Fix joinOn

pmd-core/src/main/java/net/sourceforge/pmd/util/CollectionUtil.java

@@ -626,12 +626,12 @@ public final class CollectionUtil {
                                            String delimiter) {
         boolean first = true;
         for (T t : iterable) {
-            appendItem.accept(sb, t);
             if (first) {
                 first = false;
             } else {
                 sb.append(delimiter);
             }
+            appendItem.accept(sb, t);
         }
         return sb;
     }

pmd-core/src/test/java/net/sourceforge/pmd/util/CollectionUtilTest.java

@@ -0,0 +1,37 @@
+/*
+ * BSD-style license; for more info see http://pmd.sourceforge.net/license.html
+ */
+
+package net.sourceforge.pmd.util;
+
+import static net.sourceforge.pmd.util.CollectionUtil.listOf;
+import static org.junit.Assert.assertEquals;
+
+import java.util.Collections;
+import java.util.List;
+
+import org.junit.Test;
+
+import net.sourceforge.pmd.lang.document.Chars;
+
+/**
+ * @author Clément Fournier
+ */
+public class CollectionUtilTest {
+
+    @Test
+    public void testJoinOn() {
+        testJoinOn(listOf("a", "b", "c"), ".",
+                   "a.b.c");
+        testJoinOn(Collections.emptyList(), ".",
+                   "");
+    }
+
+    private void testJoinOn(List<String> toJoin, String delimiter, String expected) {
+        String actual = CollectionUtil.joinCharsIntoStringBuilder(
+            CollectionUtil.map(toJoin, Chars::wrap),
+            delimiter
+        ).toString();
+        assertEquals(expected, actual);
+    }
+}