String Calculator

pom.xml

@@ -94,5 +94,15 @@
         </plugin>
       </plugins>
     </pluginManagement>
+      <plugins>
+          <plugin>
+              <groupId>org.apache.maven.plugins</groupId>
+              <artifactId>maven-compiler-plugin</artifactId>
+              <configuration>
+                  <source>10</source>
+                  <target>10</target>
+              </configuration>
+          </plugin>
+      </plugins>
   </build>
 </project>

src/main/java/com/serenitydojo/Exceptions/IllegalMathsOperatorException.java

@@ -0,0 +1,7 @@
+package com.serenitydojo.Exceptions;
+
+public class IllegalMathsOperatorException extends RuntimeException {
+    public IllegalMathsOperatorException(String message) {
+        super(message);
+    }
+}

src/main/java/com/serenitydojo/calculator/Calculator.java

@@ -1,7 +1,43 @@
 package com.serenitydojo.calculator;
 
+import com.serenitydojo.Exceptions.IllegalMathsOperatorException;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Collectors;
+
 public class Calculator {
     public int evaluate(String expression) {
-        return 0;
+        List<String> splittedExpression = Arrays.stream(expression.split(" ")).collect(Collectors.toList());
+        List<String> operands = List.of("1","2","3","4","5","6","7","8","9","0");
+
+        int expressionTotal =0;
+        String operand ="+";
+
+        if (expression.isEmpty()) {
+            return 0;
+        }
+        for(String character : splittedExpression) {
+            if (!operands.contains(character)) {
+                operand = character;
+            } else {
+                expressionTotal = calculateOperation(expressionTotal, operand, character);
+            }
+        }
+        return expressionTotal;
+    }
+
+    private int calculateOperation(int total, String operand, String token) {
+        switch (operand) {
+            case "+":
+                return total + Integer.parseInt(token);
+            case "-":
+                return total - Integer.parseInt(token);
+            case "*":
+                return total * Integer.parseInt(token);
+            default:
+                throw new IllegalMathsOperatorException(operand + "Does not exist");
+        }
     }
 }
+

src/test/java/com/serenitydojo/calculator/WhenDoingMaths.java

@@ -1,5 +1,6 @@
 package com.serenitydojo.calculator;
 
+import com.serenitydojo.Exceptions.IllegalMathsOperatorException;
 import org.apache.commons.lang3.StringUtils;
 import org.junit.Test;
 
@@ -13,38 +14,38 @@ public void shouldReturnZeroForAnEmptyString() {
         assertThat(calculator.evaluate("")).isEqualTo(0);
     }
 
-//    @Test(expected = IllegalMathsOperatorException.class)
-//    public void shouldReportNonSupportedOperations() {
-//        calculator.evaluate("1 ^ 2");
-//    }
-//
-//    @Test
-//    public void shouldReturnTheValueOfASingleNumber() {
-//        assertThat(calculator.evaluate("1")).isEqualTo(1);
-//    }
-//
-//    @Test
-//    public void shouldAddTwoNumbers() {
-//        assertThat(calculator.evaluate("1 + 1")).isEqualTo(2);
-//    }
-//
-//    @Test
-//    public void shouldAddThreeNumbers() {
-//        assertThat(calculator.evaluate("1 + 2 + 3")).isEqualTo(6);
-//    }
-//
-//    @Test
-//    public void shouldAlsoSubtract() {
-//        assertThat(calculator.evaluate("10 - 6")).isEqualTo(4);
-//    }
-//
-//    @Test
-//    public void shouldAddAndSubtract() {
-//        assertThat(calculator.evaluate("10 + 5 - 6")).isEqualTo(9);
-//    }
-//
-//    @Test
-//    public void shouldMultiplyNumbers() {
-//        assertThat(calculator.evaluate("10 * 5")).isEqualTo(50);
-//    }
+    @Test(expected = IllegalMathsOperatorException.class)
+    public void shouldReportNonSupportedOperations() {
+        calculator.evaluate("1 ^ 2");
+    }
+
+    @Test
+    public void shouldReturnTheValueOfASingleNumber() {
+        assertThat(calculator.evaluate("1")).isEqualTo(1);
+    }
+
+   @Test
+    public void shouldAddTwoNumbers() throws IllegalMathsOperatorException {
+        assertThat(calculator.evaluate("1 + 1")).isEqualTo(2);
+    }
+
+    @Test
+    public void shouldAddThreeNumbers() {
+        assertThat(calculator.evaluate("1 + 2 + 3")).isEqualTo(6);
+    }
+
+    @Test
+    public void shouldAlsoSubtract() {
+        assertThat(calculator.evaluate("10 - 6")).isEqualTo(4);
+    }
+
+    @Test
+    public void shouldAddAndSubtract() {
+        assertThat(calculator.evaluate("10 + 5 - 6")).isEqualTo(9);
+    }
+
+    @Test
+    public void shouldMultiplyNumbers() {
+        assertThat(calculator.evaluate("10 * 5")).isEqualTo(50);
+    }
 }