NullPointerException: setDisplayHomeAsUpEnabled(boolean) on a null object reference

Question

I have the standard setup with an AppCompatActivity. To enable the 'up-button' to the parent activity I need to call getSupportActionBar().setDisplayHomeAsUpEnabled(true);

There are many similiar threads. unfortunately I was not able to find my mistake because I am using AppCompatActivity instead Activity and getSupportActionBar() instead of getActionBar()

activity:

public class GalleryActivity extends AppCompatActivity {

   (...)

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_gallery_view);

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        setSupportActionBar(toolbar);

        (...)
    }
}

activity_gallery_view.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">

    <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_gallery"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:popupTheme="@style/AppTheme.PopupOverlay" />

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

    <include layout="@layout/gallery_content" />

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

styles.xml

<resources>

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

    <style name="AppTheme.AppBarOverlay" parent="ThemeOverlay.AppCompat.Dark.ActionBar" />

    <style name="AppTheme.PopupOverlay" parent="ThemeOverlay.AppCompat.Light" />

</resources>

stacktrace:

 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.stack.overflow/com.stack.overflow.reviewmaschine.gallery.GalleryActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
                                                                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
                                                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
                                                                     at android.app.ActivityThread.-wrap11(ActivityThread.java)
                                                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
                                                                     at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                     at android.os.Looper.loop(Looper.java:148)
                                                                     at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                                     at java.lang.reflect.Method.invoke(Native Method)
                                                                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
                                                                  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.stack.overflow.reviewmaschine.gallery.GalleryActivity.onCreate(GalleryActivity.java:56)
                                                                     at android.app.Activity.performCreate(Activity.java:6251)
                                                                     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
                                                                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
                                                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                                                                     at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                                                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
                                                                     at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                     at android.os.Looper.loop(Looper.java:148) 
                                                                     at android.app.ActivityThread.main(ActivityThread.java:5417) 
                                                                     at java.lang.reflect.Method.invoke(Native Method) 
                                                                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                                                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

 

Answer 1

You have given different id for Toolbar in xml and in java file.Make them same and most probably your error should be gone. Here is your error

android:id="@+id/toolbar_gallery"

And in Activity java file you have done

Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);

Solve this issue