nth-prime: make use instance method

exercises/nth-prime/src/example/java/Prime.java → exercises/nth-prime/src/example/java/PrimeCalculator.java

 import java.util.stream.IntStream;
 
-public final class Prime {
-    public static int nth(int nth) {
+public final class PrimeCalculator {
+    public int nth(int nth) {
         if (nth < 1) {
             throw new IllegalArgumentException();
         }
         return possiblePrime;
     }
 
-    private static boolean isPrime(int n) {
+    private boolean isPrime(int n) {
         if (n == 1) {
             return false;
         }

exercises/nth-prime/src/test/java/PrimeTest.java → exercises/nth-prime/src/test/java/PrimeCalculatorTest.java

-import org.junit.Test;
+import org.junit.Before;
 import org.junit.Ignore;
 import org.junit.Rule;
+import org.junit.Test;
 import org.junit.rules.ExpectedException;
 
 import static org.hamcrest.CoreMatchers.*;
 import static org.junit.Assert.*;
 
-public class PrimeTest {
+public class PrimeCalculatorTest {
+    private PrimeCalculator primeCalculator;
+
+    @Before
+    public void setup() {
+        primeCalculator = new PrimeCalculator();
+    }
 
     @Rule
     public ExpectedException thrown = ExpectedException.none();
-    
+
     @Test
     public void testFirstPrime() {
-        assertThat(Prime.nth(1), is(2));
+        assertThat(primeCalculator.nth(1), is(2));
     }
 
     @Ignore
     @Test
     public void testSecondPrime() {
-        assertThat(Prime.nth(2), is(3));
+        assertThat(primeCalculator.nth(2), is(3));
     }
 
     @Ignore
     @Test
     public void testSixthPrime() {
-        assertThat(Prime.nth(6), is(13));
+        assertThat(primeCalculator.nth(6), is(13));
     }
 
     @Ignore
     @Test
     public void testBigPrime() {
-        assertThat(Prime.nth(10001), is(104743));
+        assertThat(primeCalculator.nth(10001), is(104743));
     }
 
     @Ignore
     @Test
     public void testUndefinedPrime() {
         thrown.expect(IllegalArgumentException.class);
-        Prime.nth(0);
+        primeCalculator.nth(0);
     }
-    
+
 }