主界面代码

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

   <Button
       android:id="@+id/change"
       android:layout_width="match_parent"
       android:layout_height="wrap_content"
       android:text="改变内容"/>
   <TextView
       android:id="@+id/text"
       android:layout_width="match_parent"
       android:layout_height="wrap_content"
       android:layout_centerInParent="true"
       android:text="你好，世界！"
       android:textSize="20sp"/>

</RelativeLayout>

主活动代码：

import android.os.Handler;
import android.os.Message;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity implements View.OnClickListener{
    private TextView text;
    public static final int UPDATE_TEXT=1;
    private Handler handler=new Handler(){
        public void handleMessage(Message msg){
            switch (msg.what){
                case UPDATE_TEXT:
                    text.setText("遇见你真好");
                    break;
                    default:
                        break;

            }
        }



    };
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        text=(TextView)findViewById(R.id.text);
        Button ChangeText=(Button)findViewById(R.id.change);
        ChangeText.setOnClickListener(this);
    }
    public void onClick(View v)
    {
        switch (v.getId())
        {
            case R.id.change:
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        Message message=new Message();
                        message.what=UPDATE_TEXT;
                        handler.sendMessage(message);

                    }
                }).start();
                break;
                default:
                    break;
        }

    }
}