Add checkstyle rule for StringBuilder.append(char)

And replace all instances where String.Builder.append() is called with a
String of length one with append(char).

Also adds StringUtils.toStringBuilder(Collection, String).

smack-core/src/integration-test/java/org/jivesoftware/smack/MessageTest.java

@@ -160,7 +160,7 @@ public class MessageTest extends SmackTestCase {
         Message msg = new Message(getFullJID(1), Message.Type.chat);
         StringBuilder sb = new StringBuilder(5000);
         for (int i = 0; i <= 4000; i++) {
-            sb.append("X");
+            sb.append('X');
         }
         msg.setBody(sb.toString());
 

smack-core/src/main/java/org/jivesoftware/smack/filter/AbstractListFilter.java

@@ -19,10 +19,10 @@ package org.jivesoftware.smack.filter;
 
 import java.util.ArrayList;
 import java.util.Arrays;
-import java.util.Iterator;
 import java.util.List;
 
 import org.jivesoftware.smack.util.Objects;
+import org.jivesoftware.smack.util.StringUtils;
 
 public abstract class AbstractListFilter implements StanzaFilter {
 
@@ -67,14 +67,8 @@ public abstract class AbstractListFilter implements StanzaFilter {
         StringBuilder sb = new StringBuilder();
         sb.append(getClass().getSimpleName());
         sb.append(": (");
-        for (Iterator<StanzaFilter> it = filters.iterator(); it.hasNext();) {
-            StanzaFilter filter = it.next();
-            sb.append(filter.toString());
-            if (it.hasNext()) {
-                sb.append(", ");
-            }
-        }
-        sb.append(")");
+        sb.append(StringUtils.toStringBuilder(filters, ", "));
+        sb.append(')');
         return sb.toString();
     }
 }

smack-core/src/main/java/org/jivesoftware/smack/util/StringUtils.java

@@ -222,21 +222,32 @@ public class StringUtils {
     }
 
     /**
-     * Transform a collection of CharSequences to a whitespace delimited String.
+     * Transform a collection of objects to a whitespace delimited String.
      *
      * @param collection the collection to transform.
      * @return a String with all the elements of the collection.
      */
-    public static String collectionToString(Collection<? extends CharSequence> collection) {
+    public static String collectionToString(Collection<? extends Object> collection) {
+        return toStringBuilder(collection, " ").toString();
+    }
+
+    /**
+     * Transform a collection of objects to a delimited String.
+     *
+     * @param collection the collection to transform.
+     * @param delimiter the delimiter used to delimit the Strings.
+     * @return a StringBuilder with all the elements of the collection.
+     */
+    public static StringBuilder toStringBuilder(Collection<? extends Object> collection, String delimiter) {
         StringBuilder sb = new StringBuilder(collection.size() * 20);
-        for (Iterator<? extends CharSequence> it = collection.iterator(); it.hasNext();) {
-            CharSequence cs = it.next();
+        for (Iterator<? extends Object> it = collection.iterator(); it.hasNext();) {
+            Object cs = it.next();
             sb.append(cs);
             if (it.hasNext()) {
-                sb.append(' ');
+                sb.append(delimiter);
             }
         }
-        return sb.toString();
+        return sb;
     }
 
     public static String returnIfNotEmptyTrimmed(String string) {