java.lang.NullPointerException:Android Studio
java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法“void android.widget.ListView.setAdapter(android.widget.ListAdapter)” 当我试图用聊天适配器设置listView时,第67行出现了一个NullPointerException。下面是课堂: 我已将**放置在获取NullPointerException的行的周围。当我将messageAdapter设置为一个名为ChatAdapter(this)的内部类的新对象时,我并不理解为什么它返回nulljava.lang.NullPointerException:Android Studio,java,android,nullpointerexception,Java,Android,Nullpointerexception,java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法“void android.widget.ListView.setAdapter(android.widget.ListAdapter)” 当我试图用聊天适配器设置listView时,第67行出现了一个NullPointerException。下面是课堂: 我已将**放置在获取NullPointerException的行的周围。当我将messageAdapter设置为一个名为ChatAdapter(thi
导入android.content.Context;
导入android.content.Intent;
导入android.database.Cursor;
导入android.os.Bundle;
导入android.support.annotation.NonNull;
导入android.support.v7.app.AppActivity;
导入android.support.v7.widget.LinearLayoutManager;
导入android.support.v7.widget.RecyclerView;
导入android.support.v7.widget.Toolbar;
导入android.support.design.widget.FloatingActionButton;
导入android.support.design.widget.Snackbar;
导入android.view.LayoutInflater;
导入android.view.view;
导入android.view.ViewGroup;
导入android.widget.ArrayAdapter;
导入android.widget.Button;
导入android.widget.EditText;
导入android.widget.ListView;
导入android.widget.TextView;
导入android.widget.Toast;
导入java.util.ArrayList;
导入java.util.List;
公共类MessageListActivity扩展了AppCompatActivity{
/**
*活动是否处于双窗格模式,即在平板电脑上运行
*装置。
*/
列表视图列表视图;
按钮发送按钮;
ArrayList ArrayList=新的ArrayList();
ChatDatabaseHelper Cdb;
私有布尔值窗格;
ChatAdapter messageAdapter=null;
@凌驾
创建时受保护的void(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
setContentView(R.layout.activity\u消息\u列表);
Toolbar Toolbar=(Toolbar)findViewById(R.id.Toolbar);
设置支持操作栏(工具栏);
setTitle(getTitle());
listView=(listView)findViewById(R.id.chatListView);
final EditText EditText=(EditText)findViewById(R.id.messageText);
sendButton=(按钮)findViewById(R.id.sendButton);
messageAdapter=new ChatAdapter(this);//ChatAdapter是内置适配器
**setAdapter(messageAdapter)**
Cdb=新的ChatDatabaseHelper(此);
Cursor Cursor=Cdb.getMessages();//get messages方法的类型为数据库帮助器类中的Cursor
//光标将在数据库中移动以查找下一个文本(如果有)。
while(cursor.moveToNext(){arrayList.add(cursor.getString(cursor.getColumnIndex(Cdb.KEY_MESSAGE)));}
sendButton.setOnClickListener(新视图.OnClickListener(){
@凌驾
public void onClick(View v){//单击send按钮的侦听器
String chatText=editText.getText().toString();//将editText更改为字符串
add(chatText);//将EditTest中的字符串添加到arrayList
布尔值isInserted=Cdb.insertData(ChatterText);//将消息文本插入数据库
如果(isInserted=true)
{Toast.makeText(MessageListActivity.this,“Message Sent”,Toast.LENGTH_SHORT).show();}//如果将消息插入数据库,则此Toast将显示
否则{
Toast.makeText(MessageListActivity.this,“消息未发送”,Toast.LENGTH_SHORT).show();}//如果消息未进入数据库,则此Toast将显示
messageAdapter.notifyDataSetChanged();//通知适配器消息已发送,从传入更改为传出
editText.setText(“”;//将发送按钮上的文本设置为空白。
}//onClick视图的结束
});//onclick侦听器的结尾
if(findViewById(R.id.message\u detail\u container)!=null){
//详图容器视图将仅显示在
//大屏幕布局(res/values-w900dp)。
//如果存在此视图,则
//活动应处于双窗格模式。
mTwoPane=true;
}
}
类ChatAdapter扩展了ArrayAdapter{//自定义适配器类//当您使用它为您形成for循环的所有适配器时。
公共聊天适配器(MessageListActivity ctx){
super(ctx,0);
}//默认构造函数
//方法返回数组中的行数
//将显示运行for循环的次数
public int getCount(){return arrayList.size();}//将返回数组的大小
公共字符串getItem(int-position){return arrayList.get(position);}//将返回位置处的项
//getview方法
@凌驾
公共视图getView(int位置,最终视图convertView,视图组父级){//内部类
LayoutInflater充气器=MessageListActivity.this.getLayoutInflater();//充气器将xml布局充气到视图中。
查看结果=空;
如果(位置%2==0){//如果数组中的位置号为奇数,则执行此操作;如果位置号为偶数,则执行此操作。
结果=充气机。充气(R.layout.chat_row_incoming,null);//根据位置显示布局传入
}否则{
结果=充气器。充气(R.layout.chat_row_outing,null);//根据位置显示布局传出
}
TextView message=(TextView)result.findViewById(R.id.messageText);//创建连接到messageText的TextView类型的消息
最终字符串messageText=getItem(位置);
message.setText(messageText);
result.setOnClickListener(新视图.OnClickListener(){
@凌驾
公共void onClick(视图v){
如果(mTwoPane){
Bundle参数=新Bundle();
arguments.putString(MessageDetailFragment.ARG_ITEM_ID,messageText);
MessageDetailFragment=新建MessageDetailFragment();
fragment.setArguments(参数);
getSupportFragmentMana
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() + "'";
}
}
}
<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>
messageAdapter = new ChatAdapter(MessageListActivity .this);