How to work Button action in android studio

Create XML code for the design

activity_main.xml

<?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:layout_width="match_parent"

    android:layout_height="match_parent"

    tools:context=".MainActivity"

    >

    <EditText

        android:id="@+id/F"

        android:layout_width="151dp"

        android:layout_height="wrap_content"

        android:layout_alignParentStart="true"

        android:layout_alignParentTop="true"

        android:layout_marginTop="39dp"

        android:ems="10"

        android:inputType="numberDecimal"

        android:textSize="30dp"

        android:importantForAutofill="no" />


    <EditText

        android:id="@+id/S"

        android:layout_width="151dp"

        android:layout_height="wrap_content"

        android:layout_alignTop="@+id/F"

        android:layout_alignParentEnd="true"

        android:ems="10"

        android:inputType="numberDecimal"

        android:textSize="30dp"/>


    <Button

        android:id="@+id/Add"

        android:layout_width="151dp"

        android:layout_height="wrap_content"

        android:layout_alignParentStart="true"

        android:layout_alignParentTop="true"

        android:layout_marginTop="123dp"

        android:text="Add"

        android:textSize="25dp"

        />


    <Button

        android:id="@+id/Sub"

        android:layout_width="151dp"

        android:layout_height="wrap_content"

        android:layout_alignStart="@+id/S"

        android:layout_alignParentTop="true"

        android:layout_marginStart="5dp"

        android:layout_marginTop="118dp"

        android:text="Sub"

        android:textSize="25dp"

        />


    <TextView

        android:id="@+id/Result"

        android:layout_width="151dp"

        android:layout_height="47dp"

        android:layout_alignParentTop="true"

        android:layout_centerHorizontal="true"

        android:layout_marginTop="192dp"

        android:text="Result"

        android:textSize="30sp"

        />

</RelativeLayout>

Create Class

But use your own package this creat default  package

MainActivity.class

import android.support.v7.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 {
    private EditText F,S;
    private Button Add, Sub;
    private TextView Result;
    Double Num1,Num2,Sum;

    @Override    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Add =(Button)findViewById(R.id.Add);
        Sub =(Button)findViewById(R.id.Sub);
        Add.setOnClickListener(this);
        Sub.setOnClickListener(this);


    }

    @Override    public void onClick(View v) {
        F =(EditText) findViewById(R.id.F);
        S =(EditText) findViewById(R.id.S);
        Result = (TextView) findViewById(R.id.Result);
        Num1 =Double.parseDouble(F.getText().toString());
        Num2 = Double.parseDouble(S.getText().toString());
        if (v.getId()==R.id.Add){
            Sum=Num1+Num2;
            Result.setText(Double.toString(Sum));
        }
       else if (v.getId()==R.id.Sub){
            Sum=Num1-Num2;
            Result.setText(Double.toString(Sum));
        }
    }
}