How to add overflow menu to Toolbar?

Define a Menu for your Toolbar in the res/menu resource folder, for example:

toolbar_menu.xml

<?xml version="1.0" encoding="utf-8"?>
<menu
    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"
    tools:context=".activity.MainActivity">

    <item
        android:id="@+id/action_sign_out"
        android:title="@string/toolbar_sign_out"
        app:showAsAction="never"/>
</menu>

Setting app:showAsAction="never" ensures that this MenuItem will not be shown in the Toolbar, but placed in the overflow menu instead.

The theme of your Activity should be (or derive from) one of the NoActionBar themes (Theme.AppCompat.NoActionBar for example, or Theme.MaterialComponents.NoActionBar if you're using Material Components).

In your Activity, set up your Toolbar:

Toolbar myToolbar = (Toolbar) findViewById(R.id.my_toolbar);
setSupportActionBar(myToolbar);

And override onCreateOptionsMenu() to inflate your previously defined menu resource:

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.toolbar_menu, menu);
    return true;
}

You can override onOptionsItemSelected() to define the onClick behaviour of your MenuItem(s):

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
        case R.id.action_sign_out: {
            // do your sign-out stuff
            break;
        }
        // case blocks for other MenuItems (if any)
    }
    return true;
}

in manifest file declare

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

like this :

<activity
    android:name=".ActivityName"
    android:label="@string/label"
    android:theme="@style/AppTheme.NoActionBar" />

and add this to your style :

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

and call this in Activity onCreate() :

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

override this method in activity:

 @Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.product_list, menu);
    //U can find item set icon and stuff...
    MenuItem item= menu.findItem(R.id.action_search);

    return true;
}

and declare your menu like this for overflow menu:

  <?xml version="1.0" encoding="utf-8"?>
<menu >
    <group>
        <item
            android:id="@+id/sign_out"
            android:title="@string/sign_out" />
        <item
            android:id="@+id/about"
            android:title="@string/about" />
    </group>
</menu>

and for handle item selection call this

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    int id = item.getItemId();
    switch (id) { 
       case R.id.sign_out:
       //do stuff
           break;
           }  
        return super.onOptionsItemSelected(item);
    }

done :)