所以我想用片段在我的活动中创建一个listView，我从教程中实现了以下代码。它没有告诉我错误在哪里，我不确切地知道哪个对象或东西是null。
package com.example.aland.intrigueme;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v7.app.ActionBarActivity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;
public class listActivity extends ActionBarActivity {
ListView listView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_list);
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment())
.commit();
}
//haal listView van XML
listView = (ListView) findViewById(R.id.list);
//waarden in Array definiëren om lijst te tonen
String[] values = new String[] {"Park","Café","Nachtclub","Straat","School/Opleiding"};
// Definieert nieuwe Adapter
// Eerste paramater = context
// Tweede parameter = layout voor rijen
// Derde parameter = ID voor de textView
// Vierde = data array
ArrayAdapter <String> adapter = new ArrayAdapter<String>(this,
android.R.layout.simple_list_item_1,android.R.id.text1, values);
//Adapter instellen aan lijst
listView.setAdapter(adapter);
//onClick event voor lijst
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
//listview clicked item index
int itemPosition = position;
//listview clicked item value
String itemValue = (String) listView.getItemAtPosition(position);
//show alert
Toast.makeText(getApplicationContext(),
"Position: "+itemPosition+" Listitem: " +itemValue, Toast.LENGTH_LONG).show();
}
});
}
@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_list, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
/**
* A placeholder fragment containing a simple view.
*/
public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_list, container, false);
return rootView;
}
}
}
这是我的XML文件
<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:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin"
tools:context="com.example.aland.intrigueme.listActivity$PlaceholderFragment">
<ListView android:layout_width="match_parent"
android:layout_height="fill_parent"
android:id="@+id/list" />
</LinearLayout>
这是我得到的错误
01-17 19:02:43.080 2830-2830/com.example.aland.intrigueme E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.aland.intrigueme, PID: 2830
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.aland.intrigueme/com.example.aland.intrigueme.listActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
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:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
at com.example.aland.intrigueme.listActivity.onCreate(listActivity.java:49)
at android.app.Activity.performCreate(Activity.java:5933)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
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:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
01-17 19:03:50.460 2830-2830/com.example.aland.intrigueme I/Process﹕ Sending signal. PID: 2830 SIG: 9
01-17 19:04:16.630 2870-2870/com.example.aland.intrigueme D/AndroidRuntime﹕ Shutting down VM
01-17 19:04:16.630 2870-2870/com.example.aland.intrigueme E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.aland.intrigueme, PID: 2870
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.aland.intrigueme/com.example.aland.intrigueme.listActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
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:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
at com.example.aland.intrigueme.listActivity.onCreate(listActivity.java:49)
at android.app.Activity.performCreate(Activity.java:5933)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
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:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
01-17 19:04:19.400 2870-2870/com.example.aland.intrigueme I/Process﹕ Sending signal. PID: 2870 SIG: 9
答案 0 :(得分：1)
好吧，空指针表示您正在尝试访问尚未初始化的对象（可能）。我遇到了同样的问题，我的建议是在代码的每一行基本上使用Log.i（String a，String b）方法，看看你的对象何时被初始化以及何时被查询。
字符串a是日志文件中的“标记”，保持相同，以便更轻松地找到您的面包屑。 字符串b应该用于描述特定代码行的内容。
祝你好运。