ServiceからActivityへ情報を渡す

import android.content.BroadcastReceiver;

import android.content.Context;

import android.content.Intent;

import android.content.IntentFilter;

import android.os.Bundle;

import android.support.v7.app.AppCompatActivity;

import android.view.View;

import android.widget.Button;

import android.widget.LinearLayout;

import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        LinearLayout ll = new LinearLayout(this);

        Button btn = new Button(this);

        btn.setText("start");

        ll.addView(btn);

        setContentView(ll);

        btn.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View v) {

                Intent service = new Intent(getApplicationContext(), TestService.class);

                startService(service);

            }

        });

        // receiver

        UpdateReceiver receiver = new UpdateReceiver();

        IntentFilter filter = new IntentFilter();

        filter.addAction("DO_ACTION");

        registerReceiver(receiver, filter);

    }

    protected class UpdateReceiver extends BroadcastReceiver {

        @Override

        public void onReceive(Context context, Intent intent){

            Bundle extras = intent.getExtras();

            String msg = extras.getString("message");

            Toast.makeText(getApplicationContext(), msg, Toast.LENGTH_SHORT).show();

        }

    }

}

import android.app.Service;

import android.content.Intent;

import android.os.IBinder;

import android.support.annotation.Nullable;

public class TestService extends Service {

    @Nullable

    @Override

    public IBinder onBind(Intent intent) {

        return null;

    }

    public int onStartCommand(Intent intent, int flags, int startId){

        sendMessage("こんにちは");

        stopSelf();

        return START_NOT_STICKY;

    }

    protected void sendMessage(String msg){

        Intent broadcast = new Intent();

        broadcast.putExtra("message", msg);

        broadcast.setAction("DO_ACTION");

        getBaseContext().sendBroadcast(broadcast);

    }

}