearLayoutManager(getActivity()); 
       mCurrentLayoutManagerType = LayoutManagerType.LINEAR_LAYOUT_MANAGER; 
     } 

     mRecyclerView.setLayoutManager(mLayoutManager); 
     mRecyclerView.scrollToPosition(scrollPosition); 
    } 

    @Override 
    public void onSaveInstanceState(Bundle savedInstanceState) { 
     // Save currently selected layout manager. 
     savedInstanceState.putSerializable(KEY_LAYOUT_MANAGER, mCurrentLayoutManagerType); 
     super.onSaveInstanceState(savedInstanceState); 
    } 

    /** 
    * Generates Strings for RecyclerView's adapter. This data would usually come 
    * from a local content provider or remote server. 
    */ 
    private void initDataset() { 
//  mDataset = new String[DATASET_COUNT]; 
//  for (int i = 0; i < DATASET_COUNT; i++) { 
//   mDataset[i] = "This is element #" + i; 
//  } 
     mDataset = new ArrayList<Task>(); 
     Task task1 = new Task(0, "task1", "01-09-2016", "10:10"); 
     Task task2 = new Task(0, "task2", "01-09-2016", "10:10"); 
     Task task3 = new Task(0, "task3", "01-09-2016", "10:10"); 
     mDataset.add(task1); 
     mDataset.add(task2); 
     mDataset.add(task3); 
    } 
} 

und Adapter:

package com.example.narehh.todolist.Adapters; 

import android.support.v7.widget.RecyclerView; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.CheckBox; 
import android.widget.TextView; 

import com.example.narehh.todolist.JavaClasses.Task; 
import com.example.narehh.todolist.R; 

import java.util.ArrayList; 
import java.util.List; 

/** 
* Created by Narehh on 7/10/2016. 
*/ 
public class TaskListAdapter extends RecyclerView.Adapter<TaskListAdapter.TaskViewHolder> { 

    private ArrayList<Task> taskList = new ArrayList<Task>(); 

    public TaskListAdapter(ArrayList<Task> taskList) { 
     this.taskList = taskList; 
    } 


    @Override 
    public TaskListAdapter.TaskViewHolder onCreateViewHolder(ViewGroup parent, int viewType) { 
     return null; 
    } 

    @Override 
    public void onBindViewHolder(TaskViewHolder holder, int position) { 
     Task task = taskList.get(position); 
     holder.title.setText(task.getName()); 
     holder.date.setText(task.getDate() + task.getTime()); 
     holder.isDone.setChecked(false); 
    } 

    @Override 
    public int getItemCount() { 
     return taskList.size(); 
    } 


    public static class TaskViewHolder extends RecyclerView.ViewHolder { 
     protected TextView title; 
     protected TextView date; 
     protected CheckBox isDone; 

     public TaskViewHolder(View v) { 
      super(v); 
      title = (TextView) v.findViewById(R.id.title); 
      date = (TextView) v.findViewById(R.id.date); 
      isDone = (CheckBox) v.findViewById(R.id.isDone); 
     } 
    } 
} 

Ich erhalte diese Ausnahme

FATAL EXCEPTION: main 
       Process: com.example.narehh.todolist, PID: 16871 
                     java.lang.NullPointerException 
                      at android.support.v7.widget.RecyclerView$Adapter.createViewHolder(RecyclerView.java:5780) 
                      at android.support.v7.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:5003) 
                      at android.support.v7.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:4913) 
                      at android.support.v7.widget.LinearLayoutManager$LayoutState.next(LinearLayoutManager.java:2029) 
                      at android.support.v7.widget.LinearLayoutManager.layoutChunk(LinearLayoutManager.java:1414) 
                      at android.support.v7.widget.LinearLayoutManager.fill(LinearLayoutManager.java:1377) 
                      at android.support.v7.widget.LinearLayoutManager.onLayoutChildren(LinearLayoutManager.java:578) 
                      at android.support.v7.widget.RecyclerView.dispatchLayoutStep2(RecyclerView.java:3260) 
                      at android.support.v7.widget.RecyclerView.dispatchLayout(RecyclerView.java:3069) 
                      at android.support.v7.widget.RecyclerView.onLayout(RecyclerView.java:3518) 
                      at android.view.View.layout(View.java:15781) 
                      at android.view.ViewGroup.layout(ViewGroup.java:4884) 
                      at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453) 
                      at android.widget.FrameLayout.onLayout(FrameLayout.java:388) 
                      at android.view.View.layout(View.java:15781) 
                      at android.view.ViewGroup.layout(ViewGroup.java:4884) 
                      at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453) 
                      at android.widget.FrameLayout.onLayout(FrameLayout.java:388) 
                      at android.view.View.layout(View.java:15781) 
                      at android.view.ViewGroup.layout(ViewGroup.java:4884) 
                      at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1055) 
                      at android.view.View.layout(View.java:15781) 
                      at android.view.ViewGroup.layout(ViewGroup.java:4884) 
                      at android.support.design.widget.HeaderScrollingViewBehavior.layoutChild(HeaderScrollingViewBehavior.java:120) 
                      at android.support.design.widget.ViewOffsetBehavior.onLayoutChild(ViewOffsetBehavior.java:42) 
                      at android.support.design.widget.AppBarLayout$ScrollingViewBehavior.onLayoutChild(AppBarLayout.java:1319) 
                      at android.support.design.widget.CoordinatorLayout.onLayout(CoordinatorLayout.java:815) 
                      at android.view.View.layout(View.java:15781) 
                      at android.view.ViewGroup.layout(ViewGroup.java:4884) 
                      at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453) 
                      at android.widget.FrameLayout.onLayout(FrameLayout.java:388) 
                      at android.view.View.layout(View.java:15781) 
                      at android.view.ViewGroup.layout(ViewGroup.java:4884) 
                      at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1677) 
                      at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1531) 
                      at android.widget.LinearLayout.onLayout(LinearLayout.java:1440) 
                      at android.view.View.layout(View.java:15781) 
                      at android.view.ViewGroup.layout(ViewGroup.java:4884) 
                      at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453) 
                      at android.widget.FrameLayout.onLayout(FrameLayout.java:388) 
                      at android.view.View.layout(View.java:15781) 
                      at android.view.ViewGroup.layout(ViewGroup.java:4884) 
                      at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1677) 
                      at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1531) 
                      at android.widget.LinearLayout.onLayout(LinearLayout.java:1440) 
                      at android.view.View.layout(View.java:15781) 
                      at android.view.ViewGroup.layout(ViewGroup.java:4884) 
                      at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453) 
                      at android.widget.FrameLayout.onLayout(FrameLayout.java:388) 
                      at android.view.View.layout(View.java:15781) 
                      at android.view.ViewGroup.layout(ViewGroup.java:4884) 
                      at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2361) 
                      at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2074) 
                      at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1256) 
                      at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6658) 
                      at android.view.Choreographer$CallbackRecord.run(Choreographer.java:813) 
                      at android.view.Choreographer.doCallbacks(Choreographer.java:613) 
                      at android.view.Choreographer.doFrame(Choreographer.java:583) 

Haben Sie jemals diese Art von Problem gehabt?

Answer 1

Der Fehler ist, weil Sie keine Ansicht

Zuerst tun dies auf der onCreateViewHolder

View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.your_individual_layout_name, null); 
final TaskViewHolder holder = new TaskViewHolder(view); 
// this is where the each item is inflated. 

return holder; 

Rückkehr Dies das Problem lösen ..

Answer 2

Sie sollten nicht null zurück von oncreateviewholder () Sie sollten immer Ihr Viewholder-Objekt zurückgeben, das die Ansicht pro Zeile für Ihre RecyclerView-Liste aufgepumpt hat.