Merge ListUtil into StringUtil; refactor HelpSyntaxHelper + create test

The HelpSyntaxHelper had suppressed warnings for string concatenation within StringBuilder - the point of the StringBuilder is that it is faster when you use it to concatenate many elements. If you still use string concatenation with + within these calls it beats the purpose.
(cherry picked from commit bb00be2)

src/main/java/fr/xephi/authme/command/CommandParts.java

@@ -3,7 +3,6 @@ package fr.xephi.authme.command;
 import java.util.ArrayList;
 import java.util.List;
 
-import fr.xephi.authme.util.ListUtils;
 import fr.xephi.authme.util.StringUtils;
 
 /**
@@ -165,8 +164,8 @@ public class CommandParts {
      *
      * @param other The other reference.
      *
-    
-     * @return The result from zero to above. A negative number will be returned on error. */
+     * @return The result from zero to above. A negative number will be returned on error.
+     */
     public double getDifference(CommandParts other) {
         return getDifference(other, false);
     }
@@ -177,8 +176,8 @@ public class CommandParts {
      * @param other The other reference.
      * @param fullCompare True to compare the full references as far as the range reaches.
      *
-    
-     * @return The result from zero to above. A negative number will be returned on error. */
+     * @return The result from zero to above. A negative number will be returned on error.
+     */
     public double getDifference(CommandParts other, boolean fullCompare) {
         // Make sure the other reference is correct
         if(other == null)
@@ -196,10 +195,10 @@ public class CommandParts {
     /**
      * Convert the parts to a string.
      *
-    
-     * @return The part as a string. */
+     * @return The part as a string.
+     */
     @Override
     public String toString() {
-        return ListUtils.implode(this.parts, " ");
+        return StringUtils.join(" ", this.parts);
     }
 }

src/main/java/fr/xephi/authme/command/help/HelpSyntaxHelper.java

@@ -1,13 +1,15 @@
 package fr.xephi.authme.command.help;
 
+import fr.xephi.authme.util.StringUtils;
 import org.bukkit.ChatColor;
 
 import fr.xephi.authme.command.CommandArgumentDescription;
 import fr.xephi.authme.command.CommandDescription;
 import fr.xephi.authme.command.CommandParts;
-import fr.xephi.authme.util.ListUtils;
 
 /**
+ * Helper class for formatting a command's structure (name and arguments)
+ * for a Minecraft user.
  */
 public final class HelpSyntaxHelper {
 
@@ -16,52 +18,63 @@ public final class HelpSyntaxHelper {
     }
 
     /**
-     * Get the proper syntax for a command.
+     * Get the formatted syntax for a command.
      *
-     * @param commandDescription The command to get the syntax for.
+     * @param commandDescription The command to build the syntax for.
      * @param commandReference The reference of the command.
      * @param alternativeLabel The alternative label to use for this command syntax.
      * @param highlight True to highlight the important parts of this command.
      *
      * @return The command with proper syntax.
      */
-    @SuppressWarnings("StringConcatenationInsideStringBufferAppend")
-    public static String getCommandSyntax(CommandDescription commandDescription, CommandParts commandReference, String alternativeLabel, boolean highlight) {
-        // Create a string builder to build the command
-        StringBuilder sb = new StringBuilder();
-
-        // Set the color and prefix a slash
-        sb.append(ChatColor.WHITE + "/");
+    public static String getCommandSyntax(CommandDescription commandDescription, CommandParts commandReference,
+                                          String alternativeLabel, boolean highlight) {
+        // Create a string builder with white color and prefixed slash
+        StringBuilder sb = new StringBuilder()
+            .append(ChatColor.WHITE)
+            .append("/");
 
         // Get the help command reference, and the command label
         CommandParts helpCommandReference = commandDescription.getCommandReference(commandReference);
-        final String parentCommand = (new CommandParts(helpCommandReference.getRange(0, helpCommandReference.getCount() - 1))).toString();
-        String commandLabel = helpCommandReference.get(helpCommandReference.getCount() - 1);
+        final String parentCommand = new CommandParts(
+                helpCommandReference.getRange(0, helpCommandReference.getCount() - 1)).toString();
 
         // Check whether the alternative label should be used
-        if (alternativeLabel != null && alternativeLabel.trim().length() > 0) {
+        String commandLabel;
+        if (StringUtils.isEmpty(alternativeLabel)) {
+            commandLabel = helpCommandReference.get(helpCommandReference.getCount() - 1);
+        } else {
             commandLabel = alternativeLabel;
         }
 
         // Show the important bit of the command, highlight this part if required
-        sb.append(ListUtils.implode(parentCommand, (highlight ? ChatColor.YELLOW + "" + ChatColor.BOLD : "") + commandLabel, " "));
-        if(highlight)
-            sb.append(ChatColor.YELLOW);
+        sb.append(parentCommand)
+            .append(" ")
+            .append(highlight ? ChatColor.YELLOW.toString() + ChatColor.BOLD : "")
+            .append(commandLabel);
 
-        // Add each command arguments
-        for(CommandArgumentDescription arg : commandDescription.getArguments()) {
-            // Add the argument as optional or non-optional argument
-            if(!arg.isOptional())
-                sb.append(ChatColor.ITALIC + " <" + arg.getLabel() + ">");
-            else
-                sb.append(ChatColor.ITALIC + " [" + arg.getLabel() + "]");
+        if (highlight) {
+            sb.append(ChatColor.YELLOW);
+        }
+
+        // Add each command argument
+        for (CommandArgumentDescription arg : commandDescription.getArguments()) {
+            sb.append(ChatColor.ITALIC).append(formatArgument(arg));
         }
 
         // Add some dots if the command allows unlimited arguments
-        if(commandDescription.getMaximumArguments() < 0)
-            sb.append(ChatColor.ITALIC + " ...");
+        if (commandDescription.getMaximumArguments() < 0) {
+            sb.append(ChatColor.ITALIC).append(" ...");
+        }
 
         // Return the build command syntax
         return sb.toString();
     }
+
+    private static String formatArgument(CommandArgumentDescription argument) {
+        if (argument.isOptional()) {
+            return " [" + argument.getLabel() + "]";
+        }
+        return " <" + argument.getLabel() + ">";
+    }
 }