child_guard/app/src/main/java/com/example/childguard/MainActivity.java

package com.example.childguard;

import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.core.app.NotificationCompat;
import androidx.core.app.NotificationManagerCompat;

import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.content.Context;
import android.content.pm.PackageManager;
import android.os.Bundle;
import android.os.Vibrator;

import com.google.android.material.bottomnavigation.BottomNavigationView;

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //通知のやつ↓
        int importance = NotificationManager.IMPORTANCE_DEFAULT;

        NotificationChannel channel = new NotificationChannel("CHANNEL_ID", "サンプルアプリ", importance);
        channel.setDescription("説明・説明　ここに通知の説明を書くことができる");

        NotificationManager notificationManager = getSystemService(NotificationManager.class);
        notificationManager.createNotificationChannel(channel);
        //通知のやつ↑


        BottomNavigationView bottomNavigationView = findViewById(R.id.nav_view);

        bottomNavigationView.setOnNavigationItemSelectedListener(v -> {
            if (v.getItemId() == findViewById(R.id.navigation_home).getId()) {
                getSupportFragmentManager().beginTransaction()
                        .replace(findViewById(R.id.fragmentContainerView).getId(), HomeFragment.newInstance("test", "tset"))
                        .commit();
            } else if (v.getItemId() == findViewById(R.id.navigation_QR).getId()) {
                getSupportFragmentManager().beginTransaction()
                        .replace(findViewById(R.id.fragmentContainerView).getId(), QRFragment.newInstance("test", "tset"))
                        .commit();
            } else if (v.getItemId() == findViewById(R.id.navigation_notification).getId()) {
                getSupportFragmentManager().beginTransaction()
                        .replace(findViewById(R.id.fragmentContainerView).getId(), NotificationFragment.newInstance("test", "test"))
                        .commit();
            }

            return true;

        });
    }

    //↓通知のやつ
    public void notifyTest() {
        ((Vibrator)getSystemService(Context.VIBRATOR_SERVICE)).vibrate(1000);
        NotificationCompat.Builder builder = new NotificationCompat
                .Builder(this, "CHANNEL_ID")
                .setSmallIcon(android.R.drawable.ic_menu_info_details)
                .setContentTitle("タイトル")
                .setContentText("メッセージ・メッセージ")
                .setPriority(NotificationCompat.PRIORITY_DEFAULT);

        NotificationManagerCompat notificationManager = NotificationManagerCompat.from(this);

        if (ActivityCompat.checkSelfPermission(this, android.Manifest.permission.POST_NOTIFICATIONS) != PackageManager.PERMISSION_GRANTED) {
            // TODO: Consider calling
            //    ActivityCompat#requestPermissions
            // here to request the missing permissions, and then overriding
            //   public void onRequestPermissionsResult(int requestCode, String[] permissions,
            //                                          int[] grantResults)
            // to handle the case where the user grants the permission. See the documentation
            // for ActivityCompat#requestPermissions for more details.
            return;
        }
        notificationManager.notify(R.string.app_name, builder.build());
    }
}