How to show back button in toolbar

import android.os.Bundle;

import android.support.v7.app.ActionBar;

import android.support.v7.app.AppCompatActivity;

import android.support.v7.widget.Toolbar;

import com.vivenns.mvptrainingss.R;

public class ChildActivity extends AppCompatActivity {

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_child);

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

        setSupportActionBar(toolbar);

        ActionBar actionBar = getSupportActionBar();

        actionBar.setDisplayHomeAsUpEnabled(true);

    }

}

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout android:layout_width="match_parent"

android:layout_height="match_parent"

xmlns:android="http://schemas.android.com/apk/res/android"

xmlns:app="http://schemas.android.com/apk/res-auto">

<android.support.v7.widget.Toolbar

    android:id="@+id/my_toolbar"

    android:layout_width="match_parent"

    android:layout_height="?attr/actionBarSize"

    android:background="?attr/colorPrimary"

    android:elevation="4dp"

    android:theme="@style/ThemeOverlay.AppCompat.ActionBar"

    app:popupTheme="@style/ThemeOverlay.AppCompat.Light"/>

</LinearLayout>

public class MainActivity extends AppCompatActivity {

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

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

        setSupportActionBar(myToolbar);

    }

    public void hideToolbar(View v) {

        startActivity(new Intent(this, ChildActivity.class));

    }

}

<?xml version="1.0" encoding="utf-8"?>

<RelativeLayout android:layout_width="match_parent"

    android:layout_height="match_parent"

    xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:app="http://schemas.android.com/apk/res-auto">

    <android.support.v7.widget.Toolbar

        android:id="@+id/my_toolbar"

        android:layout_width="match_parent"

        android:layout_height="?attr/actionBarSize"

        android:background="?attr/colorPrimary"

        android:elevation="4dp"

        android:theme="@style/ThemeOverlay.AppCompat.ActionBar"

        app:popupTheme="@style/ThemeOverlay.AppCompat.Light"/>

    <Button

        android:layout_below="@+id/my_toolbar"

        android:onClick="hideToolbar"

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        android:text="click me"/>

</RelativeLayout>