Minor adaptions after review

* Add missing since annotation in javadoc
* Made class PipelineElementResolved immutable (as stated by its
  annotation) and added a test for this

Signed-off-by: Yannic Klem <Yannic.Klem@bosch.io>

placeholders/src/main/java/org/eclipse/ditto/placeholders/PipelineElement.java

@@ -63,6 +63,7 @@ public interface PipelineElement extends Iterable<String> {
      *
      * @param pipelineElement the pipeline element to concatenate
      * @return the pipeline element which holds the concatenated resolved vales.
+     * @since 2.4.0
      */
     PipelineElement concat(PipelineElement pipelineElement);
 

placeholders/src/main/java/org/eclipse/ditto/placeholders/PipelineElementResolved.java

@@ -12,7 +12,9 @@
  */
 package org.eclipse.ditto.placeholders;
 
+import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Objects;
@@ -31,10 +33,10 @@
 @Immutable
 final class PipelineElementResolved implements PipelineElement {
 
-    private final Collection<String> values;
+    private final List<String> values;
 
     private PipelineElementResolved(final Collection<String> values) {
-        this.values = ConditionChecker.checkNotEmpty(values, "values");
+        this.values = Collections.unmodifiableList(new ArrayList<>(ConditionChecker.checkNotEmpty(values, "values")));
     }
 
     static PipelineElement of(final Collection<String> values) {

placeholders/src/test/java/org/eclipse/ditto/placeholders/PipelineElementResolvedTest.java

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2022 Contributors to the Eclipse Foundation
+ *
+ * See the NOTICE file(s) distributed with this work for additional
+ * information regarding copyright ownership.
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-2.0
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ */
+package org.eclipse.ditto.placeholders;
+
+import static org.mutabilitydetector.unittesting.MutabilityAssert.assertInstancesOf;
+import static org.mutabilitydetector.unittesting.MutabilityMatchers.areImmutable;
+
+import org.junit.Test;
+
+public final class PipelineElementResolvedTest {
+
+    @Test
+    public void assertImmutability() {
+        assertInstancesOf(PipelineElementResolved.class, areImmutable());
+    }
+
+}