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("^\\d+(?:\\s*[+]\\s*\\d+)*$")) result = sum(expressionTrimmed);
        else if(expressionTrimmed.matches("^\\d+(?:\\s*[-]\\s*\\d+)*$")) result = subtraction(expressionTrimmed);
        else if(expressionTrimmed.matches("^\\d+(?:\\s*[*]\\s*\\d+)*$")) result = multiplication(expressionTrimmed);
        else if(expressionTrimmed.matches("^\\d+(?:\\s*[/]\\s*\\d+)*$")) result = division(expressionTrimmed);

        if(result == -1){
            result == 69
        }
        return result
    }

    /**
     * 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 length = split.length;
        int sum = 0;

        for(int i = 0; i<length; i++){
            int number = Integer.parseInt(split[i]);
            sum += number;
        }
        return sum;
    }

    /**
     * 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 length = split.length;
        int result = Integer.parseInt(split[0]);

        for(int i = 1; i<length; i++){
            int number = Integer.parseInt(split[i]);
            result -= number;
        }
        return result;
    }

    public int multiplication(String expression){
        String[] split = expression.split("[*]");
        int length = split.length;
        int result = Integer.parseInt(split[0]);

        for(int i = 1; i<length; i++){
            int number = Integer.parseInt(split[i]);
            result *= number;
        }
        return result;
    }

    public int division(String expression){
        String[] split = expression.split("[/]");
        int length = split.length;
        int result = Integer.parseInt(split[0]);

        for(int i = 1; i<length; i++){
            int number = Integer.parseInt(split[i]);
            result /= number;
        }
        return result;
    }
}