[英]Unable to start activity: Attempt to invoke virtual method (NullPointerException)
[英]NullPointerException - attempt to invoke virtual method `notifyDataSetChanged()` - can't see list created
我正在按照本教程的方式工作,并且一直在向列表添加名称并填充列表。
(带下划线的“Darryl”是输入框,它在顶部显示该名称(点击按钮后),然后将其添加到列表中)。
虽然我收到此错误:
08-24 18:17:50.903 21935-21935/com.example.batman.myapplication E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.batman.myapplication, PID: 21935
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ArrayAdapter.notifyDataSetChanged()' on a null object reference
at com.example.batman.myapplication.MainActivity.onClick(MainActivity.java:65)
at android.view.View.performClick(View.java:5217)
at android.view.View$PerformClick.run(View.java:20983)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:6141)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)
这是我的MainActivity.java
:
package com.example.batman.myapplication;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.TextView;
import java.util.ArrayList;
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
TextView mainTextView;
EditText mainEditText;
ListView mainListView;
ArrayAdapter mArrayAdapter;
// ArrayList<String> mNameList = new ArrayList<String>();
ArrayList mNameList = new ArrayList();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// 1. Access the TextView defined in layout XML
// and then set its text
mainTextView = (TextView) findViewById(R.id.main_textview);
mainTextView.setText("Set in Java!");
Button mainButton;
mainButton = (Button) findViewById(R.id.main_button);
mainButton.setOnClickListener(this);
// 3. Access the EditText defined in layout XML
mainEditText = (EditText) findViewById(R.id.main_edittext);
// 4. Access the ListView
mainListView = (ListView) findViewById(R.id.main_listview);
// Create an ArrayAdapter for the ListView
mArrayAdapter = new ArrayAdapter(this, // removed ArrayAdapter before mArrayAdapter
android.R.layout.simple_list_item_1,
mNameList);
// Set the ListView to use the ArrayAdapter
mainListView.setAdapter(mArrayAdapter);
}
@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 void onClick(View v) {
// Take what was typed into the EditText
// and use in TextView
mainTextView.setText(mainEditText.getText().toString() + ".");
// Also add that value to the list shown in the ListView
mNameList.add(mainEditText.getText().toString());
mArrayAdapter.notifyDataSetChanged();
}
} // end class
这是ActivityMain.XML
:
<LinearLayout 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:orientation="vertical"
tools:context=".MainActivity">
<TextView
android:id="@+id/main_textview"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginLeft="20dp"
android:layout_marginTop="20dp"
android:text="textView"/>
<!-- Displays keyboard when touched -->
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content">
<!-- List whose dataset is defined in code with an adapter -->
<ListView
android:id="@+id/main_listview"
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1"
android:layout_marginTop="20dp"/>
<!-- Set OnClickListener to trigger results when pressed -->
<Button
android:id="@+id/main_button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="20dp"
android:layout_marginLeft="20dp"
android:text="button" />
<!-- Shows an image from your drawable resources -->
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="20dp"
android:layout_marginLeft="20dp"
android:src="@drawable/ic_launcher" />
<!-- Closing tag for the horizontal nested layout -->
</LinearLayout>
<EditText
android:id="@+id/main_edittext"
android:inputType="textCapSentences"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="20dp"
android:layout_marginLeft="20dp"
android:hint="hint"/>
<!-- Closing tag for the horizontal nested layout -->
</LinearLayout>
我还在new ArrayAdapter(this, android.R.layout.simple_list_item_1, mNameList);
上得到了一些突出显示(Android Studio new ArrayAdapter(this, android.R.layout.simple_list_item_1, mNameList);
行,说有一个“未经检查的电话”。
正如您在我的一条评论中看到的那样,我是否需要通过诸如ArrayList<String> mNameList = new ArrayList<String>();
类的东西使mNameList
成为字符串ArrayList<String> mNameList = new ArrayList<String>();
?
(如果您需要清单或我的MainActivity.xml
,请告诉我)。
感谢您的任何想法!
编辑:感谢评论,我从 onClick 部分删除了ArrayAdapter
。 问题的最后一部分是获取要显示的列表。 它现在显示输入,但不会显示在下面的列表中。 我也附上了 XML。
你没有初始化全局变量 mArrayAdapter
改变 :
ArrayAdapter mArrayAdapter = new ArrayAdapter(this,
android.R.layout.simple_list_item_1,
mNameList);
到 :
mArrayAdapter = new ArrayAdapter(this,
android.R.layout.simple_list_item_1,
mNameList);
在您的活动的 onCreate 方法中
您的 mArrayAdapter 始终为空,因为您正在使用以下内容创建本地 mArrayAdapter:
ArrayAdapter mArrayAdapter = new ArrayAdapter(this,
android.R.layout.simple_list_item_1,
mNameList);
所以......只需删除ArrayAdapter
这样你就可以初始化适配器的全局版本,你应该没问题。
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