Do not request Window.FEATURE_SUPPORT_ACTION_BAR and set windowActionBar to false in your theme to use a Toolbar instead
<p>I'm trying to set up a toolbar with a burger menu, this is my code,and
the error is:</p>

<blockquote>
  <p>This Activity already has an action bar supplied by the window decor.
  Do not request <code>Window.FEATURE_SUPPORT_ACTION_BAR</code> and set
  windowActionBar to false in your theme to use a Toolbar instead.</p>
</blockquote>

<p>The problem is that I don't know where I am getting the error from, and
also the fact that I tried everything.</p>

<p><strong>MainActivity.java</strong></p>

<pre><code>    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    txResult = (TextView) findViewById(R.id.txResult);
    toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    mDrawer = (DrawerLayout) findViewById(R.id.drawer_layout);}

// Set a Toolbar to replace the ActionBar.


// Find our drawer view



private void setupDrawerContent(NavigationView navigationView) {
    navigationView.setNavigationItemSelectedListener(
            new NavigationView.OnNavigationItemSelectedListener() {
                @Override
                public boolean onNavigationItemSelected(MenuItem menuItem) {
                    selectDrawerItem(menuItem);
                    return true;
                }
            });
}

public void selectDrawerItem(MenuItem menuItem) {
    // Create a new fragment and specify the planet to show based on
    // position
    Fragment fragment = null;

    Class fragmentClass;
    switch(menuItem.getItemId()) {
        case R.id.nav_first_fragment:
            fragmentClass = MainActivity.class;
            break;
        case R.id.nav_second_fragment:
            fragmentClass = SecondActivity.class;
            break;
        case R.id.nav_third_fragment:
            fragmentClass = ThirdActivity.class;
            break;
        default:
            fragmentClass = MainActivity.class;
    }

    try {
        fragment = (Fragment) fragmentClass.newInstance();
    } catch (Exception e) {
        e.printStackTrace();
    }

    // Insert the fragment by replacing any existing fragment
    FragmentManager fragmentManager = getSupportFragmentManager();
    fragmentManager.beginTransaction().replace(R.id.flContent, fragment).commit();

    // Highlight the selected item, update the title, and close the drawer
    menuItem.setChecked(true);
    setTitle(menuItem.getTitle());
    mDrawer.closeDrawers();
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // The action bar home/up action should open or close the drawer.
    switch (item.getItemId()) {
        case android.R.id.home:
            mDrawer.openDrawer(GravityCompat.START);
            return true;
    }

    return super.onOptionsItemSelected(item);
}
</code></pre>

<p><strong>MainActivity.xml</strong></p>

<pre><code>&lt;!-- This DrawerLayout has two children at the root  --&gt;
 &lt;android.support.v4.widget.DrawerLayout
     xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:app="http://schemas.android.com/apk/res-auto"
     android:id="@+id/drawer_layout"
     android:layout_width="match_parent"
     android:layout_height="match_parent"&gt;

     &lt;!-- This LinearLayout represents the contents of the screen  --&gt;
     &lt;LinearLayout
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         android:orientation="vertical"&gt;

         &lt;include
             layout="@layout/toolbar"
             android:layout_width="match_parent"
             android:layout_height="wrap_content" /&gt;

         &lt;View
             android:layout_width="match_parent"
             android:layout_height="1dp"
             android:layout_marginTop="10dp"
             android:layout_marginBottom="10dp"
             android:background="#ddd"
            android:visibility="invisible" /&gt;

         &lt;View
             android:layout_width="match_parent"
             android:layout_height="1dp"
             android:layout_marginTop="10dp"
             android:layout_marginBottom="10dp"
             android:background="#ddd"
             android:visibility="invisible" /&gt;

         &lt;TextView
             android:id="@+id/txResult"
             android:layout_width="match_parent"
             android:layout_height="wrap_content"
             android:text="Rezultat:"
             android:textSize="20sp"
             android:visibility="invisible" /&gt;

         &lt;Button
             android:layout_width="207dp"
             android:layout_height="wrap_content"
             android:text="Scan"
             android:radius="3dp"
             android:shape="oval"
             android:width="5px"
             android:color="#1a1a1a"
             android:onClick="callZXing"
             android:id="@+id/Button"
             android:layout_centerHorizontal="true"
             android:background="#F39C12"
             android:textColor="#ffffff"
             android:layout_above="@+id/button2"
             android:singleLine="false"
             android:soundEffectsEnabled="false" /&gt;

         &lt;Button
             style="@style/CaptureTheme"
             android:layout_width="112dp"
             android:layout_height="68dp"
             android:text="No"
             android:id="@+id/button1"
             android:layout_weight="0.10"
             android:layout_gravity="center_horizontal"
             android:onClick="sendFirst"
             android:visibility="visible"
             android:layout_alignParentBottom="true"
             android:layout_centerHorizontal="true"
             android:layout_marginBottom="60dp" /&gt;

         &lt;Button
             style="@style/CaptureTheme"
             android:layout_width="112dp"
             android:layout_height="20dp"
             android:text="Yes"
             android:id="@+id/button2"
             android:layout_gravity="center_horizontal"
             android:onClick="sendSecond"
             android:visibility="visible"
             android:layout_alignParentBottom="true"
             android:layout_centerHorizontal="true"
             android:layout_marginBottom="165dp"
             android:layout_weight="0.01" /&gt;


         &lt;!-- The ActionBar displayed at the top --&gt;

         &lt;!-- The main content view where fragments are loaded --&gt;
         &lt;FrameLayout
             android:id="@+id/flContent"
             android:layout_width="match_parent"
             android:layout_height="match_parent" /&gt;
     &lt;/LinearLayout&gt;

     &lt;!-- The navigation drawer that comes from the left --&gt;
     &lt;!-- Note that `android:layout_gravity` needs to be set to 'start' --&gt;
     &lt;android.support.design.widget.NavigationView
         android:id="@+id/nvView"
         android:layout_width="wrap_content"
         android:layout_height="match_parent"
         android:layout_gravity="start"
         android:background="@android:color/white"
         app:menu="@menu/drawer_view" /&gt; &lt;/android.support.v4.widget.DrawerLayout&gt;
</code></pre>

<p><strong>AndroidManifest.xml</strong></p>

<pre><code>&lt;Activity
    android:name="com.google.zxing.client.android.CaptureActivity"
    android:configChanges="orientation|keyboardHidden"
    android:label="ZXing ScanBar"
    android:theme="@style/Theme.AppCompat.Light.NoActionBar"
    android:windowSoftInputMode="stateAlwaysHidden"&gt;
</code></pre>

<p><strong>Styles.xml</strong></p>

<pre><code>&lt;style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar"&gt;
    &lt;item name="windowActionBar"&gt;false&lt;/item&gt;
    &lt;item name="colorPrimary"&gt;#673AB7&lt;/item&gt;
    &lt;item name="colorPrimaryDark"&gt;#512DA8&lt;/item&gt;
    &lt;item name="colorAccent"&gt;#FF4081&lt;/item&gt;
    &lt;item name="windowNoTitle"&gt;true&lt;/item&gt;

&lt;/style&gt;


&lt;!-- Application theme. --&gt;
&lt;style name="AppBaseTheme" parent="Theme.AppCompat.Light.NoActionBar"&gt;
    &lt;item name="windowActionBar"&gt;false&lt;/item&gt;
    &lt;item name="colorPrimary"&gt;#673AB7&lt;/item&gt;
    &lt;item name="windowNoTitle"&gt;true&lt;/item&gt;         
    &lt;item name="colorPrimaryDark"&gt;#512DA8&lt;/item&gt;
    &lt;item name="colorAccent"&gt;#FF4081&lt;/item&gt;
    &lt;!-- All customizations that are NOT specific to a particular API-level can go here. --&gt;    
&lt;/style&gt;
</code></pre>

<p><strong>Toolbar.xml</strong></p>

<pre><code>&lt;android.support.v7.widget.Toolbar
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/toolbar"
    android:layout_height="wrap_content"
    android:layout_width="match_parent"
    android:fitsSystemWindows="true"
    android:minHeight="?attr/actionBarSize"
    app:theme="@style/Theme.AppCompat.Light.NoActionBar"
    android:background="?attr/colorPrimaryDark"&gt;  
&lt;/android.support.v7.widget.Toolbar&gt;
</code></pre>