A simple android calculator (XML + JAVA)

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    android:background="#000000"
    android:orientation="vertical"
    tools:context=".MainActivity"
    tools:ignore="HardcodedText">

    <EditText
        android:id="@+id/editText"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_marginBottom="1pt"
        android:layout_weight="1"
        android:background="#000000"
        android:ems="10"
        android:gravity="right"
        android:inputType="textPersonName"
        android:text="0"
        android:textColor="#ffffff"
        android:textSize="20pt" />

    <TableLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        tools:ignore="ButtonStyle">


        <TableRow
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_marginBottom="1pt">

            <Button
                android:id="@+id/bu7"
                android:layout_width="30pt"
                android:layout_height="20pt"
                android:layout_margin="5dp"
                android:layout_marginEnd="1pt"
                android:layout_marginRight="1pt"
                android:layout_weight="1"
                android:onClick="padButtonOnClick"
                android:background="#C0C0C0"
                android:text="7" />

            <Button
                android:id="@+id/bu8"
                android:layout_width="30pt"
                android:layout_height="20pt"
                android:layout_margin="5dp"
                android:layout_marginEnd="1pt"
                android:layout_marginRight="1pt"
                android:layout_weight="1"
                android:onClick="padButtonOnClick"
                android:background="#C0C0C0"
                android:text="8" />

            <Button
                android:id="@+id/bu9"
                android:layout_width="30pt"
                android:layout_height="20pt"
                android:layout_margin="5dp"
                android:layout_marginEnd="1pt"
                android:layout_marginRight="1pt"
                android:layout_weight="1"
                android:onClick="padButtonOnClick"
                android:background="#C0C0C0"
                android:text="9" />

            <Button
                android:id="@+id/buAC"
                android:layout_width="5pt"
                android:layout_height="20pt"
                android:layout_margin="5dp"
                android:layout_marginEnd="1pt"
                android:layout_marginRight="1pt"
                android:layout_weight="1"
                android:onClick="ACOnClick"
                android:background="#C0C0C0"
                android:text="AC" />
        </TableRow>

        <TableRow
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_marginBottom="1pt">

            <Button
                android:id="@+id/bu4"
                android:layout_width="30pt"
                android:layout_height="20pt"
                android:layout_margin="5dp"
                android:layout_marginEnd="1pt"
                android:layout_marginRight="1pt"
                android:layout_weight="1"
                android:onClick="padButtonOnClick"
                android:background="#C0C0C0"
                android:text="4" />

            <Button
                android:id="@+id/bu5"
                android:layout_width="30pt"
                android:layout_height="20pt"
                android:layout_margin="5dp"
                android:layout_marginEnd="1pt"
                android:layout_marginRight="1pt"
                android:layout_weight="1"
                android:onClick="padButtonOnClick"
                android:background="#C0C0C0"
                android:text="5" />

            <Button
                android:id="@+id/bu6"
                android:layout_width="30pt"
                android:layout_height="20pt"
                android:layout_margin="5dp"
                android:layout_marginEnd="1pt"
                android:layout_marginRight="1pt"
                android:layout_weight="1"
                android:onClick="padButtonOnClick"
                android:background="#C0C0C0"
                android:text="6" />

            <Button
                android:id="@+id/buPlus"
                android:layout_width="5pt"
                android:layout_height="20pt"
                android:layout_margin="5dp"
                android:layout_marginEnd="1pt"
                android:layout_marginRight="1pt"
                android:layout_weight="1"
                android:onClick="padButtonOnClick"
                android:background="#C0C0C0"
                android:text="+"
                android:textColor="#000000" />
        </TableRow>

        <TableRow
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_marginBottom="1pt">

            <Button
                android:id="@+id/bu1"
                android:layout_width="30pt"
                android:layout_height="20pt"
                android:layout_margin="5dp"
                android:layout_marginRight="1pt"
                android:layout_weight="1"
                android:onClick="padButtonOnClick"
                android:background="#C0C0C0"
                android:text="1"
                android:layout_marginEnd="1pt" />

            <Button
                android:id="@+id/bu2"
                android:layout_width="30pt"
                android:layout_height="20pt"
                android:layout_margin="5dp"
                android:layout_marginRight="1pt"
                android:layout_weight="1"
                android:onClick="padButtonOnClick"
                android:background="#C0C0C0"
                android:text="2"
                android:layout_marginEnd="1pt" />

            <Button
                android:id="@+id/bu3"
                android:layout_width="30pt"
                android:layout_height="20pt"
                android:layout_margin="5dp"
                android:layout_marginRight="1pt"
                android:layout_weight="1"
                android:onClick="padButtonOnClick"
                android:background="#C0C0C0"
                android:text="3"
                android:layout_marginEnd="1pt" />

            <Button
                android:id="@+id/buMinus"
                android:layout_width="5pt"
                android:layout_height="20pt"
                android:layout_margin="5dp"
                android:layout_marginRight="1pt"
                android:layout_weight="1"
                android:onClick="padButtonOnClick"
                android:background="#C0C0C0"
                android:text="-"
                android:textColor="#000000"
                android:layout_marginEnd="1pt" />
        </TableRow>
    </TableLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">


        <TableRow
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            tools:ignore="UselessParent">

            <Button
                android:id="@+id/bu0"
                android:layout_width="60pt"
                android:layout_height="20pt"
                android:layout_margin="5dp"
                android:layout_marginRight="1pt"
                android:layout_weight="2"
                android:onClick="padButtonOnClick"
                android:background="#C0C0C0"
                android:text="0"
                android:layout_marginEnd="1pt" />

            <Button
                android:id="@+id/buDot"
                android:layout_width="30pt"
                android:layout_height="20pt"
                android:layout_margin="5dp"
                android:layout_marginRight="1pt"
                android:layout_weight="1"
                android:onClick="padButtonOnClick"
                android:background="#C0C0C0"
                android:text="."
                android:layout_marginEnd="1pt" />

            <Button
                android:id="@+id/buEqual"
                android:layout_width="5pt"
                android:layout_height="20pt"
                android:layout_margin="5dp"
                android:layout_marginRight="1pt"
                android:layout_weight="1"
                android:background="#C0C0C0"
                android:text="="
                android:textColor="#000000"
                android:layout_marginEnd="1pt" />
        </TableRow>


    </LinearLayout>
</LinearLayout>

MainActivity.java

package com.example.calculator;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

public class MainActivity extends AppCompatActivity {

    String text = "0";
    EditText screenText;
    public static String TAG =  "FirstFragment";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        screenText = findViewById(R.id.editText);
    }

    public void padButtonOnClick(View source) {
        Log.d(TAG, "padButton: " + ((Button) source).getText());
        updateText(((Button) source).getText().toString());
    }

    public void updateText(String text) {
        if (this.text.trim().equals("0")) {
            this.text = text;
        } else {
            this.text = this.text.concat(text);
        }

        screenText.setText(this.text);
    }

    public void ACOnClick(View view) {
        this.text = "0";
        screenText.setText(this.text);
    }
}