Question

I keep getting this error on running but no compile errors? here is the logcat and code for main.

05-25 12:29:25.543: W/dalvikvm(384): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
05-25 12:29:25.582: E/AndroidRuntime(384): FATAL EXCEPTION: main
05-25 12:29:25.582: E/AndroidRuntime(384): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.tablayout/com.example.tablayout.TablayoutActivity}: java.lang.NullPointerException
05-25 12:29:25.582: E/AndroidRuntime(384):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2585)
05-25 12:29:25.582: E/AndroidRuntime(384):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
05-25 12:29:25.582: E/AndroidRuntime(384):  at android.app.ActivityThread.access$2300(ActivityThread.java:125)
05-25 12:29:25.582: E/AndroidRuntime(384):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
05-25 12:29:25.582: E/AndroidRuntime(384):  at android.os.Handler.dispatchMessage(Handler.java:99)
05-25 12:29:25.582: E/AndroidRuntime(384):  at android.os.Looper.loop(Looper.java:123)
05-25 12:29:25.582: E/AndroidRuntime(384):  at android.app.ActivityThread.main(ActivityThread.java:4627)
05-25 12:29:25.582: E/AndroidRuntime(384):  at java.lang.reflect.Method.invokeNative(Native Method)
05-25 12:29:25.582: E/AndroidRuntime(384):  at java.lang.reflect.Method.invoke(Method.java:521)
05-25 12:29:25.582: E/AndroidRuntime(384):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
05-25 12:29:25.582: E/AndroidRuntime(384):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
05-25 12:29:25.582: E/AndroidRuntime(384):  at dalvik.system.NativeStart.main(Native Method)
05-25 12:29:25.582: E/AndroidRuntime(384): Caused by: java.lang.NullPointerException
05-25 12:29:25.582: E/AndroidRuntime(384):  at android.app.Activity.setContentView(Activity.java:1647)
05-25 12:29:25.582: E/AndroidRuntime(384):  at android.app.TabActivity.ensureTabHost(TabActivity.java:114)
05-25 12:29:25.582: E/AndroidRuntime(384):  at android.app.TabActivity.getTabHost(TabActivity.java:136)
05-25 12:29:25.582: E/AndroidRuntime(384):  at com.example.tablayout.TablayoutActivity.<init>(TablayoutActivity.java:15)
05-25 12:29:25.582: E/AndroidRuntime(384):  at java.lang.Class.newInstanceImpl(Native Method)
05-25 12:29:25.582: E/AndroidRuntime(384):  at java.lang.Class.newInstance(Class.java:1429)
05-25 12:29:25.582: E/AndroidRuntime(384):  at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
05-25 12:29:25.582: E/AndroidRuntime(384):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2577)
05-25 12:29:25.582: E/AndroidRuntime(384):  ... 11 more






    <?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="horizontal" 
      android:layout_width="fill_parent" 
      android:layout_height="fill_parent"
      android:baselineAligned="false">
    <FrameLayout android:layout_width="0dip" 
        android:layout_height="fill_parent" 
        android:layout_weight="0.2">
    <TabWidget android:id="@android:id/tabs" 
            android:layout_width="fill_parent" 
            android:layout_height="wrap_content"
            android:visibility="invisible"/>
        <LinearLayout android:layout_width="fill_parent"  
            android:layout_height="fill_parent"
            android:orientation="vertical">
            <Button android:layout_height="0dip" 
                android:layout_width="fill_parent" 
                android:layout_weight="1.0"
            android:background="@drawable/icon_photos_tab"               
                android:id="@+id/artist_id" 
                android:onClick="tabHandler"/>
            <Button android:layout_height="0dip" 
                android:layout_width="fill_parent" 
                android:layout_weight="1.0"
            android:background="@drawable/icon_videos_tab"  
                android:id="@+id/album_id" 
                android:onClick="tabHandler"/>
            <Button android:layout_height="0dip" 
                android:layout_width="fill_parent" 
                android:layout_weight="1.0"
            android:background="@drawable/icon_songs_tab"   
                android:id="@+id/song_id" 
                android:onClick="tabHandler"/>
    </LinearLayout> 
    </FrameLayout>       
    <FrameLayout 
     android:id="@android:id/tabcontent" 
     android:layout_width="0dip" 
     android:layout_height="fill_parent" 
     android:layout_weight="0.8"/>
    </LinearLayout>
    </TabHost>

here is the main.java file, this way you can see how it was a global variable and where i removed it from. Also added the buttons globally instead of locally to that one spot.

    import android.app.TabActivity;
    import android.content.Intent;
   import android.os.Bundle;
   import android.view.View;
   import android.widget.Button;
   import android.widget.TabHost;
   import android.widget.TabHost.TabSpec;

   public class TablayoutActivity extends TabActivity {
    /** Called when the activity is first created. */

Button artistButton, songButton, videosButton;
TabHost tabHost = getTabHost();

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

    //TabHost tabHost = getTabHost();

    // Tab for Photos
    TabSpec photospec = tabHost.newTabSpec("Photos");
    // setting Title and Icon for the Tab
    photospec.setIndicator("Photos",   getResources().getDrawable(R.drawable.icon_photos_tab));
    Intent photosIntent = new Intent(this, PhotosActivity.class);
    photospec.setContent(photosIntent);

    // Tab for Songs
    TabSpec songspec = tabHost.newTabSpec("Songs");
    songspec.setIndicator("Songs", getResources().getDrawable(R.drawable.icon_songs_tab));
    Intent songsIntent = new Intent(this, SongsActivity.class);
    songspec.setContent(songsIntent);

    // Tab for Videos
    TabSpec videospec = tabHost.newTabSpec("Videos");
    videospec.setIndicator("Videos",         getResources().getDrawable(R.drawable.icon_videos_tab));
    Intent videosIntent = new Intent(this, VideosActivity.class);
    videospec.setContent(videosIntent);

    // Adding all TabSpec to TabHost
    tabHost.addTab(photospec); // Adding photos tab
    tabHost.addTab(songspec); // Adding songs tab
    tabHost.addTab(videospec); // Adding videos tab




   }

    public void tabHandler(View target){
    artistButton.setSelected(false);
    videosButton.setSelected(false);
    songButton.setSelected(false);
    if(target.getId() == R.id.artist_id){
        tabHost.setCurrentTab(0);
        artistButton.setSelected(true);
    } else if(target.getId() == R.id.album_id){
        tabHost.setCurrentTab(1);
        videosButton.setSelected(true);
    } else if(target.getId() == R.id.song_id){
        tabHost.setCurrentTab(2);
        songButton.setSelected(true);
    }
   };

    }

Answer 1

You are probably trying to call getTabHost() somewhere in your constructor or in setting a field on your class. You have to call it from within onCreate() since getTabHost() down deep tries to get the current window, which isn’t created yet at constructor time.

Any time after onCreate() has been called will be fine.