Erro: Required 'android.app.Fragment'

Programação Java
#1

Olá programadores,

Sou iniciante e gostaria se possível na ajuda para resolver esse erro de compilação ao instanciar um FragmentTransation, desde já agradeço a ajuda:

package com.example.backstack;
import androidx.appcompat.app.AppCompatActivity;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.app.Fragment;

public class MainActivity extends AppCompatActivity {

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

    Fragment_1 f1 = new Fragment_1();
    Fragment_2 f2 = new Fragment_2();
    Fragment_3 f3 = new Fragment_3();

    FragmentManager fm = getFragmentManager();


    FragmentTransaction ft = fm.beginTransaction();

    ft.add(R.id.lay_top,  f1, "f1");
    ft.add(R.id.lay_middle, f2, "f2");
    ft.add(R.id.lay_bottom, f3, "f3");
    ft.commit();


}

}

Esse é o código de uma das fragments:

<?xml version="1.0" encoding="utf-8"?> 
<TextView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_margin="5dp"
    android:background="@android:color/holo_green_light"
    android:text="Fragment 1"
    android:gravity="center"/>

E aqui o da activityMain:

<?xml version="1.0" encoding="utf-8"?> 
<FrameLayout
    android:id="@+id/lay_top"
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:layout_weight="1"/>

<FrameLayout
    android:id="@+id/lay_middle"
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:layout_weight="1"/>

<FrameLayout
    android:id="@+id/lay_bottom"
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:layout_weight="1"/>