Shwing Error : Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)

Question

I want to implement the Android ActionBar back button in my StreamActivity but I'm getting this error :

    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.schoolteacher/com.example.schoolteacher.StreamActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference

I don't know why this is not working, despite I followed all instructions from the documentation! plus, I did exactly this in other projects and it works.

Here is my code :

StreamActvivty.java

public class StreamActivity extends AppCompatActivity {

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

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

        setSupportActionBar(toolbar);
        ActionBar actionBar= getSupportActionBar();
        actionBar.setDisplayHomeAsUpEnabled(true); 

    }

}

styles.xml

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

manifest

<activity android:name=".StreamActivity"
            android:label="@string/stream"
            android:parentActivityName=".ClassActivity">
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value=".ClassActivity" />
        </activity>

Answer 1

Try this

Add this in onCreate

 ActionBar actionBar = this.getSupportActionBar();
 if (actionBar != null) {
    actionBar.setDisplayHomeAsUpEnabled(true);
 }

Add this outside of the onCreate, this is used to navigate back to the parent activity

@Override
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
   if (id == android.R.id.home) {
    NavUtils.navigateUpFromSameTask(this);
}
    return super.onOptionsItemSelected(item);
}

And also set Parent activity

  <activity
        android:name=".activity.CreateAccountActivity"
        android:parentActivityName=".activity.LoginActivity" />