OnlyDialog.app

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.onlydialog">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.OnlyDialog">
        <activity
            android:name=".MainActivity"
            android:theme="@style/Theme.AppCompat.Light.Dialog.Alert">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

MainActivity.java

package com.example.onlydialog;

import android.os.Bundle;
import android.widget.Toast;

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

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        //setContentView(何もセットしない);
        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        String dialog_title = "Dialog w/o Activity UI";
        String dialog_message = "実験は";
        builder.setTitle(dialog_title)
                .setMessage(dialog_message)
                .setPositiveButton("成功だ！", (dialogInterface, i) -> {
                    Toast.makeText(this, "success!", Toast.LENGTH_SHORT).show();
                    finish();
                })
                .setNegativeButton("失敗だ！", (dialogInterface, i) -> {
                    Toast.makeText(this, "failure!", Toast.LENGTH_SHORT).show();
                    finish();
                })
                .show();
    }
}