CalculatorResource.java

package resources;

import javax.ws.rs.Consumes;
import javax.ws.rs.POST;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;

/**
 * Calculator resource exposed at '/calculator' path
 */
@Path("/calculator")
public class CalculatorResource {

/**
 * Method handling HTTP POST requests. The calculated answer to the expression will be sent to the client as
 * plain/text.
 *
 * @param expression the expression to be solved as plain/text.
 * @return solution to the expression as plain/text or -1 on error
 */
@POST
@Consumes(MediaType.TEXT_PLAIN)
@Produces(MediaType.TEXT_PLAIN)
public int calculate(String expression) {
    // Removes all whitespaces
    String expressionTrimmed = expression.replaceAll("\\s+", "");

    int result = -1;

    /*
     * .matches use regex expression to decide if a String matches a given pattern.
     * [0-9]+[+][0-9]+ explained:
     * [0-9]+: a number from 0-9 one or more times. For example 1, 12 and 123.
     * [+]: the operator + one time.
     * The total regex expression accepts for example:
     * 12+34,
     * 1+2,
     * 10000+1000
     */
    if (expressionTrimmed.matches("[0-9]+[+][0-9]+")) result = sum(expressionTrimmed);
    else if (expressionTrimmed.matches("[0-9]+[-][0-9]+")) result = subtraction(expressionTrimmed);

    return result;
}

    public int multiplication(String equation) {
    String[] split = equation.split("[*]");

    int number1 = Integer.parseInt(split[0]);
    int number2 = Integer.parseInt(split[1]);

    return number1 * number2;
}

    public int division(String equation) {
    String[] split = equation.split("[/]");

    int number1 = Integer.parseInt(split[0]);
    int number2 = Integer.parseInt(split[1]);

    return number1 / number2;
}

    /**
     * Method used to calculate a sum expression.
     * @param expression the equation to be calculated as a String
     * @return the answer as an int
     */
    public int sum (String expression){
        String[] split = expression.split("[+]");

        int number1 = Integer.parseInt(split[0]);
        int number2 = Integer.parseInt(split[1]);

        return number1 + number2;
    }

    /**
     * Method used to calculate a subtraction expression.
     * @param expression the expression to be calculated as a String
     * @return the answer as an int
     */
    public int subtraction (String expression){
        String[] split = expression.split("[-]");

        int number1 = Integer.parseInt(split[0]);
        int number2 = Integer.parseInt(split[1]);

        return number1 - number2;
    }
}