在service中调用MainActivity的静态方法：该静态方法会开启子线程产生随机数，将随机数放在Message中传回主线程的handler中去更新UI

package com.example.threadrandomdemo;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity{

    private final String TAG = this.getClass().getSimpleName();
    private static final int UPDATE_TEXT = 1;

    Button button_startService;
    Button button_stopService;
    private static TextView textView_showRandom;

    private static Handler handler = new Handler(new Handler.Callback() {
        //静态handler
        @Override
        public boolean handleMessage(@NonNull Message message) {
            switch (message.what){
                case UPDATE_TEXT:{
                    textView_showRandom.setText(String.valueOf(message.obj));
                    
                    break;
                }
                default:break;
            }
            return false;
        }
    });
    private static double randomInText;
    public static void UpdateText(double doubleRandom){
        //静态方法，开启一个子线程（该静态方法将被service调用来产生随机数）
        //在子线程中根据传入的double类型参数来发送message给静态handler（在主线程中），
        //在handler中根据收到的message来更新UI
        randomInText = doubleRandom;
        Thread myThread = new Thread(new Runnable() {
            @Override
            public void run() {
                Message msg = new Message();
                msg.what = UPDATE_TEXT;
                msg.obj = randomInText;
                handler.sendMessage(msg);
                
            }
        });
        myThread.start();
        Log.d("MainActivity","the ThreadId is " + myThread.getId() + "in Start");
    }

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

        button_startService = (Button) findViewById(R.id.startService);
        button_stopService = (Button) findViewById(R.id.stopService);
        textView_showRandom = (TextView) findViewById(R.id.text_view);

        //设置按键控制service的关闭和开启
        final Intent intent_Service = new Intent(this,RandomService.class);
        button_startService.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                startService(intent_Service);
            }
        });
        button_stopService.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                stopService(intent_Service);
            }
        });

        Log.d(TAG,"the TaskId is " + getTaskId());
    }
}

package com.example.threadrandomdemo;

import android.app.Service;
import android.content.Intent;
import android.os.IBinder;
import android.util.Log;
import android.widget.Toast;

public class RandomService extends Service {

    private String TAG = this.getClass().getSimpleName();
    private Thread randomThread;

    public RandomService() {
    }

    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }


    @Override
    public void onCreate() {
        super.onCreate();
        Toast.makeText(this, TAG + "has already executed onCreate method",
                Toast.LENGTH_SHORT).show();
        //service被创建之后就实例化一个子线程
        randomThread = new Thread(runnable);//实例化一个子线程
        //runnable作为参数
    }

    private Runnable runnable = new Runnable() {
        @Override
        public void run() {
            try {
                while (!randomThread.isInterrupted()) {
                    double doubleRandom = Math.random() * 100.0;
                    MainActivity.UpdateText(doubleRandom);
                    //调用MainActivity的静态方法产生随机数
                    Thread.sleep(1000);
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    };


    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Toast.makeText(this, TAG + "has already executed onStartCommand method",
                Toast.LENGTH_SHORT).show();
        //service启动时让子线程启动
        if (!randomThread.isAlive()) {
            randomThread.start();//如果子线程没有启动则启动它并在log中做上记录
            Log.d(TAG, "the ThreadId is " + randomThread.getId() + "in onStartCommand");
        }
        return super.onStartCommand(intent, flags, startId);
    }

    @Override
    public void onDestroy() {
        Toast.makeText(this, TAG + "has already executed onDestory method",
                Toast.LENGTH_SHORT).show();
        super.onDestroy();
        randomThread.interrupt();
        //service结束被销毁之后阻塞子线程
    }

}