getActionBar().setDisplayHomeAsUpEnabled(true); throws NullPointerException on new activity creation (Google - Basic Tutorial)

Question

I am following this tutorial and getting a NullPointerException at the onCreate method of the DisplayMessageActivity at this block of code:

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
// Show the Up button in the action bar.
getActionBar().setDisplayHomeAsUpEnabled(true); //Exception here
}

I am running the app at an emulator following the NexusOne AVD template.
When I click the send button with some text typed in I get the exception.

Here is the log:

09-03 23:02:07.586: E/AndroidRuntime(7095): FATAL EXCEPTION: main
09-03 23:02:07.586: E/AndroidRuntime(7095): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.mypackage.myfirstapp/com.mypackage.myfirstapp.DisplayMessageActivity}: java.lang.NullPointerException
09-03 23:02:07.586: E/AndroidRuntime(7095):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
09-03 23:02:07.586: E/AndroidRuntime(7095):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
09-03 23:02:07.586: E/AndroidRuntime(7095):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
09-03 23:02:07.586: E/AndroidRuntime(7095):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
09-03 23:02:07.586: E/AndroidRuntime(7095):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-03 23:02:07.586: E/AndroidRuntime(7095):     at android.os.Looper.loop(Looper.java:137)
09-03 23:02:07.586: E/AndroidRuntime(7095):     at android.app.ActivityThread.main(ActivityThread.java:5041)
09-03 23:02:07.586: E/AndroidRuntime(7095):     at java.lang.reflect.Method.invokeNative(Native Method)
09-03 23:02:07.586: E/AndroidRuntime(7095):     at java.lang.reflect.Method.invoke(Method.java:511)
09-03 23:02:07.586: E/AndroidRuntime(7095):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
09-03 23:02:07.586: E/AndroidRuntime(7095):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
09-03 23:02:07.586: E/AndroidRuntime(7095):     at dalvik.system.NativeStart.main(Native Method)
09-03 23:02:07.586: E/AndroidRuntime(7095): Caused by: java.lang.NullPointerException
09-03 23:02:07.586: E/AndroidRuntime(7095):     at com.mypackage.myfirstapp.DisplayMessageActivity.onCreate(DisplayMessageActivity.java:40)
09-03 23:02:07.586: E/AndroidRuntime(7095):     at android.app.Activity.performCreate(Activity.java:5104)
09-03 23:02:07.586: E/AndroidRuntime(7095):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
09-03 23:02:07.586: E/AndroidRuntime(7095):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
09-03 23:02:07.586: E/AndroidRuntime(7095):     ... 11 more

By commenting this block the app runs successfuly.

Why is a NullPointerException thrown?

Answer 1

You are probably using a Theme that doesn't support ActionBar. Hence getActionBar() method throws NullPointerException.

Trying using this theme:

android:theme="@android:style/Theme.Holo.Light"

Answer 2

Try changing getActionBar() to getSupportActionBar() or ((ActionBarActivity)getActivity()).getSupportActionBar().

Answer 3

I had the same problem.

In the activity of the manifest i had declared

android:theme="@android:style/Theme.Black.NoTitleBar

which caused the error. After removing this line, my actionbar worked fine.

Answer 4

It shouldn't be necessary but there are some behavior inconsistencies between API versions even after API level 14.

Behavior:

Back "<" image is displayed but it doesn't work when pressed. As a good practice I use to implement the switch case in order to handle back event.

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
    // Respond to the action bar's Up/Home button
    case android.R.id.home:
        NavUtils.navigateUpFromSameTask(this);
        return true;
    }
    return super.onOptionsItemSelected(item);
}

It worked for me.

Answer 5

I encountered this error when I was trying out the navigation drawer sample app available at the android developer site. It turns out the MainActivity did not extend the ActionBarActivity and thus getActionBar() returns null causing the exception.

Answer 6

None of the other answers worked for me, really. I just commented out that whole if and it worked. From the method name (and documentation), you don't need that functionality anyway (unless you want it), so no harm done.

Relevant documentation excerpt:

Set whether home should be displayed as an "up" affordance. Set this to true if selecting "home" returns up by a single level in your UI rather than back to the top level or front page.

To set several display options at once, see the setDisplayOptions methods.

Parameters

showHomeAsUp true to show the user that selecting home will return one level up rather than to the top level of the app.

Answer 7

You must have set some theme with your Activity which is not compatible with the Action bar.

So just check the theme you are using with in your manifest file and remove it

or if you have defined custom theme go to your res->values->style.xml and make WindowActionBar to true.