Cara Membuat AlertDialog Android Studio

<?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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="20sp"
        android:layout_centerHorizontal="true"
        android:text="@string/contoh_alertdialog"
        android:textSize="25sp"
        android:textStyle="bold"/>

    <Button
        android:id="@+id/btnAlert"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/textView"
        android:layout_marginTop="20sp"
        android:layout_centerHorizontal="true"
        android:text="@string/munculin_alertdialog"/>

</RelativeLayout>

package com.example.alertdialog;

import androidx.appcompat.app.AlertDialog;
import androidx.appcompat.app.AppCompatActivity;

import android.content.DialogInterface;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    AlertDialog.Builder builder;

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

        builder = new AlertDialog.Builder(this);
        builder.setTitle("Ini Judul AlertDialog");
        builder.setMessage("Apakah Kamu Mengerti?");
        builder.setCancelable(false);
        builder.setPositiveButton("Tentu", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                Toast.makeText(getApplicationContext(), "Tentu", Toast.LENGTH_SHORT).show();
            }
        });
        builder.setNegativeButton("Belum", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                Toast.makeText(getApplicationContext(), "Belum", Toast.LENGTH_SHORT).show();
            }
        });

        Button btnAlert = (Button)findViewById(R.id.btnAlert);
        btnAlert.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                builder.show();
            }
        });
    }
}