Calculator:

Same as SetOnclickListener. Only addition ,subtraction operation

are performed.
ActivityMain.xml

<?xml version="1.0" encoding="utf-8"?>

<android.support.constraint.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"
tools:context=".MainActivity">

<RelativeLayout
android:layout_width="368dp"
android:layout_height="495dp"
android:layout_marginBottom="8dp"
android:layout_marginEnd="8dp"
android:layout_marginTop="8dp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintTop_toTopOf="parent">

<Button
android:id="@+id/btn_1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:layout_below="@+id/edText1"
android:layout_marginTop="60dp"
android:onClick="PressOne"
android:text="1"
android:textSize="18sp" />

<Button
android:id="@+id/btn_0"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/btn_8"
android:layout_toEndOf="@+id/btn_7"
android:layout_toRightOf="@+id/btn_7"
android:text="0"
android:textSize="18sp" />

<Button
android:id="@+id/btn_9"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/btn_6"
android:layout_toEndOf="@+id/btn_5"
android:layout_toRightOf="@+id/btn_5"
android:text="9"
android:textSize="18sp" />

<Button
android:id="@+id/btn_8"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/btn_5"
 android:layout_toEndOf="@+id/btn_7"
android:layout_toRightOf="@+id/btn_7"
android:text="8"
android:textSize="18sp" />

<Button
android:id="@+id/btn_7"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/btn_4"
android:layout_alignStart="@+id/btn_4"
android:layout_below="@+id/btn_4"
android:text="7"
android:textSize="18sp" />

<Button
android:id="@+id/btn_6"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/btn_5"
android:layout_alignBottom="@+id/btn_5"
android:layout_toEndOf="@+id/btn_5"
android:layout_toRightOf="@+id/btn_5"
android:text="6"
android:textSize="18sp" />

<Button
android:id="@+id/btn_5"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/btn_2"
android:layout_toEndOf="@+id/btn_4"
android:layout_toRightOf="@+id/btn_4"
android:text="5"
android:textSize="18sp" />

<Button
android:id="@+id/btn_4"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/btn_1"
android:layout_alignStart="@+id/btn_1"
android:layout_below="@+id/btn_1"
android:text="4"
android:textSize="18sp" />

<Button
android:id="@+id/btn_3"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/btn_2"
android:layout_alignBottom="@+id/btn_2"
android:layout_toEndOf="@+id/btn_2"
android:layout_toRightOf="@+id/btn_2"
android:text="3"
android:textSize="18sp" />

<Button
android:id="@+id/btn_2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/btn_1"
android:layout_alignBottom="@+id/btn_1"
 android:layout_toEndOf="@+id/btn_1"
android:layout_toRightOf="@+id/btn_1"
android:text="2"
android:textSize="18sp" />

<Button
android:id="@+id/btn_Add"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_above="@+id/btn_6"
android:layout_alignParentEnd="true"
android:layout_alignParentRight="true"
android:backgroundTint="@android:color/darker_gray"
android:text="+"
android:textColor="@android:color/background_light"
android:textSize="18sp" />

<Button
android:id="@+id/btn_Sub"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/btn_Add"
android:layout_alignStart="@+id/btn_Add"
android:layout_below="@+id/btn_Add"
android:backgroundTint="@android:color/darker_gray"
android:text="-"
android:textColor="@android:color/background_light"
android:textSize="18sp" />

<Button
android:id="@+id/btn_Mul"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/btn_Sub"
android:layout_alignStart="@+id/btn_Sub"
android:layout_below="@+id/btn_6"
android:backgroundTint="@android:color/darker_gray"
android:text="*"
android:textColor="@android:color/background_light"
android:textSize="18sp" />

<Button
android:id="@+id/btn_Div"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/btn_Mul"
android:layout_alignStart="@+id/btn_Mul"
android:layout_below="@+id/btn_9"
android:backgroundTint="@android:color/darker_gray"
android:text="/"
android:textColor="@android:color/background_light"
android:textSize="18sp" />

<EditText
android:id="@+id/edText1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentEnd="true"
android:layout_alignParentLeft="true"
android:layout_alignParentRight="true"
android:layout_alignParentStart="true"
android:layout_alignParentTop="true"
android:layout_marginTop="22dp"
 android:ems="10"
android:inputType="textPersonName"
android:textAlignment="textEnd"
android:textSize="24sp" />

<Button
android:id="@+id/btn_calc"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/btn_0"
android:layout_toEndOf="@+id/btn_0"
android:layout_toRightOf="@+id/btn_0"
android:backgroundTint="@android:color/holo_green_light"
android:text="="
android:textColor="@android:color/background_light"
android:textSize="18sp" />

<Button
android:id="@+id/btn_dec"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/btn_7"
android:layout_toLeftOf="@+id/btn_8"
android:layout_toStartOf="@+id/btn_8"
android:text="."
android:textSize="18sp" />

<Button
android:id="@+id/btn_clear"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentEnd="true"
android:layout_alignParentRight="true"
android:layout_below="@+id/btn_Div"
android:backgroundTint="@android:color/holo_blue_dark"
android:text="clear"
android:textColor="@android:color/background_light"
android:textSize="18sp" />

</RelativeLayout>

</android.support.constraint.ConstraintLayout>

MainActivity.kt
package com.example.nikita.calculator

import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.view.View
import android.widget.Button
import android.widget.EditText

class MainActivity : AppCompatActivity() {

internal lateinit var btn_1: Button
internal lateinit var btn_2: Button
internal lateinit var btn_3: Button
internal lateinit var btn_4: Button
internal lateinit var btn_5: Button
internal lateinit var btn_6: Button
internal lateinit var btn_7: Button
internal lateinit var btn_8: Button
internal lateinit var btn_9: Button
internal lateinit var btn_0: Button
internal lateinit var btn_Add: Button
internal lateinit var btn_Sub: Button
internal lateinit var btn_Mul: Button
internal lateinit var btn_Div: Button
internal lateinit var btn_calc: Button
internal lateinit var btn_dec: Button
internal lateinit var btn_clear: Button
internal var ed1: EditText? = null

internal var Value1: Float = 0.toFloat()

internal var Value2: Float = 0.toFloat()
internal var mAddition: Boolean = false
internal var mSubtract: Boolean = false
internal var mMultiplication: Boolean = false
internal var mDivision: Boolean = false
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
btn_0 = findViewById<View>(R.id.btn_0) as Button
btn_1 = findViewById<View>(R.id.btn_1) as Button
btn_2 = findViewById<View>(R.id.btn_2) as Button
btn_3 = findViewById<View>(R.id.btn_3) as Button
btn_4 = findViewById<View>(R.id.btn_4) as Button
btn_5 = findViewById<View>(R.id.btn_5) as Button
btn_6 = findViewById<View>(R.id.btn_6) as Button
btn_7 = findViewById<View>(R.id.btn_7) as Button
btn_8 = findViewById<View>(R.id.btn_8) as Button
btn_9 = findViewById<View>(R.id.btn_9) as Button
btn_Add = findViewById<View>(R.id.btn_Add) as Button
btn_Div = findViewById<View>(R.id.btn_Div) as Button
btn_Sub = findViewById<View>(R.id.btn_Sub) as Button
btn_Mul = findViewById<View>(R.id.btn_Mul) as Button
btn_calc = findViewById<View>(R.id.btn_calc) as Button
btn_dec = findViewById<View>(R.id.btn_dec) as Button
btn_clear = findViewById<View>(R.id.btn_clear) as Button
ed1 = findViewById<View>(R.id.edText1) as EditText

btn_0.setOnClickListener { ed1!!.setText(ed1!!.text.toString() + "0") }

btn_1.setOnClickListener { ed1!!.setText(ed1!!.text.toString() + "1") }

btn_2.setOnClickListener { ed1!!.setText(ed1!!.text.toString() + "2") }

btn_3.setOnClickListener { ed1!!.setText(ed1!!.text.toString() + "3") }

btn_4.setOnClickListener { ed1!!.setText(ed1!!.text.toString() + "4") }

btn_5.setOnClickListener { ed1!!.setText(ed1!!.text.toString() + "5") }

btn_6.setOnClickListener { ed1!!.setText(ed1!!.text.toString() + "6") }

btn_7.setOnClickListener { ed1!!.setText(ed1!!.text.toString() + "7") }

btn_8.setOnClickListener { ed1!!.setText(ed1!!.text.toString() + "8") }

btn_9.setOnClickListener { ed1!!.setText(ed1!!.text.toString() + "9") }

btn_dec.setOnClickListener { ed1!!.setText(ed1!!.text.toString() + ".") }

 btn_Add.setOnClickListener {
if (ed1 == null) {
ed1!!.setText("")
} else {
Value1 = java.lang.Float.parseFloat(ed1!!.text.toString() + "")
mAddition = true
ed1!!.text = null
}
}

btn_Sub.setOnClickListener {
Value1 = java.lang.Float.parseFloat(ed1!!.text.toString() + "")
mSubtract = true
ed1!!.text = null
}

btn_Mul.setOnClickListener {
Value1 = java.lang.Float.parseFloat(ed1!!.text.toString() + "")
mMultiplication = true
ed1!!.text = null
}

btn_Div.setOnClickListener {
Value1 = java.lang.Float.parseFloat(ed1!!.text.toString() + "")
mDivision = true
ed1!!.text = null
}

btn_calc.setOnClickListener {
Value2 = java.lang.Float.parseFloat(ed1!!.text.toString() + "")

if (mAddition == true) {

ed1!!.setText((Value1 + Value2).toString() + "")

mAddition = false
}

if (mSubtract == true) {
ed1!!.setText((Value1 - Value2).toString() + "")
mSubtract = false
}

if (mMultiplication == true) {
ed1!!.setText((Value1 * Value2).toString() + "")
mMultiplication = false
}

if (mDivision == true) {
ed1!!.setText((Value1 / Value2).toString() + "")
mDivision = false
}
}

btn_clear.setOnClickListener { ed1!!.setText("") }
}