android 子线程使用handle修改主线线程内容

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".HandleActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="129dp"
        android:layout_marginTop="69dp"
        android:text="@string/显示内容"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="116dp"
        android:layout_marginTop="248dp"
        android:text="@string/button2"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

package com.example.myapplication;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class HandleActivity extends AppCompatActivity {
    private Button btn;
    private TextView tv;

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

        btn = findViewById(R.id.button3);
        tv = findViewById(R.id.textView);

        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                new Thread(){

                    @Override
                    public void run() {
                            super.run();
                        Message msg=new Message();
                        msg.what = 1;
                        handler.sendMessage(msg);
                    }
                }.start();
            }
        });
    }

    Handler handler = new Handler(){
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            if(msg.what==1){
                tv.setText("子线程使用handle修改主线程控件内容");
            }
        }
    };
}