启动新活动时出现空指针异常

Question

Okay, I'm getting a null pointer exception when I start my third activity. Here is the LogCat message:

12-28 04:38:00.350: ERROR/AndroidRuntime(776): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.acithium.main/com.acithium.rss.ShowDescription}: java.lang.NullPointerException
12-28 04:38:00.350: ERROR/AndroidRuntime(776):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2401)
12-28 04:38:00.350: ERROR/AndroidRuntime(776):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2417)
12-28 04:38:00.350: ERROR/AndroidRuntime(776):     at android.app.ActivityThread.access$2100(ActivityThread.java:116)
12-28 04:38:00.350: ERROR/AndroidRuntime(776):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1794)
12-28 04:38:00.350: ERROR/AndroidRuntime(776):     at android.os.Handler.dispatchMessage(Handler.java:99)
12-28 04:38:00.350: ERROR/AndroidRuntime(776):     at android.os.Looper.loop(Looper.java:123)
12-28 04:38:00.350: ERROR/AndroidRuntime(776):     at android.app.ActivityThread.main(ActivityThread.java:4203)
12-28 04:38:00.350: ERROR/AndroidRuntime(776):     at java.lang.reflect.Method.invokeNative(Native Method)
12-28 04:38:00.350: ERROR/AndroidRuntime(776):     at java.lang.reflect.Method.invoke(Method.java:521)
12-28 04:38:00.350: ERROR/AndroidRuntime(776):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
12-28 04:38:00.350: ERROR/AndroidRuntime(776):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:549)
12-28 04:38:00.350: ERROR/AndroidRuntime(776):     at dalvik.system.NativeStart.main(Native Method)
12-28 04:38:00.350: ERROR/AndroidRuntime(776): Caused by: java.lang.NullPointerException
12-28 04:38:00.350: ERROR/AndroidRuntime(776):     at com.acithium.rss.ShowDescription.onCreate(ShowDescription.java:48)
12-28 04:38:00.350: ERROR/AndroidRuntime(776):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1123)
12-28 04:38:00.350: ERROR/AndroidRuntime(776):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2364)
12-28 04:38:00.350: ERROR/AndroidRuntime(776):     ... 11 more

Here is the section of code where I call the activity:

public void onItemClick(AdapterView parent, View v, int position, long id)
 {
     Log.i(tag,"item clicked! [" + feed.getItem(position).getTitle() + "]");

     Intent itemintent = new Intent(this,com.acithium.rss.ShowDescription.class);
     //Intent itemintent = new Intent();
     //itemintent.setClassName("com.acithium.main", "com.acithium.rss.ShowDescription");
     Bundle b = new Bundle();
     b.putString("title", feed.getItem(position).getTitle());
     b.putString("description", feed.getItem(position).getDescription());
     b.putString("link", feed.getItem(position).getLink());
     itemintent.putExtra("android.intent.extra.INTENT", b);

     startActivityForResult(itemintent,0);
 }

And here is new activity class that is called:

public class ShowDescription extends Activity 
{

    public void onCreate(Bundle icicle) 
    {
        super.onCreate(icicle);
        setContentView(R.layout.showdescription);

        String theStory = null;


        Intent startingIntent = getIntent();


        if (startingIntent != null)
        {
            Bundle b = startingIntent.getBundleExtra("android.intent.extra.INTENT");
            if (b == null)
            {
                theStory = "bad bundle?";
            }
            else
            {
                theStory =  b.getString("title") + "\n\n" + b.getString("description") + "\n\nMore information:\n" + b.getString("link");
            }
        }
        else
        {
            theStory = "Information Not Found.";

        }
        TextView db= (TextView) findViewById(R.id.storybox);
        db.setText(theStory);

        Button backbutton = (Button) findViewById(R.id.back);

        backbutton.setOnClickListener(new Button.OnClickListener() 
        {
            public void onClick(View v) 
            {
                finish();
            }
        });        
    }
}

here is the showdescription.xml file:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
    <ScrollView android:layout_width="fill_parent"
            android:layout_height="fill_parent" >
            <LinearLayout 
                android:orientation="vertical"
                android:layout_width="fill_parent" 
                android:layout_height="fill_parent">
        <TextView  
             android:layout_width="fill_parent" 
                 android:layout_height="wrap_content" 
                 android:autoLink="all"
                 android:text="story goes here ...."
             android:id="@+id/storybox" />
        <Button
                android:layout_height="wrap_content" 
                android:text="Back"
            android:id="@+id/back" 
            android:layout_width="100px" 
            android:layout_marginLeft="100px"/>   
        </LinearLayout>
    </ScrollView>
</LinearLayout>

Answer 1

This is your clue, from the stack trace:

12-28 03:47:21.670: ERROR/AndroidRuntime(862): Caused by: java.lang.NullPointerException
12-28 03:47:21.670: ERROR/AndroidRuntime(862): at com.acithium.rss.ShowDescription.onCreate(ShowDescription.java:47)

What's line 47 in ShowDescription.java?

Answer 2

If I understood the code right, you should invoke the findViewById() of the showdescription view that you set for the activity and not of the activity, because both, button and textview, seem to be part of that view:

super.onCreate(icicle);
View showdescription = this.findViewById(R.id.showdescription);
setContentView(showdescription);

// ..

TextView db = (TextView)showdescription.findViewById(R.id.storybox);
db.setText(theStory);

Button backbutton = (Button)showdescription.findViewById(R.id.back);
backbutton.setOnClickListener(new Button.OnClickListener() 
{
    public void onClick(View v) 
    {
        finish();
    }
});

Answer 3

what is the 47 line in your ShowDescription code? I think you are passing a null cursor.