Add Sum of Squares exercise

(cherry picked from commit d66e448)

Author: tboychuk

pom.xml

@@ -12,6 +12,7 @@
         <module>math-functions</module>
         <module>account-analytics</module>
         <module>account-data</module>
+        <module>sum-of-squares</module>
     </modules>
 
     <properties>

sum-of-squares/README.MD

@@ -0,0 +1,18 @@
+# <img src="https://raw.githubusercontent.com/bobocode-projects/resources/master/image/logo_transparent_background.png" height=50/>Sum of squares exercise :muscle:
+Improve your functional programming skills
+### Task
+`SumOfSquares` is a class that allows you to calculate a sum of squares in a provided range. It is implemented using 
+ OO approach. Your job is to **refactor the todo section using functional approach.** So the **implementation will not use 
+ mutable variables**, and all test will pass.
+ 
+### Pre-conditions :heavy_exclamation_mark:
+You're supposed to be familiar with Java 8
+
+### How to start :question:
+* Just clone the repository and start working on the **todo** section, verify your changes by running tests
+* If you don't have enough knowledge about this domain, check out the [links below](#related-materials-information_source)
+* Don't worry if you got stuck, checkout the branch **exercise/completed** and see the final implementation
+ 
+### Related materials :information_source:
+ * [Functional programming tutorial](https://github.com/bobocode-projects/java-functional-features-tutorial/tree/master/functional-programming-basics) <img src="https://raw.githubusercontent.com/bobocode-projects/resources/master/image/logo_transparent_background.png" height=20/>
+

sum-of-squares/pom.xml

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <parent>
+        <artifactId>java-functional-features-exercises</artifactId>
+        <groupId>com.bobocode</groupId>
+        <version>1.0-SNAPSHOT</version>
+    </parent>
+    <modelVersion>4.0.0</modelVersion>
+
+    <artifactId>sum-of-squares</artifactId>
+
+
+</project>

sum-of-squares/src/main/java/com/bobocode/SumOfSquares.java

@@ -0,0 +1,35 @@
+package com.bobocode;
+
+
+import com.bobocode.exception.InvalidRangeException;
+
+
+/**
+ * This class allow to calculate a sum of squares of integer number in a certain range. It was implemented using
+ * OO approach. Your job is to refactor it using functional approach. E.g. avoid using mutable variables
+ */
+public class SumOfSquares {
+    public static void main(String[] args) {
+        System.out.println("Sum of squares from 5 to 10 is " + calculateSumOfSquaresInRange(5, 10));
+    }
+
+    /**
+     * This method calculates the sum of squares of integer in the range
+     *
+     * @param startInclusive first element in range
+     * @param endInclusive last element in range
+     * @return the sum of squares of each element in the range
+     */
+    static int calculateSumOfSquaresInRange(int startInclusive, int endInclusive) {
+        if (endInclusive < startInclusive) {
+            throw new InvalidRangeException();
+        }
+
+        // todo: refactor using functional approach
+        int sumOfSquares = 0;
+        for (int i = startInclusive; i <= endInclusive; i++) {
+            sumOfSquares += i * i;
+        }
+        return sumOfSquares;
+    }
+}

sum-of-squares/src/main/java/com/bobocode/exception/InvalidRangeException.java

@@ -0,0 +1,4 @@
+package com.bobocode.exception;
+
+public class InvalidRangeException extends RuntimeException {
+}

sum-of-squares/src/test/java/com/bobocode/SumOfSquareTest.java

@@ -0,0 +1,46 @@
+package com.bobocode;
+
+import com.bobocode.exception.InvalidRangeException;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+import static org.junit.Assert.assertEquals;
+
+@RunWith(JUnit4.class)
+public class SumOfSquareTest {
+
+    @Test
+    public void testCalculateSumOfSquaresOfZero() {
+        int sumOfSquares = SumOfSquares.calculateSumOfSquaresInRange(0, 0);
+
+        assertEquals(0, sumOfSquares);
+    }
+
+    @Test
+    public void testCalculateSumOfSquaresOfOne() {
+        int sumOfSquares = SumOfSquares.calculateSumOfSquaresInRange(0, 1);
+
+        assertEquals(1, sumOfSquares);
+    }
+
+    @Test
+    public void testCalculateSumOfSquares() {
+        int sumOfSquares = SumOfSquares.calculateSumOfSquaresInRange(1, 5); // 1*1 + 2*2 + 3*3 + 4*4 + 5*5 = 55
+
+        assertEquals(55, sumOfSquares);
+    }
+
+    @Test
+    public void testCalculateSumOfSquaresOnNegative() {
+        int sumOfSquares = SumOfSquares.calculateSumOfSquaresInRange(-4, -2); // -4*(-4) + -3*(-3) + -2*(-2) = 29
+
+        assertEquals(29, sumOfSquares);
+    }
+
+    @Test(expected = InvalidRangeException.class)
+    public void testWithInvalidRange() {
+        SumOfSquares.calculateSumOfSquaresInRange(4, 1);
+    }
+
+}