Почему stack оказывается пустым?

import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.widget.Toast;

import java.util.Scanner;
import java.util.Stack;

/**
 * Created by Admin on 02.12.2017.
 */

class PostfixCalculator extends AppCompatActivity{

    private Stack<String> stack;
    private String regular;
    private double result;

    public PostfixCalculator(String regular) {
        this.regular = regular;
        stack = new Stack<>();

        // Parse and calculate
        parse(regular);
    }

    public PostfixCalculator() {
        stack = new Stack<>();
    }

    public void startCalculate(String regular) {
        this.regular = regular;
        parse(regular);
    }

    private void parse(String s) {
        String currentElement = "";
        String[] elements = s.split(" ");
        double number1, number2, interAns = 0;

        for (int i = 0; i < elements.length; i++) {
            currentElement = elements[i];

           
            // Check if current element is number
            try {
                double element = Double.parseDouble(currentElement);
                stack.push(String.valueOf(element));
            } catch (NumberFormatException e) {
                if (!currentElement.equals("") ) {

                        number2 = Double.valueOf(stack.pop());
                        number1 = Double.valueOf(stack.pop());


            new Scanner(System.in);
                    // Make calculations
                    if (currentElement.equals("+"))
                        interAns = number1 + number2;
                    else if (currentElement.equals("-"))
                        interAns = number1 - number2;
                    else if (currentElement.equals("×"))
                        interAns = number1 * number2;
                    else if (currentElement.equals("/"))
                        interAns = number1 / number2;
                    else interAns = 0;

                    stack.push(String.valueOf(interAns));
                }
            }
            Log.d("DEB-A", "" + interAns);
        }
        interAns = Double.valueOf(stack.pop());
        result = interAns;
    }


    public double getResult() {
        return result;
    }

}

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.example.admin.calculater, PID: 4147
                  java.lang.IllegalStateException: Could not execute method for android:onClick
                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:293)
                      at android.view.View.performClick(View.java:5198)
                      at android.view.View$PerformClick.run(View.java:21147)
                      at android.os.Handler.handleCallback(Handler.java:739)
                      at android.os.Handler.dispatchMessage(Handler.java:95)
                      at android.os.Looper.loop(Looper.java:148)
                      at android.app.ActivityThread.main(ActivityThread.java:5417)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
                   Caused by: java.lang.reflect.InvocationTargetException
                      at java.lang.reflect.Method.invoke(Native Method)
                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
                      at android.view.View.performClick(View.java:5198) 
                      at android.view.View$PerformClick.run(View.java:21147) 
                      at android.os.Handler.handleCallback(Handler.java:739) 
                      at android.os.Handler.dispatchMessage(Handler.java:95) 
                      at android.os.Looper.loop(Looper.java:148) 
                      at android.app.ActivityThread.main(ActivityThread.java:5417) 
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 
                   Caused by: java.util.EmptyStackException
                      at java.util.Stack.pop(Stack.java:73)
                      at com.example.admin.calculater.PostfixCalculator.parse(PostfixCalculator.java:53)
                      at com.example.admin.calculater.PostfixCalculator.startCalculate(PostfixCalculator.java:33)
                      at com.example.admin.calculater.CalculaterActivity.onClickEqual(CalculaterActivity.java:188)
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288) 
                      at android.view.View.performClick(View.java:5198) 
                      at android.view.View$PerformClick.run(View.java:21147) 
                      at android.os.Handler.handleCallback(Handler.java:739) 
                      at android.os.Handler.dispatchMessage(Handler.java:95) 
                      at android.os.Looper.loop(Looper.java:148) 
                      at android.app.ActivityThread.main(ActivityThread.java:5417) 
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

number2 = Double.valueOf(stack.pop());
                        number1 = Double.valueOf(stack.pop());