<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/tv1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:gravity="center"
        android:text="计算器"
        android:textColor="#FF2196F3"
        android:textSize="25sp"
        android:textStyle="italic" />

    <LinearLayout
        android:id="@+id/l1_number"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/tv1"
        android:layout_centerVertical="true"
        android:layout_marginLeft="10dp"
        android:layout_marginTop="15dp"
        android:layout_marginRight="10dp"
        android:layout_marginBottom="5dp"
        android:background="#FFFFFF">

        <TextView
            android:id="@+id/tv_number"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:padding="10dp"
            android:text="数值1:"
            android:textColor="#000"
            android:textSize="20sp" />

        <EditText
            android:id="@+id/et1"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:layout_below="@id/tv1" />
    </LinearLayout>

    <LinearLayout
        android:id="@+id/l2_number"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/l1_number"
        android:layout_centerVertical="true"
        android:layout_marginLeft="10dp"
        android:layout_marginTop="15dp"
        android:layout_marginRight="10dp"
        android:layout_marginBottom="5dp"
        android:background="#FFFFFF">

        <TextView
            android:id="@+id/tv_number2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:padding="10dp"
            android:text="数值2:"
            android:textColor="#000"
            android:textSize="20sp" />

        <EditText
            android:id="@+id/et2"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:layout_below="@id/tv2" />
    </LinearLayout>
     <TextView
        android:id="@+id/tv2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/l2_number"
        android:layout_centerHorizontal="true"
        android:layout_marginLeft="10dp"
        android:layout_marginTop="50dp"
        android:layout_marginRight="10dp"
        android:layout_marginBottom="50dp"
        android:gravity="center"
        android:text="显示计算结果"
        android:textSize="20sp"
        android:textStyle="italic" />

    <Button
        android:id="@+id/btn1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/tv2"
        android:gravity="center"
        android:onClick="click"
        android:text="+"
        android:textColor="#FFF44336"
        android:textSize="25sp"
        android:textStyle="italic" />

    <Button
        android:id="@+id/btn2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/tv2"
        android:layout_toRightOf="@id/btn1"
        android:gravity="center"
        android:onClick="click"
        android:text="-"
        android:textColor="#FFE91E63"
        android:textSize="25sp"
        android:textStyle="italic" />

    <Button
        android:id="@+id/btn3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/tv2"
        android:layout_toRightOf="@id/btn2"
        android:gravity="center"
        android:onClick="click"
        android:text="*"
        android:textColor="#FFF79E38"
        android:textSize="25sp"
        android:textStyle="italic" />

    <Button
        android:id="@+id/btn4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/tv2"
        android:layout_toRightOf="@id/btn3"
        android:gravity="center"
        android:onClick="click"
        android:text="/"
        android:textColor="#FFF79E38"
        android:textSize="25sp"
        android:textStyle="italic" />


</RelativeLayout>

package com.example.myapplication;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.app.Activity;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;

import com.example.myapplication.R;


public class MainActivity extends Activity {

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

    public void click(View v) {
        TextView tv2 = (TextView) findViewById(R.id.tv2);
        EditText et1 = (EditText) findViewById(R.id.et1);
        EditText et2 = (EditText) findViewById(R.id.et2);
        double num1,num2,result=0;
        num1=Integer.parseInt(et1.getText().toString());
        num2=Integer.parseInt(et2.getText().toString());

        switch (v.getId()) {
            case R.id.btn1:
                result=num1+num2;
                break;
            case R.id.btn2:
                result=num1-num2;
                break;
            case R.id.btn3:
                result=num1*num2;
                break;
            case R.id.btn4:
                result=num1/num2;
                break;
        }
        tv2.setText(result+"");
    }

}