<?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="com.example.administrator.test.MainActivity">

    <EditText
        android:layout_width="250dp"
        android:layout_height="wrap_content"
        android:hint="First Number"
        android:textSize="20dp"
        android:layout_marginTop="27dp"
        android:id="@+id/n1"
        android:layout_alignParentTop="true"
        android:layout_alignLeft="@+id/plus"
        android:layout_alignStart="@+id/plus"
        android:layout_alignRight="@+id/n2"
        android:layout_alignEnd="@+id/n2" />

    <EditText
        android:layout_width="250dp"
        android:layout_height="wrap_content"
        android:hint="Second Number"
        android:textSize="20dp"
        android:id="@+id/n2"
        android:layout_marginTop="25dp"
        android:layout_below="@+id/n1"
        android:layout_alignLeft="@+id/plus"
        android:layout_alignStart="@+id/plus"
        android:layout_alignRight="@+id/minus"
        android:layout_alignEnd="@+id/minus" />

    <Button
        android:id="@+id/plus"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="33dp"
        android:text="+"
        android:textSize="30dp"
        android:layout_below="@+id/n2"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_marginLeft="46dp"
        android:layout_marginStart="46dp" />

    <Button
        android:id="@+id/minus"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBottom="@+id/plus"
        android:layout_alignParentEnd="true"
        android:layout_alignParentRight="true"
        android:layout_marginEnd="73dp"
        android:layout_marginRight="73dp"
        android:textSize="30dp"
        android:text="-" />

    <Button
        android:id="@+id/mul"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignEnd="@+id/plus"
        android:layout_alignRight="@+id/plus"
        android:layout_below="@+id/plus"
        android:layout_marginTop="24dp"
        android:textSize="30dp"
        android:text="*" />

    <Button
        android:id="@+id/div"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/minus"
        android:layout_alignStart="@+id/minus"
        android:layout_alignTop="@+id/mul"
        android:textSize="30dp"
        android:text="/" />

    <TextView
        android:id="@+id/ans"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="30dp"
        android:text="Result"
        android:layout_below="@+id/div"
        android:layout_toLeftOf="@+id/div"
        android:layout_toStartOf="@+id/div"
        android:layout_marginTop="75dp" />


</RelativeLayout>








package com.example.administrator.test;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    public EditText e1;
    public EditText e2;
    public Button b1;
    public Button b2;
    public Button b3;
    public Button b4;
    public TextView res;

    public double result;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        e1 = (EditText)findViewById(R.id.n1);
        e2 = (EditText)findViewById(R.id.n2);
        b1 = (Button)findViewById(R.id.plus);
        b2 = (Button)findViewById(R.id.minus);
        b3 = (Button)findViewById(R.id.mul);
        b4 = (Button)findViewById(R.id.div);
        res =(TextView)findViewById(R.id.ans);
    }
}