Create tests for StringUtils#getStackTrace and StringUtils#getDifference

- Create tests
- Make StringUtils final with private constructor (utility class)

src/main/java/fr/xephi/authme/util/StringUtils.java

@@ -10,10 +10,14 @@ import java.io.StringWriter;
 /**
  * Utility class for String operations.
  */
-public class StringUtils {
+public final class StringUtils {
 
     public static final String newline = System.getProperty("line.separator");
 
+    private StringUtils() {
+        // Utility class
+    }
+
     /**
      * Get the difference of two strings.
      *

src/test/java/fr/xephi/authme/util/StringUtilsTest.java

@@ -6,7 +6,9 @@ import java.net.MalformedURLException;
 import java.util.Arrays;
 import java.util.List;
 
+import static org.hamcrest.Matchers.greaterThan;
 import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.stringContainsInOrder;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
@@ -97,4 +99,32 @@ public class StringUtilsTest {
         // then
         assertThat(result, equalTo("[MalformedURLException]: Unrecognized URL format"));
     }
+
+    @Test
+    public void shouldPrintStackTrace() {
+        // given
+        MalformedURLException ex = new MalformedURLException("Unrecognized URL format");
+
+        // when
+        String result = StringUtils.getStackTrace(ex);
+
+        // then
+        assertThat(result, stringContainsInOrder(getClass().getName()));
+    }
+
+    @Test
+    public void shouldGetDifferenceWithNullString() {
+        // given/when/then
+        assertThat(StringUtils.getDifference(null, "test"), equalTo(1.0));
+        assertThat(StringUtils.getDifference("test", null), equalTo(1.0));
+        assertThat(StringUtils.getDifference(null, null), equalTo(1.0));
+    }
+
+    @Test
+    public void shouldGetDifferenceBetweenTwoString() {
+        // given/when/then
+        assertThat(StringUtils.getDifference("test", "taste"), equalTo(0.4));
+        assertThat(StringUtils.getDifference("test", "bear"), equalTo(0.75));
+        assertThat(StringUtils.getDifference("test", "something"), greaterThan(0.88));
+    }
 }