致命异常：主要是java.lang.NullPointerException

Question

我是一个不太了解编程的新手

我正在尝试字典申请，而且由于

而感到困惑

01-27 06:27:57.561: E/AndroidRuntime(2439): FATAL EXCEPTION: main 
01-27 06:27:57.561: E/AndroidRuntime(2439): Process: taejung.kim.dic, PID: 2439
    01-27 06:27:57.561: E/AndroidRuntime(2439): java.lang.RuntimeException: Unable to start activity 
    ComponentInfo{taejung.kim.dic/taejung.kim.dic.DictionaryMainActivity}: java.lang.NullPointerException
01-27 06:27:57.561: E/AndroidRuntime(2439):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
01-27 06:27:57.561: E/AndroidRuntime(2439):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
01-27 06:27:57.561: E/AndroidRuntime(2439):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
01-27 06:27:57.561: E/AndroidRuntime(2439):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
01-27 06:27:57.561: E/AndroidRuntime(2439):     at android.os.Handler.dispatchMessage(Handler.java:102)
01-27 06:27:57.561: E/AndroidRuntime(2439):     at android.os.Looper.loop(Looper.java:136)
01-27 06:27:57.561: E/AndroidRuntime(2439):     at android.app.ActivityThread.main(ActivityThread.java:5017)
01-27 06:27:57.561: E/AndroidRuntime(2439):     at java.lang.reflect.Method.invokeNative(Native Method)
01-27 06:27:57.561: E/AndroidRuntime(2439):     at java.lang.reflect.Method.invoke(Method.java:515)
01-27 06:27:57.561: E/AndroidRuntime(2439):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
01-27 06:27:57.561: E/AndroidRuntime(2439):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
01-27 06:27:57.561: E/AndroidRuntime(2439):     at dalvik.system.NativeStart.main(Native Method)
01-27 06:27:57.561: E/AndroidRuntime(2439): Caused by: java.lang.NullPointerException
01-27 06:27:57.561: E/AndroidRuntime(2439):     at taejung.kim.dic.DictionaryMainActivity.onCreate(DictionaryMainActivity.java:45)
01-27 06:27:57.561: E/AndroidRuntime(2439):     at android.app.Activity.performCreate(Activity.java:5231)
01-27 06:27:57.561: E/AndroidRuntime(2439):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
01-27 06:27:57.561: E/AndroidRuntime(2439):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
01-27 06:27:57.561: E/AndroidRuntime(2439):     ... 11 more

DictionaryMainActivity.java

package taejung.kim.dic;

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import java.util.ArrayList;
import java.util.HashMap;

import android.app.Activity;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.ListView;
`enter code here`public class DictionaryMainActivity extends ActionBarActivity {

 private ListView lv;

    // Listview Adapter
    ArrayAdapter<String> adapter;

    // Search EditText


    // ArrayList for Listview
    ArrayList<HashMap<String, String>> productList;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_list_main);

        // Listview Data
        String products[] = {"Dell Inspiron", "HTC One X", "HTC Wildfire S", "HTC Sense", "HTC Sensation XE",
                                "iPhone 4S", "Samsung Galaxy Note 800",
                                "Samsung Galaxy S3", "MacBook Air", "Mac Mini", "MacBook Pro"};



        // Adding items to listview
        adapter = new ArrayAdapter<String>(this, R.layout.list_item, R.id.product_name, products);
        lv.setAdapter(adapter);
    }

}

activity_list_main.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="taejung.kim.dic.DictionaryMainActivity" >


   <!--  모음리스트 -->

   <HorizontalScrollView
       android:id="@+id/MoumScroll"
       android:layout_width="match_parent"
       android:layout_height="wrap_content"
       android:fillViewport="true" >

    <LinearLayout
        android:id="@+id/MoumList"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal" >

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="ㅏ" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="ㅑ" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="ㅓ" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="ㅕ" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="ㅗ" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="ㅛ" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="ㅜ" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="ㅠ" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="ㅡ" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="ㅣ" />
    </LinearLayout>
    </HorizontalScrollView>


    <!-- 자음리스트 -->
     <HorizontalScrollView 
       android:id="@+id/JaumScroll"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
        android:fillViewport="true">
    <LinearLayout
        android:id="@+id/JaumList"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal" >

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/ga" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/na" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/da" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/ra" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/ma" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/ba" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/sa" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/aa" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/ja" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/cha" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/ka" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/ta" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/pa" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="@string/ha" />
    </LinearLayout>
    </HorizontalScrollView>

     <!--화면이동  -->

    <LinearLayout
        android:id="@+id/BtLayout1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:layout_weight="8" >

        <Button 
            android:id="@+id/BtHelp1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="도움말"
            android:layout_weight="1"/>

         <Button 
            android:id="@+id/BtSearch1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="검색"
            android:layout_weight="1"/>
    </LinearLayout>

    <ListView
        android:id="@+id/listview"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content" />



    </LinearLayout>

list_item.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >

<!-- Single ListItem -->

<!-- Product Name -->
<TextView android:id="@+id/product_name"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:padding="10dip"
    android:textSize="16dip"
    android:textStyle="bold"/>    

</LinearLayout>

清单

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="taejung.kim.dic"
android:versionCode="1"
android:versionName="1.0" >

<uses-sdk
    android:minSdkVersion="14"
    android:targetSdkVersion="21" />

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".DictionaryMainActivity"

        android:label="@string/app_name" 
        android:windowSoftInputMode="stateHidden">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>


</application>

</manifest>

我认为问题出现在主要活动的OnCreate中，但我无法理解这个问题是什么。

Answer 1

您忘记初始化lv变量。添加

lv = (ListView)findViewById(R.id.listview);

在setContentView()之后和lv上调用方法之前

。

Answer 2

您的ListView lv永远不会被初始化。所以你在这一行获得了一个NPE：

lv.setAdapter(adapter);

Answer 3

您的ListView lv;变量未初始化，因此您尝试为尚未创建的对象调用lv.setAdapter(adapter);。