出来るだけコードが少なくなるように電卓アプリを作成しました。
改善点などご教授いただけますと幸いです。
YouTubeに作っている動画をアップしています。
https://www.youtube.com/watch?v=1c0LvmCW5EU&list=PLhg2PHSq8bjjjBpOPll39ZAUgtbnlDOvU

strings.xml

<resources>
    <string name="app_name">CalculateApp</string>
    <string name="btn_zero">0</string>
    <string name="btn_one">1</string>
    <string name="btn_two">2</string>
    <string name="btn_three">3</string>
    <string name="btn_four">4</string>
    <string name="btn_five">5</string>
    <string name="btn_six">6</string>
    <string name="btn_seven">7</string>
    <string name="btn_eight">8</string>
    <string name="btn_nine">9</string>
    <string name="btn_plus">+</string>
    <string name="btn_minus">-</string>
    <string name="btn_times">×</string>
    <string name="btn_divide">÷</string>
    <string name="btn_clear">C</string>
    <string name="btn_point">.</string>
    <string name="btn_equal">=</string>
</resources>

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:weightSum="7"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/text_formula"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:gravity="right"
        android:textSize="36sp"></TextView>

    <TextView
        android:id="@+id/text_result"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:gravity="right"
        android:textSize="36sp"
        android:textStyle="bold"></TextView>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:orientation="horizontal"
        android:weightSum="4">

        <Space
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"></Space>

        <Space
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"></Space>

        <Space
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"></Space>

        <Button
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"
            android:text="@string/btn_clear"
            android:onClick="tapClear"
            android:textSize="36sp"></Button>
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:orientation="horizontal"
        android:weightSum="4">

        <Button
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"
            android:text="@string/btn_seven"
            android:onClick="tapSeven"
            android:textSize="36sp"></Button>

        <Button
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"
            android:text="@string/btn_eight"
            android:onClick="tapEight"
            android:textSize="36sp"></Button>

        <Button
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"
            android:text="@string/btn_nine"
            android:onClick="tapNine"
            android:textSize="36sp"></Button>

        <Button
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"
            android:text="@string/btn_divide"
            android:onClick="tapDivide"
            android:textSize="36sp"></Button>
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:orientation="horizontal"
        android:weightSum="4">

        <Button
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"
            android:text="@string/btn_four"
            android:onClick="tapFour"
            android:textSize="36sp"></Button>

        <Button
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"
            android:text="@string/btn_five"
            android:onClick="tapFive"
            android:textSize="36sp"></Button>

        <Button
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"
            android:text="@string/btn_six"
            android:onClick="tapSix"
            android:textSize="36sp"></Button>

        <Button
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"
            android:text="@string/btn_times"
            android:onClick="tapTimes"
            android:textSize="36sp"></Button>
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:orientation="horizontal"
        android:weightSum="4">

        <Button
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"
            android:text="@string/btn_one"
            android:onClick="tapOne"
            android:textSize="36sp"></Button>

        <Button
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"
            android:text="@string/btn_two"
            android:onClick="tapTwo"
            android:textSize="36sp"></Button>

        <Button
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"
            android:text="@string/btn_three"
            android:onClick="tapThree"
            android:textSize="36sp"></Button>

        <Button
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"
            android:text="@string/btn_minus"
            android:onClick="tapMinus"
            android:textSize="36sp"></Button>
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:orientation="horizontal"
        android:weightSum="4">

        <Button
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"
            android:text="@string/btn_zero"
            android:onClick="tapZero"
            android:textSize="36sp"></Button>

        <Button
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"
            android:text="@string/btn_point"
            android:onClick="tapPoint"
            android:textSize="36sp"></Button>

        <Button
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"
            android:text="@string/btn_equal"
            android:onClick="tapEqual"
            android:textSize="36sp"></Button>

        <Button
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_margin="2dp"
            android:layout_weight="1"
            android:text="@string/btn_plus"
            android:onClick="tapPlus"
            android:textSize="36sp"></Button>
    </LinearLayout>

</LinearLayout>

MainActivity.java

package com.example.calculateapp;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.TextView;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class MainActivity extends AppCompatActivity {

    TextView formula, result;
    List<String> symbols;

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

        formula = findViewById(R.id.text_formula);
        result = findViewById(R.id.text_result);
        symbols = new ArrayList<String>();
    }

    public void tapZero(View view) {
        formula.setText(formula.getText().toString() + "0");
    }

    public void tapOne(View view) {
        formula.setText(formula.getText().toString() + "1");
    }

    public void tapTwo(View view) {
        formula.setText(formula.getText().toString() + "2");
    }

    public void tapThree(View view) {
        formula.setText(formula.getText().toString() + "3");
    }

    public void tapFour(View view) {
        formula.setText(formula.getText().toString() + "4");
    }

    public void tapFive(View view) {
        formula.setText(formula.getText().toString() + "5");
    }

    public void tapSix(View view) {
        formula.setText(formula.getText().toString() + "6");
    }

    public void tapSeven(View view) {
        formula.setText(formula.getText().toString() + "7");
    }

    public void tapEight(View view) {
        formula.setText(formula.getText().toString() + "8");
    }

    public void tapNine(View view) {
        formula.setText(formula.getText().toString() + "9");
    }

    public void tapPoint(View view) {
        formula.setText(formula.getText().toString() + ".");
    }

    public void tapPlus(View view) {
        formula.setText(formula.getText().toString() + "+");
        symbols.add("+");
    }

    public void tapMinus(View view) {
        formula.setText(formula.getText().toString() + "-");
        symbols.add("-");
    }

    public void tapTimes(View view) {
        formula.setText(formula.getText().toString() + "×");
        symbols.add("×");
    }

    public void tapDivide(View view) {
        formula.setText(formula.getText().toString() + "÷");
        symbols.add("÷");
    }

    public void tapClear(View view) {
        formula.setText("");
        result.setText("");
        symbols.clear();
    }

    public void tapEqual(View view) {
        BigDecimal tmp;
        BigDecimal numResult = new BigDecimal(0);
        String TextFormula = formula.getText().toString();
        List<String> nums = Arrays.asList(TextFormula.split("[\\+\\-×÷]", -1));
        List<BigDecimal> numbers = new ArrayList<BigDecimal>();
        try {
            if ((nums.get(0).length() == 0) && (symbols.get(0) == "-")) {
                nums.set(0, "0");
            }
            for (String num : nums){
                numbers.add(new BigDecimal(num));
            }
            for (int i = 0; i < symbols.size(); i++) {
                switch (symbols.get(i)) {
                    case "×":
                        tmp = numbers.get(i).multiply(numbers.get(i+1));
                        numbers.set(i,new BigDecimal(0));
                        numbers.set(i + 1, tmp);
                        break;
                    case "÷":
                        tmp = numbers.get(i).divide(numbers.get(i+1),2,BigDecimal.ROUND_HALF_UP);
                        numbers.set(i,new BigDecimal(0));
                        numbers.set(i + 1, tmp);
                        break;
                    case "-":
                        numbers.set(i+1,numbers.get(i+1).multiply(new BigDecimal(-1)));
                        break;
                }
            }
            for (BigDecimal num : numbers) {
                numResult = numResult.add(num);
            }
            result.setText(numResult.toString());
        } catch (Exception e) {
            Log.e("エラー", e.getMessage());
        }
    }
}

縦画面に固定したいので、マニフェストに追記

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.calculateapp">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.CalculateApp">
        <activity
            android:name=".MainActivity"
            android:screenOrientation="portrait">//縦画面固定
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

YouTubeに作っている動画をアップしています。
https://www.youtube.com/watch?v=kvz7gLP3SHI&list=PLhg2PHSq8bjisIZGg-cLe4TegqJTCBXhS
以上。