p2

 activity_main.xml

<?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"
    android:background="#FAF7F7"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/butSub"
        android:layout_width="89dp"
        android:layout_height="62dp"
        android:text="sub"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.353"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.656" />

    <Button
        android:id="@+id/butDiv"
        android:layout_width="89dp"
        android:layout_height="62dp"
        android:text="DIV"
        android:textSize="16sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.96"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.656" />

    <EditText
        android:id="@+id/txtResult1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:ems="10"
        android:inputType="text"
        android:textColor="#FF5722"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.771"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/txtNum2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:ems="10"
        android:inputType="text"
        android:textColor="#3F51B5"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.766"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.37" />

    <TextView
        android:id="@+id/textView5"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Result:"
        android:textAlignment="center"
        android:textColor="#FF5722"
        android:textSize="24sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.21"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.499" />

    <TextView
        android:id="@+id/textView3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Enter Num2:"
        android:textAlignment="center"
        android:textColor="#3F51B5"
        android:textSize="20sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.11"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.372" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="255dp"
        android:layout_height="38dp"
        android:text="SIMPLE CALCULATOR"
        android:textAlignment="center"
        android:textColor="#9C27B0"
        android:textSize="24sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.077" />

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Enter Num1:"
        android:textAlignment="center"
        android:textColor="#3F51B5"
        android:textSize="20sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.11"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.241" />

    <EditText
        android:id="@+id/txtNum1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:ems="10"
        android:inputType="text"
        android:textColor="#673AB7"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.761"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.233" />

    <Button
        android:id="@+id/butAdd"
        android:layout_width="89dp"
        android:layout_height="62dp"
        android:text="ADD"
        android:textSize="16sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.043"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.658" />

    <Button
        android:id="@+id/butMul"
        android:layout_width="89dp"
        android:layout_height="62dp"
        android:text="MUL"
        android:textSize="16sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.666"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.656" />

</androidx.constraintlayout.widget.ConstraintLayout>

mainacitivity.java

package com.example.calculator;


import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
    Button but_Add,but_Sub,but_Mul,but_Div;
    EditText txt_Num1,txt_Num2;
    TextView txt_Res;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        but_Add = (Button) findViewById(R.id.butAdd);
        but_Sub = (Button) findViewById(R.id.butSub);
        but_Mul = (Button) findViewById(R.id.butMul);
        but_Div = (Button) findViewById(R.id.butDiv);

        txt_Num1 = (EditText) findViewById(R.id.txtNum1);
        txt_Num2 = (EditText) findViewById(R.id.txtNum2);

        txt_Res = (TextView) findViewById(R.id.txtResult1);

        but_Add.setOnClickListener(this);
        but_Sub.setOnClickListener(this);
        but_Mul.setOnClickListener(this);
        but_Div.setOnClickListener(this);
    }
    @Override
    public void onClick(View v) {
        double op1 =
                Double.parseDouble(txt_Num1.getText().toString());
        double op2 =
                Double.parseDouble(txt_Num2.getText().toString());
        double Res;

        if(v.equals(but_Add))
            Res = op1 + op2;
        else if(v.equals(but_Sub))
            Res = op1 - op2;
        else if(v.equals(but_Mul))
            Res = op1 * op2;
        else if(v.equals(but_Div))
            Res = op1 / op2;
        else Res = 0;
        txt_Res.setText(String.valueOf(Res));

    }
}