Andoroid Studio電卓について

https://qiita.com/yanasep/items/503b378c439752f70880
現在、このサイトを参考にプログラムを作っているのですが表示させるだけで数字を表示して計算することが出来ません。

プログラム↓
package com.example.dentaku

import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import com.example.dentaku.R
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

var nStr : String = ""
val nList = ArrayList<Double>() // arraylist to store numbers
val oList = ArrayList<Char>() // arraylist to store operations

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    num0.setOnClickListener {
        formula.text = "${formula.text}0"
        nStr += "0"
    }
    num1.setOnClickListener {
        formula.text = "${formula.text}1"
        nStr += "1"
    }
    num2.setOnClickListener {
        formula.text = "${formula.text}2"
        nStr += "2"
    }
    num3.setOnClickListener {
        formula.text = "${formula.text}3"
        nStr += "3"
    }
    num4.setOnClickListener {
        formula.text = "${formula.text}4"
        nStr += "4"
    }
    num5.setOnClickListener {
        formula.text = "${formula.text}5"
        nStr += "5"
    }
    num6.setOnClickListener {
        formula.text = "${formula.text}6"
        nStr += "6"
    }
    num7.setOnClickListener {
        formula.text = "${formula.text}7"
        nStr += "7"
    }
    num8.setOnClickListener {
        formula.text = "${formula.text}8"
        nStr += "8"
    }
    num9.setOnClickListener {
        formula.text = "${formula.text}9"
        nStr += "9"
    }
    point.setOnClickListener {
        formula.text = "${formula.text}."
        nStr += "."
    }
    equal.setOnClickListener {
        formula.text = "${formula.text}="
        addList(nStr)
        var result = calcualte().toString()
        formula.text = result
        nStr = result
        nList.clear()
        oList.clear()
    }
    add.setOnClickListener {
        formula.text = "${formula.text}+"
        addList(nStr,'+')
        nStr = ""
    }
    subtract.setOnClickListener {
        formula.text = "${formula.text}-"
        addList(nStr,'-')
        nStr = ""
    }
    multiply.setOnClickListener {
        formula.text = "${formula.text}*"
        addList(nStr,'*')
        nStr = ""
    }
    divide.setOnClickListener {
        formula.text = "${formula.text}/"
        addList(nStr,'/')
        nStr = ""
    }
    delete.setOnClickListener {
        var formulaStr = formula.text.toString()
        if (!formulaStr.isEmpty()) {
            formula.text = formulaStr.subSequence(0,formulaStr.lastIndex)
        }
        if (!nStr.isEmpty()) {
            nStr = nStr.substring(0, nStr.lastIndex)
        }
    }
    percent.setOnClickListener {
        formula.text = "${formula.text}%"
    }
    sign.setOnClickListener {

    }
    clear.setOnClickListener {
        formula.text = ""
        nStr = ""
        nList.clear()
        oList.clear()
    }

} // end fun onCreate

fun addList(str : String, ope : Char) {
    try {
        var num = str.toDouble()
        nList.add(num)
        oList.add(ope)
    }catch(e:Exception){
        formula.text = "Numeric error"
    }
}

fun addList(str : String) {
    try {
        var num = str.toDouble()
        nList.add(num)
    }catch(e:Exception){
        formula.text = "Numeric error"
    }
}

fun calcualte() : Double {

    var i = 0
    while (i < oList.size) {
        //do multiplication and division first
        if(oList.get(i) == '*' || oList.get(i) == '/') {
            var result = if (oList.get(i) == '*') nList.get(i) * nList.get(i+1) else nList.get(i) / nList.get(i+1)
            nList.set(i,result)
            nList.removeAt(i+1)
            oList.removeAt(i)
            i--
        }
        // change subtraction to addition
        else if(oList.get(i) == '-'){
            oList.set(i,'+')
            nList.set(i+1,nList.get(i+1) * -1)
        }
        i++
    }

    // get sum
    var result = 0.0
    for (i in nList){
        result += i
    }

    return result
}// end fun calcualte

} // end class