Calculator.java

package com.serenitydojo.calculator;

import com.google.common.base.Splitter;

import java.util.List;

import static org.apache.commons.lang3.StringUtils.isNumeric;

public class Calculator {
    public int evaluate(String expression) {
        if (expression.isEmpty()) {
            return 0;
        }
        List<String> tokens = Splitter.on(" ").splitToList(expression);

        int runningTotal = 0;
        String nextOperator = "+";

        for(String token : tokens) {
            if (!isNumeric(token)) {
                nextOperator = token;
            } else {
                runningTotal = process(runningTotal, nextOperator, token);
            }
        }

        return runningTotal;
    }

    private int process(int runningTotal, String nextOperator, String token) {
        switch (nextOperator) {
            case "+":
                return runningTotal + Integer.parseInt(token);
            case "-":
                return runningTotal - Integer.parseInt(token);
            case "*":
                return runningTotal * Integer.parseInt(token);
            case "/":
                return runningTotal / Integer.parseInt(token);
            default:
                throw new IllegalMathsOperatorException("Unknown operator " + nextOperator);
        }
    }

}