difference-of-squares: adjust API to use instance method

Fixes issue #253.

exercises/difference-of-squares/src/example/java/Difference.java → exercises/difference-of-squares/src/example/java/DifferenceOfSquaresCalculator.java

 import java.util.stream.IntStream;
 
-public final class Difference {
+public final class DifferenceOfSquaresCalculator {
 
-    public static int computeSquareOfSumTo(final int input) {
+    public int computeSquareOfSumTo(final int input) {
         final int sum = input * (input + 1) / 2;
         return (int) Math.pow(sum, 2);
     }
 
-    public static int computeSumOfSquaresTo(final int input) {
+    public int computeSumOfSquaresTo(final int input) {
         return IntStream.rangeClosed(1, input)
                 .map(i -> (int) Math.pow(i, 2))
                 .sum();
     }
 
-    public static int betweenSquareOfSumAndSumOfSquaresTo(final int input) {
+    public int betweenSquareOfSumAndSumOfSquaresTo(final int input) {
         return computeSquareOfSumTo(input) - computeSumOfSquaresTo(input);
     }
 

exercises/difference-of-squares/src/main/java/Difference.java

-public final class Difference {
-    
-
-
-}

exercises/difference-of-squares/src/main/java/DifferenceOfSquaresCalculator.java

+public final class DifferenceOfSquaresCalculator {
+    
+
+
+}

exercises/difference-of-squares/src/test/java/DifferenceTest.java → exercises/difference-of-squares/src/test/java/DifferenceOfSquaresCalculatorTest.java

+import org.junit.Before;
 import org.junit.Ignore;
 import org.junit.Test;
 
 import static org.junit.Assert.assertEquals;
 
-public final class DifferenceTest {
+public final class DifferenceOfSquaresCalculatorTest {
+    private DifferenceOfSquaresCalculator calculator;
+
+    @Before
+    public void setUp() {
+        calculator = new DifferenceOfSquaresCalculator();
+    }
 
     @Test
     public void testSquareOfSum5() {
         final int expected = 225;
-        final int actual = Difference.computeSquareOfSumTo(5);
+        final int actual = calculator.computeSquareOfSumTo(5);
         assertEquals(expected, actual);
     }
 
     @Test
     public void testSquareOfSum10() {
         final int expected = 3025;
-        final int actual = Difference.computeSquareOfSumTo(10);
+        final int actual = calculator.computeSquareOfSumTo(10);
         assertEquals(expected, actual);
     }
 
     @Test
     public void testSquareOfSum100() {
         final int expected = 25502500;
-        final int actual = Difference.computeSquareOfSumTo(100);
+        final int actual = calculator.computeSquareOfSumTo(100);
         assertEquals(expected, actual);
     }
 
     @Test
     public void testSumOfSquares5() {
         final int expected = 55;
-        final int actual = Difference.computeSumOfSquaresTo(5);
+        final int actual = calculator.computeSumOfSquaresTo(5);
         assertEquals(expected, actual);
     }
 
     @Test
     public void testSumOfSquares10() {
         final int expected = 385;
-        final int actual = Difference.computeSumOfSquaresTo(10);
+        final int actual = calculator.computeSumOfSquaresTo(10);
         assertEquals(expected, actual);
     }
 
     @Test
     public void testSumOfSquares100() {
         final int expected = 338350;
-        final int actual = Difference.computeSumOfSquaresTo(100);
+        final int actual = calculator.computeSumOfSquaresTo(100);
         assertEquals(expected, actual);
     }
 
     @Test
     public void testDifferenceOfSquares0() {
         final int expected = 0;
-        final int actual = Difference.betweenSquareOfSumAndSumOfSquaresTo(0);
+        final int actual = calculator.betweenSquareOfSumAndSumOfSquaresTo(0);
         assertEquals(expected, actual);
     }
 
     @Test
     public void testDifferenceOfSquares5() {
         final int expected = 170;
-        final int actual = Difference.betweenSquareOfSumAndSumOfSquaresTo(5);
+        final int actual = calculator.betweenSquareOfSumAndSumOfSquaresTo(5);
         assertEquals(expected, actual);
     }
 
     @Test
     public void testDifferenceOfSquares10() {
         final int expected = 2640;
-        final int actual = Difference.betweenSquareOfSumAndSumOfSquaresTo(10);
+        final int actual = calculator.betweenSquareOfSumAndSumOfSquaresTo(10);
         assertEquals(expected, actual);
     }
 
     @Test
     public void testDifferenceOfSquares100() {
         final int expected = 25164150;
-        final int actual = Difference.betweenSquareOfSumAndSumOfSquaresTo(100);
+        final int actual = calculator.betweenSquareOfSumAndSumOfSquaresTo(100);
         assertEquals(expected, actual);
     }