のjava.lang.NullPointerException：Androidのメーカー

Question

のjava.lang.NullPointerException：nullのオブジェクト参照に '無効android.widget.ListView.setAdapter（android.widget.ListAdapter）' 仮想メソッドを呼び出そうと

チャットアダプタでlistViewを設定しようとしているときに、67行目でNullPointerExceptionが発生します。ここにクラスがあります：

**私はNullPointerExceptionを取得しています。私は本当になぜ私がChatAdapter（this）と呼ばれる内部クラスの新しいオブジェクトにmessageAdapterを設定しているときにnullとして返すかを理解していません。

import android.content.Context; 
import android.content.Intent; 
import android.database.Cursor; 
import android.os.Bundle; 
import android.support.annotation.NonNull; 
import android.support.v7.app.AppCompatActivity; 
import android.support.v7.widget.LinearLayoutManager; 
import android.support.v7.widget.RecyclerView; 
import android.support.v7.widget.Toolbar; 
import android.support.design.widget.FloatingActionButton; 
import android.support.design.widget.Snackbar; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.ArrayAdapter; 
import android.widget.Button; 
import android.widget.EditText; 
import android.widget.ListView; 
import android.widget.TextView; 
import android.widget.Toast; 

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


public class MessageListActivity extends AppCompatActivity { 

/** 
* Whether or not the activity is in two-pane mode, i.e. running on a tablet 
* device. 
*/ 

ListView listView; 
Button sendButton; 
ArrayList<String> arrayList = new ArrayList<String>(); 
ChatDatabaseHelper Cdb; 
private boolean mTwoPane; 
ChatAdapter messageAdapter = null; 


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

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

    listView = (ListView)findViewById(R.id.chatListView); 
    final EditText editText = (EditText)findViewById(R.id.messageText); 
    sendButton = (Button)findViewById(R.id.sendButton); 
    messageAdapter = new ChatAdapter(this); // chatAdapter is a built in adapater 
    **listView.setAdapter(messageAdapter);** 
    Cdb = new ChatDatabaseHelper(this); 

    Cursor cursor = Cdb.getMessages(); // get messages method is of type Cursor from database helper class 

    // cursor will move through the database to find the next text if there is any. 
    while (cursor.moveToNext()) { arrayList.add(cursor.getString(cursor.getColumnIndex(Cdb.KEY_MESSAGE))); } 

    sendButton.setOnClickListener(new View.OnClickListener() { 

     @Override 
     public void onClick(View v) { // on click listener for the send button 

      String chatText = editText.getText().toString(); // changing editText to String 
      arrayList.add(chatText); // adding the string from EditTest to arrayList 
      boolean isInserted = Cdb.insertData(chatText); // inserting the message text into the database 
      if(isInserted = true) 
      { Toast.makeText(MessageListActivity.this,"Message Sent",Toast.LENGTH_SHORT).show(); } // if the message inserts into the database this toast will show 
      else { 
       Toast.makeText(MessageListActivity.this,"Message not Sent",Toast.LENGTH_SHORT).show(); } // if message does not nter the database this toast will show 
      messageAdapter.notifyDataSetChanged(); // notifying the adapter that a message has been sent, changing from incoming to outgoing 
      editText.setText(" "); // set the text on the send button to blank. 

     } // end of onClick view 

    }); // end of onClickListener 


    if (findViewById(R.id.message_detail_container) != null) { 
     // The detail container view will be present only in the 
     // large-screen layouts (res/values-w900dp). 
     // If this view is present, then the 
     // activity should be in two-pane mode. 
     mTwoPane = true; 
    } 
} 

class ChatAdapter extends ArrayAdapter<String> { // custom adapter class // when youc all the adapter it forms the for loop for you. 

    public ChatAdapter(MessageListActivity ctx) { 
     super(ctx, 0); 
    } // default constructor 

    // method to return the number of rows that will be in your array 
    // will tell how many times to run a for loop 
    public int getCount(){ return arrayList.size(); } // will return the size of the array 
    public String getItem(int position){ return arrayList.get(position); } // will return the item at position 

    // getview method 
    @Override 
    public View getView(int position, final View convertView, ViewGroup parent) {// inner class 

     LayoutInflater Inflater = MessageListActivity.this.getLayoutInflater(); // an inflater inflates the xml layout into a view. 
     View result = null; 

     if(position%2 == 0){ // if position number in the array is odd do this, if number is even, do this. 
      result = Inflater.inflate(R.layout.chat_row_incoming, null); // depending on the position, show layout incoming 

     } else { 
      result = Inflater.inflate(R.layout.chat_row_outgoing,null); // depending on the position, show layout outgoing 

     } 

     TextView message = (TextView)result.findViewById(R.id.messageText); // creating a message of type TextView connected to messageText 
     final String messageText = getItem(position) ; 
     message.setText(messageText); 

     result.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       if (mTwoPane) { 
        Bundle arguments = new Bundle(); 
        arguments.putString(MessageDetailFragment.ARG_ITEM_ID,messageText); 
        MessageDetailFragment fragment = new MessageDetailFragment(); 
        fragment.setArguments(arguments); 
        getSupportFragmentManager().beginTransaction() 
          .replace(R.id.message_detail_container, fragment) 
          .commit(); 
       } else { 
        Context context = v.getContext(); 
        Intent intent = new Intent(context, MessageDetailActivity.class); 
        intent.putExtra(MessageDetailFragment.ARG_ITEM_ID,messageText); 

        context.startActivity(intent); 
       } 

      } 
     }); 

     return result; // return the view which is the Inflater. 

    } 
} // end of chat adapter class 

private void setupRecyclerView(@NonNull RecyclerView recyclerView) { 
    recyclerView.setAdapter(new SimpleItemRecyclerViewAdapter(DummyContent.ITEMS)); 
} 

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

    private final List<DummyContent.DummyItem> mValues; 

    public SimpleItemRecyclerViewAdapter(List<DummyContent.DummyItem> items) { 
     mValues = items; 
    } 

    @Override 
    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) { 
     View view = LayoutInflater.from(parent.getContext()) 
       .inflate(R.layout.message_list_content, parent, false); 
     return new ViewHolder(view); 
    } 

    @Override 
    public void onBindViewHolder(final ViewHolder holder, int position) { 
     holder.mItem = mValues.get(position); 
     holder.mIdView.setText(mValues.get(position).id); 
     holder.mContentView.setText(mValues.get(position).content); 

     holder.mView.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       if (mTwoPane) { 
        Bundle arguments = new Bundle(); 
        arguments.putString(MessageDetailFragment.ARG_ITEM_ID, holder.mItem.id); 
        MessageDetailFragment fragment = new MessageDetailFragment(); 
        fragment.setArguments(arguments); 
        getSupportFragmentManager().beginTransaction() 
          .replace(R.id.message_detail_container, fragment) 
          .commit(); 
       } else { 
        Context context = v.getContext(); 
        Intent intent = new Intent(context, MessageDetailActivity.class); 
        intent.putExtra(MessageDetailFragment.ARG_ITEM_ID, holder.mItem.id); 

        context.startActivity(intent); 
       } 
      } 
     }); 
    } 

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

    public class ViewHolder extends RecyclerView.ViewHolder { 
     public final View mView; 
     public final TextView mIdView; 
     public final TextView mContentView; 
     public DummyContent.DummyItem mItem; 

     public ViewHolder(View view) { 
      super(view); 
      mView = view; 
      mIdView = (TextView) view.findViewById(R.id.id); 
      mContentView = (TextView) view.findViewById(R.id.content); 
     } 

     @Override 
     public String toString() { 
      return super.toString() + " '" + mContentView.getText() + "'"; 
     } 
    } 
} 

のxml：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
xmlns:tools="http://schemas.android.com/tools" 
android:layout_width="match_parent" 
android:layout_height="match_parent" 
android:paddingBottom="@dimen/activity_vertical_margin" 
android:paddingLeft="@dimen/activity_horizontal_margin" 
android:paddingRight="@dimen/activity_horizontal_margin" 
android:paddingTop="@dimen/activity_vertical_margin" 
tools:context="com.example.ChatWindow"> 

<ListView 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:id="@+id/chatListView" 
    android:layout_alignParentStart="true" 
    android:layout_alignParentEnd="true" 
    android:layout_alignParentTop="true" 
    android:layout_alignParentBottom="false" 
    android:layout_alignWithParentIfMissing="false" 
    android:layout_above="@+id/sendButton" /> 

<Button 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:text="@string/sendButton" 
    android:id="@+id/sendButton" 
    android:layout_alignParentBottom="true" 
    android:layout_alignParentEnd="true" 
    android:layout_alignParentRight="true" 
    android:singleLine="true" /> 

<EditText 
    android:layout_width="match_parent" 
    android:layout_height="wrap_content" 
    android:id="@+id/messageText" 
    android:layout_toStartOf="@id/sendButton" 
    android:layout_alignParentStart="true" 
    android:layout_alignParentBottom="true" 
    android:layout_alignParentLeft="true" 
    android:enabled="true" 
    /> 

</RelativeLayout> 

Answer 1

ただ単に好奇心から

messageAdapter = new ChatAdapter(MessageListActivity .this);