Primeiro projeto Android Native com Fragments (parece não estar funcionando, não troca para o segundo fragmento)

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.andersonvieira.exemplo_fragments.MainActivity">

    <Button
        android:text="@string/fragmento_1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_alignParentStart="true"
        android:layout_marginStart="33dp"
        android:id="@+id/btnFragmento1" />

    <Button
        android:text="@string/fragmento_2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="17dp"
        android:id="@+id/btnFragmento2"
        android:layout_alignParentTop="true"
        android:layout_toEndOf="@+id/btnFragmento1" />

    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_marginTop="17dp"
        android:layout_below="@+id/btnFragmento1"
        android:layout_centerHorizontal="true"
        android:id="@+id/frame_container">

    </FrameLayout>
</RelativeLayout>

fragment_novo.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.andersonvieira.exemplo_fragments.NovoFragment">

    <!-- TODO: Update blank fragment layout -->

    <TextView
        android:id="@+id/textView"
        android:layout_width="256dp"
        android:layout_height="wrap_content"
        android:layout_alignParentStart="true"
        android:layout_alignParentTop="true"
        android:layout_marginStart="67dp"
        android:layout_marginTop="81dp"
        android:text="@string/este_o_primeiro_fragmento"
        android:textAppearance="@style/TextAppearance.AppCompat.Body2"
        android:textSize="18sp" />

    <Button
        android:id="@+id/Button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/textView"
        android:layout_alignStart="@+id/textView"
        android:layout_marginStart="78dp"
        android:layout_marginTop="68dp"
        android:text="@string/ok"
        tools:text="OK" />
</RelativeLayout>

fragment_segundo.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.andersonvieira.exemplo_fragments.SegundoFragment"
    android:background="@android:color/holo_green_light">

    <!-- TODO: Update blank fragment layout -->

    <TextView
        android:id="@+id/textView2"
        android:layout_width="265dp"
        android:layout_height="wrap_content"
        android:layout_alignParentStart="true"
        android:layout_alignParentTop="true"
        android:layout_marginStart="58dp"
        android:layout_marginTop="91dp"
        android:text="@string/estou_no_segundo_fragmento"
        android:textAppearance="@style/TextAppearance.AppCompat.Body2"
        android:textSize="18sp" />

    <Button
        android:id="@+id/Button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/textView2"
        android:layout_alignStart="@+id/textView2"
        android:layout_marginStart="88dp"
        android:layout_marginTop="40dp"
        android:text="@string/button"
        tools:text="OK" />
</RelativeLayout>

MainActivity.java

package com.example.andersonvieira.exemplo_fragments;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

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

        if (savedInstanceState == null) {
            getSupportFragmentManager()
                    .beginTransaction()
                    .add(R.id.frame_container, new NovoFragment())
                    .commit();
        }

        Button f1 = (Button) findViewById(R.id.btnFragmento1);
        Button f2 = (Button) findViewById(R.id.btnFragmento2);
        f1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                getSupportFragmentManager()
                        .beginTransaction()
                        .replace(R.id.frame_container, new NovoFragment())
                        .commit();
            }
        });
        f2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                getSupportFragmentManager()
                        .beginTransaction()
                        .replace(R.id.frame_container, new SegundoFragment())
                        .commit();
            }
        });
    }
}

NovoFragmento.java

package com.example.andersonvieira.exemplo_fragments;


import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.Toast;

public class NovoFragment extends Fragment {
    public NovoFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater,
                             ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_novo, container, false);
        Button b = (Button) view.findViewById(R.id.Button);
        b.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Toast.makeText(getActivity().getApplicationContext(),
                        "Estou no PRIMEIRO fragmento!!!",
                        Toast.LENGTH_LONG).show();
            }
        });

        return view;
    }
}

SegundoFragment.java

package com.example.andersonvieira.exemplo_fragments;


import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.Toast;

public class SegundoFragment extends Fragment {
    public SegundoFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater,
                             ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_novo, container, false);
        Button b = (Button) view.findViewById(R.id.Button);
        b.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Toast.makeText(getActivity().getApplicationContext(),
                        "Estou no SEGUNDO fragmento!!!",
                        Toast.LENGTH_LONG).show();
            }
        });

        return view;
    }
}