Move most API of CommandParts to util classes

Gradual transition to removing CommandParts altogether.
 - Move logic from CommandParts to utils classes

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

@@ -0,0 +1,47 @@
+package fr.xephi.authme.util;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Utils class for collections.
+ */
+public final class CollectionUtils {
+
+    private CollectionUtils() {
+    }
+
+    /**
+     * Get a range from a list based on start and count parameters in a safe way.
+     *
+     * @param start The start index
+     * @param count The number of elements to add
+     *
+     * @return The sublist consisting at most of {@code count} elements (less if the parameters
+     * exceed the size of the list)
+     */
+    public static List<String> getRange(List<String> list, int start, int count) {
+        if (start >= list.size() || count <= 0) {
+            return new ArrayList<>();
+        } else if (start < 0) {
+            start = 0;
+        }
+        int end = Math.min(list.size(), start + count);
+        return list.subList(start, end);
+    }
+
+    /**
+     * Get all elements from a list starting from the given index.
+     *
+     * @param start The start index
+     *
+     * @return The sublist of all elements from index {@code start} and on; empty list
+     * if the start index exceeds the list's size
+     */
+    public static List<String> getRange(List<String> list, int start) {
+        if (start >= list.size()) {
+            return new ArrayList<>();
+        }
+        return getRange(list, start, list.size() - start);
+    }
+}

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

@@ -29,8 +29,9 @@ public final class StringUtils {
      */
     public static double getDifference(String first, String second) {
         // Make sure the strings are valid.
-        if (first == null || second == null)
+        if (first == null || second == null) {
             return 1.0;
+        }
 
         // Create a string similarity service instance, to allow comparison
         StringSimilarityService service = new StringSimilarityServiceImpl(new LevenshteinDistanceStrategy());