Android Studio 多线程实现 每隔一秒使text的值加1

MainActivity.java

package com.example.test4;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.widget.TextView;
import android.os.Handler;
import android.os.Message;


public class MainActivity extends AppCompatActivity {
    private TextView number;
    private int num = 0;
    private boolean flag = true;

    Thread thread = new Thread(new Runnable() {
        @Override
        public void run() {
            while (flag) {
                Message message = new Message();
                message.what = 1;
                handler.sendMessage(message);
                try {
                    thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    });

    private Handler handler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case 1:
                    number.setText(String.valueOf(++num));
                    break;
                default:
                    break;
            }
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        number = findViewById(R.id.textView);
        thread.start();
    }
}

activity_main.xml

AndroidManifest.xml