android - 在使用 Button 调试 NullPointerException 时需要帮助

Question

我有一个带有一些按钮的关于屏幕，但我在第一个 setOnClickListener 行中不断收到 NullPointerException，这是什么问题？

关于.java

public class About extends Activity{

    private Button backbutton;
    private Button likebutton;
    private Button faq;
    private Button update;
    private Button feedback;
    private Button rate;
    private Button help;

    private String faqpath ="http://appkon.com/hdtvs/faq.html";
    private String faqname ="常见问题";
    private String feedbackpath="http://appkon.com/hdtvs/feedback.html" ;
    private String feedbackname="反馈问题" ;
    private String ratename="评价APP" ;
    private String ratepath="http://appkon.com/hdtvs/rate.html" ;
    private String updatepath="http://appkon.com/hdtvs/update.html" ;
    private String updatename="检查更新" ;

    private String wentiname ="";
    private String wentipath ="";
    private String gengxinname ="";
    private String gengxinpath ="";
    private String pingjianame ="";
    private String pingjiapath ="";
    private String changjianpath ="";
    private String changjianname ="";



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


        backbutton=(Button) findViewById(R.id.back);
        likebutton=(Button) findViewById(R.id.share);
        faq =(Button)findViewById(R.id.faq);
        update =(Button)findViewById(R.id.update);
        feedback =(Button)findViewById(R.id.feedback);
        rate =(Button)findViewById(R.id.rate);
        help =(Button)findViewById(R.id.child);

        try{


        backbutton.setOnClickListener(new Button.OnClickListener()
        {
          public void onClick(View v)
          {
            Intent intent = new Intent();
            intent.setClass(About.this, HDtvs.class);
            startActivity(intent);
            About.this.finish();

          }
        });

        likebutton.setOnClickListener(new Button.OnClickListener()
        {
          public void onClick(View v)
          {
            Intent intent = new Intent();
            intent.setClass(About.this, Renrenframe.class);
            startActivity(intent);
            About.this.finish();

          }
        });

        faq.setOnClickListener(new Button.OnClickListener()
        {
          public void onClick(View v)
          {
            Intent intent = new Intent();
            intent.setClass(About.this, Aboutframe.class);
            Bundle bundle = new Bundle();
            bundle.putString("changjianpath",faqpath);
            bundle.putString("changjianname",faqname);
            intent.putExtras(bundle);
            startActivity(intent);
            About.this.finish();

          }
        });

        feedback.setOnClickListener(new Button.OnClickListener()
        {
          public void onClick(View v)
          {
            Intent intent = new Intent();
            intent.setClass(About.this, Aboutframe.class);
            Bundle bundle = new Bundle();
            bundle.putString("wentipath",feedbackpath);
            bundle.putString("wentiname",feedbackname);
            intent.putExtras(bundle);
            startActivity(intent);
            About.this.finish();

          }
        });

        rate.setOnClickListener(new Button.OnClickListener()
        {
          public void onClick(View v)
          {
            Intent intent = new Intent();
            intent.setClass(About.this, Aboutframe.class);
            Bundle bundle = new Bundle();
            bundle.putString("pingjiapath",ratepath);
            bundle.putString("pingjianame",ratename);
            intent.putExtras(bundle);
            startActivity(intent);
            About.this.finish();

          }
        });

        update.setOnClickListener(new Button.OnClickListener()
        {
          public void onClick(View v)
          {
            Intent intent = new Intent();
            intent.setClass(About.this, Aboutframe.class);
            Bundle bundle = new Bundle();
            bundle.putString("gengxinpath",updatepath);
            bundle.putString("gengxinname",updatename);
            intent.putExtras(bundle);
            startActivity(intent);
            About.this.finish();

          }
        });

        help.setOnClickListener(new Button.OnClickListener()
        {
          public void onClick(View v)
          {
            Intent intent = new Intent();
            intent.setClass(About.this, WapsList.class);
            startActivity(intent);
            About.this.finish();

          }
        });
        }catch (Exception e)
        { 
            e.printStackTrace();
        }
}
}

关于.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
  xmlns:android="http://schemas.android.com/apk/res/android"
  android:orientation="vertical"
    android:gravity="center|top"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent"
  android:background="@drawable/main_background_repeat"
>

  <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="45dip"
    android:id="@+id/header"
    android:background="@drawable/header"
    >

<Button android:id="@+id/back" 
    android:textSize="14.0sp" 
    android:textColor="#ffe1ebef" 
    android:gravity="center"    
    android:background="@drawable/button_back" 
    android:layout_width="54.0dip" 
    android:layout_height="33.0dip" 
    android:layout_marginLeft="10.0dip" 
    android:text="@string/back" 
    android:layout_alignParentLeft="true" 
    android:layout_centerHorizontal="true" 
    android:layout_centerVertical="true" />

    <TextView 
    android:id="@+id/title"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:textColor="#FFFFFF"
            android:textSize="18.0sp"
    android:layout_marginTop="8dip"
    android:text="@string/about"/>


<Button android:id="@+id/share" 
    android:textSize="14.0sp" 
    android:textColor="#ffe1ebef" 
    android:gravity="center"    
    android:background="@drawable/share" 
    android:layout_width="54.0dip" 
    android:layout_height="33.0dip" 
    android:layout_marginRight="10.0dip" 
    android:text="@string/share" 
    android:layout_alignParentRight="true"
    android:layout_centerHorizontal="true" 
    android:layout_centerVertical="true" 
    android:layout_marginTop="10dip"/>

    </RelativeLayout>


    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/function"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
      android:orientation="vertical"
    android:gravity="center|top"
    android:layout_marginTop="20dip">

   <TextView android:id="@+id/funcdesc"
    android:layout_width="280dip"
    android:layout_height="wrap_content"
    android:text="@string/funcdesc"
    android:gravity="center" />


   <Button android:id="@+id/feedback"
     android:textColor="#6f789b"
     android:textSize="16.0sp"
     android:gravity="center"   
     android:background="@drawable/button" 
     android:layout_width="185.0dip" 
     android:layout_height="44.0dip" 
     android:layout_marginRight="10.0dip" 
     android:text="@string/feedback" 
     android:layout_alignParentRight="true"
     android:layout_centerHorizontal="true" 
     android:layout_centerVertical="true" 
     android:layout_marginTop="10dip"/>

   <Button android:id="@+id/rate"
     android:textColor="#6f789b"
     android:textSize="16.0sp"
     android:gravity="center"   
     android:background="@drawable/button" 
     android:layout_width="185.0dip" 
     android:layout_height="44.0dip" 
     android:layout_marginRight="10.0dip" 
     android:text="@string/rate" 
     android:layout_alignParentRight="true"
     android:layout_centerHorizontal="true" 
     android:layout_centerVertical="true" 
     android:layout_marginTop="10dip"/>

   <Button android:id="@+id/faq"
     android:textColor="#6f789b"
     android:textSize="16.0sp"
     android:gravity="center"   
     android:background="@drawable/button" 
     android:layout_width="185.0dip" 
     android:layout_height="44.0dip" 
     android:layout_marginRight="10.0dip" 
     android:text="@string/faq" 
     android:layout_alignParentRight="true"
     android:layout_centerHorizontal="true" 
     android:layout_centerVertical="true" 
     android:layout_marginTop="10dip"/>

   <Button android:id="@+id/update"
     android:textColor="#6f789b"
     android:textSize="16.0sp"
     android:gravity="center"   
     android:background="@drawable/button" 
     android:layout_width="185.0dip" 
     android:layout_height="44.0dip" 
     android:layout_marginRight="10.0dip" 
     android:text="@string/update" 
     android:layout_alignParentRight="true"
     android:layout_centerHorizontal="true" 
     android:layout_centerVertical="true"
     android:layout_marginTop="10dip" />

     </LinearLayout>


    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/donation"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
      android:orientation="vertical"
    android:gravity="center|top"
    android:layout_marginTop="20dip">

    <TextView android:id="@+id/funcdesc"
    android:layout_width="280dip"
    android:layout_height="wrap_content"
    android:text="@string/helpdesc"
    android:gravity="center" />

     <Button android:id="@+id/child"
     android:textColor="#6f789b"
     android:textSize="16.0sp"
     android:gravity="center"   
     android:background="@drawable/button" 
     android:layout_width="185.0dip" 
     android:layout_height="44.0dip" 
     android:layout_marginRight="10.0dip" 
     android:text="@string/child" 
     android:layout_alignParentRight="true"
     android:layout_centerHorizontal="true" 
     android:layout_centerVertical="true"
     android:layout_marginTop="10dip" />

     </LinearLayout>


</LinearLayout>

这是我的错误轨迹

12-05 13:36:27.466: WARN/System.err(308): java.lang.NullPointerException
12-05 13:36:27.476: WARN/System.err(308):     at com.appkon.hdtvs.About.onCreate(About.java:59)
12-05 13:36:27.486: WARN/System.err(308):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
12-05 13:36:27.506: WARN/System.err(308):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2459)
12-05 13:36:27.506: WARN/System.err(308):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512)
12-05 13:36:27.526: WARN/System.err(308):     at android.app.ActivityThread.access$2200(ActivityThread.java:119)
12-05 13:36:27.537: WARN/System.err(308):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1863)
12-05 13:36:27.546: WARN/System.err(308):     at android.os.Handler.dispatchMessage(Handler.java:99)
12-05 13:36:27.556: WARN/System.err(308):     at android.os.Looper.loop(Looper.java:123)
12-05 13:36:27.576: WARN/System.err(308):     at android.app.ActivityThread.main(ActivityThread.java:4363)
12-05 13:36:27.586: WARN/System.err(308):     at java.lang.reflect.Method.invokeNative(Native Method)
12-05 13:36:27.596: WARN/System.err(308):     at java.lang.reflect.Method.invoke(Method.java:521)
12-05 13:36:27.596: WARN/System.err(308):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
12-05 13:36:27.607: WARN/System.err(308):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
12-05 13:36:27.616: WARN/System.err(308):     at dalvik.system.NativeStart.main(Native Method)

您的帖子没有太多上下文来解释代码部分；请更清楚地解释您的情况。

Answer 1

看起来您正在将 contentView 设置为 R.layout.main，但该文件名为 about.xml。如果 main.xml 不包含 ID 为 back 的小部件，那么它将从 findViewById()

返回 null