Activity para Fragment e Fragment para Activity no Android Studio

Criar botão para abrir na tela de fragment android studio

layout.activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    android:orientation="vertical"
    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"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:id="@+id/container"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/texto1"
        android:gravity="center"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Activity 1!"
        />

    <Button
        android:id="@+id/btn1"
        android:gravity="center"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Btn 1"
        />



</LinearLayout>

MainActivity.kt

package com.app.exemploclickfragmentsemjava;

import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentTransaction;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

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

        TextView texto1 = findViewById(R.id.texto1);

        // Botao Abrir Tela Fragment
        Button btn1 = findViewById(R.id.btn1);
        btn1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                btn1.setVisibility(View.GONE);
                texto1.setVisibility(View.GONE);

                Fragment fragment = new OneFragment();
                FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
                fragmentTransaction.replace(R.id.container, fragment).commit();
            }
        });




    }
}

FragmentOne

package com.app.exemploclickfragmentsemjava;

import android.content.Intent;
import android.os.Bundle;

import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentTransaction;

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.Toast;


public class OneFragment extends Fragment {
    Button btn_activity1;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Aumente o layout deste fragmento

      // Toast.makeText(getActivity(), "fragmet 1: ", Toast.LENGTH_SHORT).show();

       // return inflater.inflate(R.layout.fragment_one, container, false);
        View view= inflater.inflate(R.layout.fragment_one, container, false);

        btn_activity1 = view.findViewById(R.id.btn_activity1);
        btn_activity1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
/*
                Fragment secondFragment = new TwoFragment();
                FragmentTransaction fm = getActivity().getSupportFragmentManager().beginTransaction();
                fm.replace(R.id.container, secondFragment).commit();
 */
                Intent intent = new Intent(getActivity(), MainActivity2.class);
                startActivity(intent);

            }
        });


        return view;






    }
}