Why do I get a null pointer exception from TabWidget?

Question

I'm writing an android program in which I have an activity that uses tabs.

The Activity

public class UnitActivity extends TabActivity {
  @Override
  public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);

  TabHost tabHost = getTabHost();
  TabSpec spec;
  Resources res = getResources();

  LayoutInflater.from(this).inflate(R.layout.unit_view, tabHost.getTabContentView(), true);

  spec = tabHost.newTabSpec("controls");
  spec.setIndicator("Control", res.getDrawable(R.drawable.ic_tab_equalizer));
  spec.setContent(R.id.txtview);
  tabHost.addTab(spec);
}

}

The XML referenced by R.layout.unit_view

<?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:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:padding="5dp">
  <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="5dp">
      <TextView android:id="@+id/txtview"
          android:layout_width="fill_parent"
          android:layout_height="fill_parent"
          android:gravity="bottom"
          android:text="nullpointer this!" />
    </FrameLayout>
  </LinearLayout>
</TabHost>

As far as I can see I'm doing the same thing I see in the tabs1 api sample from the android sdk. I've tried "getLayoutInflator()" instead of "LayoutInflator.from(this)" with the same result.

If I replace the LayoutInflater line with "setContentView(R.layout.unit_view)" my program doesn't crash with a null pointer exception but my content is completely blank and empty. I get the tab and that's it.

I've checked to make sure R.layout.unit_view and tabHost are not null when it runs the LayoutInflater line and they seem to be fine. They're defenitely not null. I've also checked to make sure LayoutInflater.from(this) returns a valid layout inflater object and it does.

The logcat indicating the error says

 E/AndroidRuntime(  541): java.lang.NullPointerException
 E/AndroidRuntime(  541):  at android.widget.TabWidget.dispatchDraw(TabWidget.java:206)
 E/AndroidRuntime(  541):  at android.view.ViewGroup.drawChild(ViewGroup.java:1529)
 E/AndroidRuntime(  541):  at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1258)
 E/AndroidRuntime(  541):  at android.view.ViewGroup.drawChild(ViewGroup.java:1529)
 E/AndroidRuntime(  541):  at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1258)
 E/AndroidRuntime(  541):  at android.view.ViewGroup.drawChild(ViewGroup.java:1529)
 E/AndroidRuntime(  541):  at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1258)
 E/AndroidRuntime(  541):  at android.view.ViewGroup.drawChild(ViewGroup.java:1529)
 E/AndroidRuntime(  541):  at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1258)
 E/AndroidRuntime(  541):  at android.view.ViewGroup.drawChild(ViewGroup.java:1529)
 E/AndroidRuntime(  541):  at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1258)
 E/AndroidRuntime(  541):  at android.view.ViewGroup.drawChild(ViewGroup.java:1529)
 E/AndroidRuntime(  541):  at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1258)
 E/AndroidRuntime(  541):  at android.view.View.draw(View.java:6538)
 E/AndroidRuntime(  541):  at android.widget.FrameLayout.draw(FrameLayout.java:352)
 E/AndroidRuntime(  541):  at android.view.ViewGroup.drawChild(ViewGroup.java:1531)
 E/AndroidRuntime(  541):  at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1258)
 E/AndroidRuntime(  541):  at android.view.ViewGroup.drawChild(ViewGroup.java:1529)
 E/AndroidRuntime(  541):  at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1258)
 E/AndroidRuntime(  541):  at android.view.View.draw(View.java:6538)
 E/AndroidRuntime(  541):  at android.widget.FrameLayout.draw(FrameLayout.java:352)
 E/AndroidRuntime(  541):  at com.android.internal.policy.impl.PhoneWindow$DecorView.draw(PhoneWindow.java:1830)
 E/AndroidRuntime(  541):  at android.view.ViewRoot.draw(ViewRoot.java:1349)
 E/AndroidRuntime(  541):  at android.view.ViewRoot.performTraversals(ViewRoot.java:1114)
 E/AndroidRuntime(  541):  at android.view.ViewRoot.handleMessage(ViewRoot.java:1633)
 E/AndroidRuntime(  541):  at android.os.Handler.dispatchMessage(Handler.java:99)
 E/AndroidRuntime(  541):  at android.os.Looper.loop(Looper.java:123)
 E/AndroidRuntime(  541):  at android.app.ActivityThread.main(ActivityThread.java:4363)
 E/AndroidRuntime(  541):  at java.lang.reflect.Method.invokeNative(Native Method)
 E/AndroidRuntime(  541):  at java.lang.reflect.Method.invoke(Method.java:521)
 E/AndroidRuntime(  541):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
 E/AndroidRuntime(  541):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
 E/AndroidRuntime(  541):  at dalvik.system.NativeStart.main(Native Method)
 I/Process (   61): Sending signal. PID: 541 SIG: 3
 I/dalvikvm(  541): threadid=7: reacting to signal 3
 I/dalvikvm(  541): Wrote stack trace to '/data/anr/traces.txt'

Anybody have any idea how I can get this content into a tab without crashing my application? My actual program is more complex and has more than one tab but I simplified it down to this in an attempt to find out why it's crashing but it still crashes and I don't know why.

If I don't use LayoutInflator my program doesn't crash but I don't get any content either, just tabs.

Answer 1

For those of you who are using a TabHost not inside a TabActivity, then be sure to call TabHost.setup(). Here's the documentation for it: http://developer.android.com/reference/android/widget/TabHost.html#setup() Not doing so will result in a NullPointerException when you try to add tabs to your tabhost.

The tutorial fails to mention that, so I thought I'd bring it to your attention to save you guys the time I spent looking for it.