adapt javadoc and delete unused code

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

gateway/service/src/main/java/org/eclipse/ditto/gateway/service/endpoints/routes/policies/OAuthTokenIntegrationSubjectIdFactory.java

@@ -52,6 +52,7 @@ public Set<SubjectId> getSubjectIds(final DittoHeaders dittoHeaders, final JsonW
                 PlaceholderFactory.newPlaceholderResolver(JwtPlaceholder.getInstance(), jwt)
         );
         return expressionResolver.resolvePartially(subjectTemplate, Set.of(JwtPlaceholder.PREFIX))
+                .toStream()
                 .map(SubjectId::newInstance)
                 .collect(Collectors.toCollection(LinkedHashSet::new));
     }

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

@@ -73,14 +73,14 @@ default PipelineElement resolve(final String expressionTemplate) {
      * @param expressionTemplate the expressionTemplate to resolve {@link Placeholder}s and execute optional
      * pipeline stages
      * @param forbiddenUnresolvedExpressionPrefixes a collection of expression prefixes which must be resolved
-     * @return the resolved String, a signifier for resolution failure, or one for deletion.
+     * @return the resolved PipelineElement.
      * @throws PlaceholderFunctionTooComplexException thrown if the {@code expressionTemplate} contains a placeholder
      * function chain which is too complex (e.g. too much chained function calls)
      * @throws UnresolvedPlaceholderException if placeholders could not be resolved which contained prefixed in the
      * provided {@code forbiddenUnresolvedExpressionPrefixes} list.
-     * @since 2.0.0
+     * @since 2.4.0
      */
-    default Stream<String> resolvePartially(final String expressionTemplate,
+    default PipelineElement resolvePartially(final String expressionTemplate,
             final Collection<String> forbiddenUnresolvedExpressionPrefixes) {
 
         return ExpressionResolver.substitute(expressionTemplate, expression -> {
@@ -102,7 +102,7 @@ default Stream<String> resolvePartially(final String expressionTemplate,
                 }
                 return PipelineElement.resolved("{{" + expression + "}}");
             });
-        }).toStream();
+        });
     }
 
     /**

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

@@ -149,14 +149,6 @@ private Pipeline getPipelineFromExpressions(final List<String> pipelineStagesExp
         return new ImmutablePipeline(ImmutableFunctionExpression.INSTANCE, pipelineStages);
     }
 
-    private ArrayPipeline getArrayPipelineFromExpressions(final List<String> pipelineStagesExpressions,
-            final int skip) {
-        final List<String> pipelineStages = pipelineStagesExpressions.stream()
-                .skip(skip) // ignore pre-processed expressions
-                .collect(Collectors.toList());
-        return new ImmutableArrayPipeline(ImmutableArrayFunctionExpression.INSTANCE, pipelineStages);
-    }
-
     private Optional<String> getPlaceholderPrefix(final String placeholder) {
         final int separatorIndex = placeholder.indexOf(SEPARATOR);
         if (separatorIndex == -1) {

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

@@ -16,7 +16,7 @@
 import java.util.stream.Collectors;
 
 /**
- * Resolves a passed in placeholder {@code name} from the {@link #getPlaceholderSources()} () resolver}.
+ * Resolves a passed in placeholder {@code name} from the {@link #getPlaceholderSources() sources}.
  * If this PlaceholderResolver is only used for validation, a constant value of {@code "valid"} is returned instead
  * of asking the resolver.
  */
@@ -27,6 +27,12 @@ public interface PlaceholderResolver<T> extends Placeholder<T> {
      */
     List<T> getPlaceholderSources();
 
+    /**
+     * Resolves the passed in {@code name} from the {@link #getPlaceholderSources() sources}.
+     *
+     * @param name the placeholder name to resolve from the resolver.
+     * @return the resolved value or an empty optional if it could not be resolved.
+     */
     default List<String> resolve(final String name) {
         return getPlaceholderSources().stream()
                 .flatMap(source -> resolve(source, name).stream())