我的RecyclerView实现有什么问题？

Question

I am following the Android Developer's explanatin on how to set RecyclerView, but I get some errors, for example android.widget.LinearLayout cannot be cast to android.widget.TextView .

The code taken from here : RecyclerView

This is my code, including all necessary files:

MainActivity.java:

package com.example.lastlocation.recyclerviewer;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;

public class MainActivity extends AppCompatActivity {
    private RecyclerView mRecyclerView;
    private RecyclerView.Adapter mAdapter;
    private RecyclerView.LayoutManager mLayoutManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mRecyclerView = (RecyclerView) findViewById(R.id.my_recycler_view);

        // use this setting to improve performance if you know that changes
        // in content do not change the layout size of the RecyclerView
        mRecyclerView.setHasFixedSize(true);

        // use a linear layout manager
        mLayoutManager = new LinearLayoutManager(this);
        mRecyclerView.setLayoutManager(mLayoutManager);
        String[] myDataset = new String[5];
        myDataset[0] = "Data0";
        myDataset[1] = "Data1";
        myDataset[2] = "Data2";
        myDataset[3] = "Data3";
        myDataset[4] = "Data4";

        // specify an adapter (see also next example)
        mAdapter = new MyAdapter(myDataset);
        mRecyclerView.setAdapter(mAdapter);
    }
}

This is MyAdapter.java:

package com.example.lastlocation.recyclerviewer;

import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.ViewGroup;
import android.widget.LinearLayout;
import android.widget.TextView;

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.MyViewHolder> {
    private String[] mDataset;

    // Provide a reference to the views for each data item
    // Complex data items may need more than one view per item, and
    // you provide access to all the views for a data item in a view holder
    public static class MyViewHolder extends RecyclerView.ViewHolder {
        // each data item is just a string in this case
        public TextView mTextView;
        public MyViewHolder(TextView v) {
            super(v);
            mTextView = v;
        }
    }

    // Provide a suitable constructor (depends on the kind of dataset)
    public MyAdapter(String[] myDataset) {
        mDataset = myDataset;
    }

    // Create new views (invoked by the layout manager)
    @Override
    public MyAdapter.MyViewHolder onCreateViewHolder(ViewGroup parent,
                                                     int viewType) {


        TextView v = (TextView) LayoutInflater.from(parent.getContext())
                .inflate(R.layout.my_text_view, parent, false);

        MyViewHolder vh = new MyViewHolder(v);
        return vh;
    }

    // Replace the contents of a view (invoked by the layout manager)
    @Override
    public void onBindViewHolder(MyViewHolder holder, int position) {
        // - get element from your dataset at this position
        // - replace the contents of the view with that element
        holder.mTextView.setText(mDataset[position]);

    }

    // Return the size of your dataset (invoked by the layout manager)
    @Override
    public int getItemCount() {
        return mDataset.length;
    }
}

this is activity_main.xml:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <View
        android:id="@+id/view"
        android:layout_width="386dp"
        android:layout_height="135dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="66dp"
        android:layout_height="64dp"
        android:layout_marginTop="24dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="@+id/view"
        app:srcCompat="@android:color/holo_orange_dark" />

    <TextView
        android:id="@+id/textView2"
        android:layout_width="68dp"
        android:layout_height="30dp"
        android:layout_marginTop="8dp"
        android:text="John Doe"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/imageView" />


    <!-- A RecyclerView with some commonly used attributes -->
    <android.support.v7.widget.RecyclerView
        android:id="@+id/my_recycler_view"
        android:layout_width="375dp"
        android:layout_height="367dp"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="4dp"
        android:scrollbars="vertical"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.428"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/view"
        app:layout_constraintVertical_bias="1.0" />



</android.support.constraint.ConstraintLayout>

And this is the xml for a single text view called my_text_view.xml:

<?xml version="1.0" encoding="utf-8"?>


<!-- Layout for a single list item -->
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="16dp">

    <TextView
        android:id="@+id/my_text_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        />


    </LinearLayout> 

As you can see I nearly copied the code from the Android Developer's website, but I still don't know where is the problem.

Answer 1

  public static class MyViewHolder extends RecyclerView.ViewHolder {
        // each data item is just a string in this case
        public TextView mTextView;
        public MyViewHolder(TextView v) {
            super(v);
            mTextView = v;
        }
    }

    // Provide a suitable constructor (depends on the kind of dataset)
    public MyAdapter(String[] myDataset) {
        mDataset = myDataset;
    }

The problem is in your ViewHolder constructor it should be like that

public static class MyViewHolder extends RecyclerView.ViewHolder {
        // each data item is just a string in this case
        public TextView mTextView;
        public MyViewHolder(View v) {
            super(v);
            mTextView = v.findViewById(R.id.your_text_view_id);
        }
    }


 @Override
    public MyAdapter.MyViewHolder onCreateViewHolder(ViewGroup parent,
                                                     int viewType) {


        View v = (TextView) LayoutInflater.from(parent.getContext())
                .inflate(R.layout.my_text_view, parent, false);

        MyViewHolder vh = new MyViewHolder(v);
        return vh;
    }

You are trying to assign LinearLayout which is the View in your constructor to your TextView and it is crashing. This will fix the problem. The view you receive as a constructor param in your ViewHolder is the root view of your ViewHolder, in your case a LinearLayout, from this root view you can find the other views in it.

Answer 2

You are inflating row view wrongly and thus not assigning views in viewholder in proper manner change your code as below

@Override
    public UserListAdapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        LayoutInflater inflater = LayoutInflater.from(parent.getContext());
        ViewHolder vh = new ViewHolder((inflater.inflate(R.layout.your_row_layout, parent, false)));
        return vh;
    }

and change your viewholder as below

public static class MyViewHolder extends RecyclerView.ViewHolder {
        // each data item is just a string in this case
        public TextView mTextView;
        public MyViewHolder(View v) {
            super(v);
            mTextView = v.findViewById(R.id.my_text_view);
        }
    }

Answer 3

Just Simply you need to change your onCreateViewHolder method by taking as

View view = LayoutInflater.from(parent.getContext())
            .inflate(R.layout.my_text_view, parent, false);
return view;