实现：EditText输入消息，通过按钮选择发送给主线程或者子线程；

以下有效果图、MainActivity.java代码和activity_main.xml代码

效果图：

MainActivity.java代码

package huan.san.handleroneapp;

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

import android.os.Bundle;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {
    public static final String TAG = "MainActivity";
    private TextView mTextView1;
    private TextView mTextView2;
    private Button mButton1;
    private Button mButton2;
    private Handler1 mSubThreadHandler;
    private Handler2 mMainThreadHandler;
    private EditText mEditText;
    String sMessage;
    private int counter = 0;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        init();
    }
    public void init() {
        mTextView1 = (TextView)findViewById(R.id.textView1);
        mTextView2 = (TextView)findViewById(R.id.textView2);
        mButton1 = (Button) findViewById(R.id.button1);
        mButton2 = (Button) findViewById(R.id.button2);
        mEditText = (EditText)findViewById(R.id.editTextTextPersonName) ;

        mButton1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                sMessage=mEditText.getText().toString();
                //主线程发送消息到子线程
                mSubThreadHandler = new Handler1(getMainLooper());
                Message message = new Message();
                message.obj = sMessage;
                mSubThreadHandler.sendMessage(message);
            }
        });
        mButton2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                sMessage=mEditText.getText().toString();
                //子线程发送消息到主线程
                mMainThreadHandler = new Handler2(getMainLooper());
                Message message = new Message();
                message.obj = sMessage;
                mMainThreadHandler.sendMessage(message);
            }
        });

        new Thread(){
            public void run(){
                Looper.prepare();
                //Looper.myLooper()获取当前线程的looper
                mSubThreadHandler = new Handler1(Looper.myLooper());
                Message message = new Message();
                message.obj = sMessage;
            };
        }.start();
    }
    
    public class Handler1 extends Handler{
        private Handler1(Looper looper){
            super(looper);
        }

        @Override
        public void handleMessage(@NonNull Message msg) {
            super.handleMessage(msg);
            //子线程消息显示
            mTextView2.setText("子线程收到：" + msg.obj);
        }
    }

    public class Handler2 extends Handler{
        private Handler2(Looper looper){
            super(looper);
        }

        @Override
        public void handleMessage(@NonNull Message msg) {
            super.handleMessage(msg);
            //主线程消息显示
            mTextView1.setText("主线程收到：" + msg.obj);

        }
    }
}

activity_main.xml代码

<?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=".MainActivity">


    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="60dp"
        android:layout_marginTop="140dp"
        android:text="主线程发送消息"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="220dp"
        android:layout_marginTop="140dp"
        android:text="子线程发送消息"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/textView2"
        android:layout_width="137dp"
        android:layout_height="97dp"
        android:layout_marginStart="220dp"
        android:layout_marginTop="216dp"
        android:text="没有收到消息"
        android:textSize="20sp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/textView1"
        android:layout_width="139dp"
        android:layout_height="94dp"
        android:layout_marginStart="60dp"
        android:layout_marginTop="216dp"
        android:text="没有收到消息"
        android:textSize="20sp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/editTextTextPersonName"
        android:layout_width="200dp"
        android:layout_height="51dp"
        android:layout_marginStart="104dp"
        android:layout_marginTop="64dp"
        android:ems="10"
        android:inputType="textPersonName"
        android:text="请输入"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />


</androidx.constraintlayout.widget.ConstraintLayout>

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