Tabhost Null Pointer Exception

问题:

Can some one please assist me in this example of TabHost? The problem i have is, When I am trying to run the application I am getting Null Pointer Exception.

Here is the code if some one need to have a look.

public class TabBarExample extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        /**
         *  TabHost will have Tabs 
         **/
        TabHost tabHost = (TabHost)findViewById(android.R.id.tabhost);

        /** TabSpec used to create a new tab.
         * By using TabSpec only we can able to setContent to the tab.
         * By using TabSpec setIndicator() we can set name to tab. 
         **/
        TabHost.TabSpec spec1, spec2;
        spec1 = tabHost.newTabSpec("tab_id_1").setIndicator("Tab One").setContent(new Intent().setClass(this, FirstTab.class));
        spec2 = tabHost.newTabSpec("tab_id_2").setIndicator("Tab Two").setContent(new Intent().setClass(this, SecondTab.class));

        /**
         *  create intent of each tab pressed 
         **/
        //Intent intent1 = new Intent().setClass(this, FirstTab.class);
        //Intent intent2 = new Intent().setClass(this, SecondTab.class);

        /**
         * add the created tab to the tab host for display
         **/
        // I am getting error at the following line
        tabHost.addTab(spec1); 
        tabHost.addTab(spec2);
    }
}

Any one to assist in any way or point me to a direction will be appreciated. Regards Shiraz

EDIT-- here is the LogCat view of the error i am getting

06-18 23:18:30.547: ERROR/AndroidRuntime(1404): FATAL EXCEPTION: main
06-18 23:18:30.547: ERROR/AndroidRuntime(1404): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.moonlight.tabbarexample/com.moonlight.tabbarexample.TabBarExample}: java.lang.NullPointerException
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1816)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1837)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at android.app.ActivityThread.access$1500(ActivityThread.java:132)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1033)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at android.os.Handler.dispatchMessage(Handler.java:99)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at android.os.Looper.loop(Looper.java:143)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at android.app.ActivityThread.main(ActivityThread.java:4196)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at java.lang.reflect.Method.invokeNative(Native Method)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at java.lang.reflect.Method.invoke(Method.java:507)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at dalvik.system.NativeStart.main(Native Method)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404): Caused by: java.lang.NullPointerException
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at android.widget.TabHost$ViewIdContentStrategy.<init>(TabHost.java:591)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at android.widget.TabHost$ViewIdContentStrategy.<init>(TabHost.java:586)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at android.widget.TabHost$TabSpec.setContent(TabHost.java:441)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at com.moonlight.tabbarexample.TabBarExample.onCreate(TabBarExample.java:26)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1093)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1780)
06-18 23:18:30.547: ERROR/AndroidRuntime(1404):     ... 11 more
06-18 23:20:06.984: ERROR/SettingsAppWidhetProvider(14282): level1 = 100.0

Thanks Shiraz

EDIT 2 -- HERE IS main.xml

<?xml version="1.0" encoding="utf-8"?>
<TabHost xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@android:id/tabhost"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
    <LinearLayout
        android:orientation="vertical"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:padding="5dip">
        <TabWidget
            android:id="@android:id/tabs"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content" />
        <FrameLayout
            android:id="@android:id/tabcontent"
            android:layout_width="fill_parent"
            android:layout_height="fill_parent"
            android:padding="5dip" />

    </LinearLayout>
</TabHost>

Here is TabbarExample manifest

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="com.moonlight.tabbarexample"
      android:versionCode="1"
      android:versionName="1.0">
    <uses-sdk android:minSdkVersion="7" />

    <application android:icon="@drawable/icon" android:label="@string/app_name">

        <activity android:name=".TabBarExample"
                  android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

    </application>
</manifest>

And here is Firsttab.java

package com.moonlight.tabbarexample;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class FirstTab extends Activity{
    public void onCreate ( Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        /* First Tab Content */
         TextView textView = new TextView(this);
         textView.setText("First Tab");
         setContentView(textView);
    }

}

last one SeconTab.java

package com.moonlight.tabbarexample;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class SecondTab extends Activity{
    public void onCreate ( Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        /* First Tab Content */
         TextView textView = new TextView(this);
         textView.setText("Second Tab");
         setContentView(textView);
    }

}

Thanks guys, pleaes healp, i am anxiously waiting. Regards. Shiraz

EDIT 3 ---

I have added these following line to my manifest file

    <activity android:name=".FirstTab"/>
    <activity android:name=".SecondTab"/>

but I am still getting the same error :(

this is the line in my main activity onCreate methid where i am getting this error

  /**
         * add the created tab to the tab host for display
         **/
        tabHost.addTab(spec1);
        tabHost.addTab(spec2);

thanks shiraz

回答1:

First, define the activities for the two tabs in your manifest:

<?xml version="1.0" encoding="utf-8"?> 
<manifest xmlns:android="http://schemas.android.com/apk/res/android" 
  package="com.moonlight.tabbarexample" 
  android:versionCode="1" 
  android:versionName="1.0"> 
<uses-sdk android:minSdkVersion="7" /> 

<application android:icon="@drawable/icon" android:label="@string/app_name"> 
     <activity android:name=".FirstTab" />
     <activity android:name=".SecondTab" /> 
     <activity android:name=".TabBarExample" 
              android:label="@string/app_name"> 
        <intent-filter> 
            <action android:name="android.intent.action.MAIN" /> 
            <category android:name="android.intent.category.LAUNCHER" /> 
        </intent-filter> 
    </activity> 

</application> 
</manifest> 

Finally, change

public class TabBarExample extends Activity {

to

public class TabBarExample extends TabActivity {

The class declarations for your tab activities are just fine.

回答2:

i just set
mlam=new LocalActivityManager(this,true);

instead of

            mlam=new LocalActivityManager(this,false);

in the oncreate event

mlam=new LocalActivityManager(this,true);
        setContentView(R.layout.main_window);

        TabHost tabHost = (TabHost) findViewById(android.R.id.tabhost);
        mlam.dispatchCreate(savedInstanceState);
        tabHost.setup(mlam );
        TabSpec inboxSpec = tabHost.newTabSpec(INBOX_SPEC);
        inboxSpec.setIndicator(INBOX_SPEC, getResources().getDrawable(R.drawable.icon_inbox));
        Intent inboxIntent = new Intent(this, InboxActivity.class);
        inboxSpec.setContent(inboxIntent);
        TabSpec outboxSpec = tabHost.newTabSpec(OUTBOX_SPEC);
        outboxSpec.setIndicator(OUTBOX_SPEC, getResources().getDrawable(R.drawable.icon_outbox));
        Intent outboxIntent = new Intent(this, OutboxActivity.class);
        outboxSpec.setContent(outboxIntent);
        TabSpec profileSpec = tabHost.newTabSpec(PROFILE_SPEC);
        profileSpec.setIndicator(PROFILE_SPEC, getResources().getDrawable(R.drawable.icon_profile));
        Intent profileIntent = new Intent(this, ProfileActivity.class);
        profileSpec.setContent(profileIntent);
        tabHost.addTab(inboxSpec);
        tabHost.addTab(outboxSpec);
        tabHost.addTab(profileSpec);

then the problem has been solved. no need to extend as tabactivity