Android: NullPointerException when using ActionBar

Question

I am getting an error saying that

java.lang.RuntimeException: Unable to start activity ComponentInfo{...}: java.lang.IllegalStateException: You need to use a Theme.AppCompat theme (or descendant) with this activity...
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference at com.myapp.contactList.MainPage.onCreate(MainPage.java:41)

And I looked up several stack overflow questions similar to this one and tried all the solutions but still having this issue. Below is my code:

MainPage.java

public class MainPage extends AppCompatActivity{
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main_page);

    android.support.v7.app.ActionBar ab = getSupportActionBar();
    assert ab != null;
    ab.setDisplayHomeAsUpEnabled(true);  // I'M GETTING THE ERROR HERE AT THIS LINE
    ab.setBackgroundDrawable(new ColorDrawable(Color.parseColor("#51c2f2")));
    ab.setDisplayUseLogoEnabled(true);

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.myapp.contactList">

 <application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:supportsRtl="true"
    android:theme="@style/AppTheme">
    <activity
        android:name=".MainPage"
        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>

styles.xml

<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
    <!-- Customize your theme here. -->
    <item name="colorPrimary">@color/colorPrimary</item>
    <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
    <item name="colorAccent">@color/colorAccent</item>
</style>

activity_main_page.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:app="http://schemas.android.com/apk/res-auto"
   xmlns:tools="http://schemas.android.com/tools"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:fitsSystemWindows="true"
   tools:context="com.myapp.contactList.MainPage">

   <android.support.design.widget.AppBarLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:theme="@style/AppTheme.AppBarOverlay">
   </android.support.design.widget.AppBarLayout>
   <include layout="@layout/content_main_page" />
</android.support.design.widget.CoordinatorLayout>

Answer 1

Basically you have two errors in your code :

First, assert keyword is not supported by Android, when you use it in your code just nothing happen. You should check that this code :

if(ab  ==  null) {
   throw new IllegalStateException();
}

or you can also use

Assert.notNull(ab);

from some lib or framework.

You can also switch on assert but only for your device via adb:

adb shell setprop debug.assert 1

Second you should use Toolbar as support action bar. Update your MainActivity class

public class MainPage extends AppCompatActivity{
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main_page);
        android.support.v7.widget.Toolbar toolbar 
             = (android.support.v7.widget.Toolbar) findViewById(R.id.toolbar)

        setSupportActionBar(toolbar)
        android.support.v7.app.ActionBar ab = getSupportActionBar();
        ab.setDisplayHomeAsUpEnabled(true);  
        ...

And update layout xml as is shown in the code below:

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout 
    ....
   tools:context="com.myapp.contactList.MainPage">

   <android.support.design.widget.AppBarLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:theme="@style/AppTheme.AppBarOverlay">

      <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        app:popupTheme="@style/AppTheme.PopupOverlay"
        android:layout_height="wrap_content"
        android:background="?attr/colorPrimary"
        android:theme="?attr/actionBarTheme"
        android:minHeight="?attr/actionBarSize">

   </android.support.design.widget.AppBarLayout>
   <include layout="@layout/content_main_page" />
</android.support.design.widget.CoordinatorLayout>

You have to also set style "@style/AppTheme.NoActionBar" for your activity

You can define it in style.xml as is shown below:

<style name="AppTheme.NoActionBar">
    <item name="windowActionBar">false</item>
    <item name="windowNoTitle">true</item>
</style>

Answer 2

I think you need to use setSupportActionBar to set it first. something like this:

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
}

this layout:

    <android.support.design.widget.AppBarLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:theme="@style/AppTheme.AppBarOverlay">

    <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:background="?attr/colorPrimary"
        app:popupTheme="@style/AppTheme.PopupOverlay" />

   </android.support.design.widget.AppBarLayout>

and this style in manifest:

android:theme="@style/AppTheme.NoActionBar">

which removes actionBar:

<style name="AppTheme.NoActionBar">
    <item name="windowActionBar">false</item>
    <item name="windowNoTitle">true</item>
</style>

I guess you are trying to get something that is not a SupportActionBar, so this happens.