SingleFragmentActivity.java

package ru.orlovph.criminalintent2020;

import android.os.Bundle;

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

public abstract class SingleFragmentActivity extends AppCompatActivity {

    // abstract method used to instantiate the fragment
    protected abstract Fragment createFragment();

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

        FragmentManager fragmentManager = getSupportFragmentManager();
        Fragment fragment = fragmentManager.findFragmentById(R.id.fragment_container);

        if (fragment == null){
            fragment = createFragment();
            fragmentManager.beginTransaction()
                    .add(R.id.fragment_container, fragment)
                    .commit();
        }
    }
}