Integer Calculator

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

@@ -1,7 +1,42 @@
 package com.serenitydojo.calculator;
 
+import com.google.common.base.Splitter;
+import org.apache.commons.lang3.StringUtils;
+
+import java.util.List;
+
 public class Calculator {
     public int evaluate(String expression) {
-        return 0;
+        if(expression.isEmpty()){
+            return 0;
+        }
+
+        List<String> characters = Splitter.on(" ").splitToList(expression);
+
+        int total = 0;
+        String operator = "+";
+
+        for(String character : characters){
+            if(StringUtils.isNumeric(character)) {
+                total = execute(total, operator, character);
+            } else {
+                operator =  character;
+            }
+        }
+
+        return total;
+    }
+
+    private int execute(int total, String operator, String character) {
+        switch (operator) {
+            case "+":
+                return total + Integer.parseInt(character);
+            case "-":
+                return total - Integer.parseInt(character);
+            case "*":
+                return total * Integer.parseInt(character);
+            default:
+                throw new IllegalMathsOperatorException("Operator " + operator + " not allowed");
+        }
     }
 }

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

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

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

@@ -13,38 +13,38 @@ 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() {
+       assertThat(calculator.evaluate("1 ^ 2")).isEqualTo(1);
+    }
+
+    @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);
+    }
 }