How to combine two streams?

koniga-ganica :

I'm trying to learn/understand streams in java and have this piece of code:

List <Tag> tags = (classA.getTags() != null ? classA.getTags() : new ArrayList<>());
List <Integer> tagsIds = new ArrayList<>(Arrays.asList(1,2,3,4));
List<Integer> ids = tags.stream().map(Tag::getId).collect(Collectors.toList());
tagIds.stream()
      .filter(tagId -> !ids.contains(tagId))
      .forEach(tagId -> {
         Tag tag = new Tag();
         tag.setId(tagId);
         tags.add(tag);
       });

Please, give me a tip how I can combine two streams into one?

------- Added 23.08.2018 --------
If we get rid of ids variable it will improve a bit performance and code below perform as we work with Set <Integer> tagsIds, so no duplicates (e.g. if tagIds contains values (5,6,7,8,5,6,7) it will work only with (5,6,7,8)). Below the modified code:

List <Tag> tags = (classA.getTags() != null ? classA.getTags() : new ArrayList<>());
List <Integer> tagIds = new ArrayList<>(Arrays.asList(5,6,7,8,5,6,7));
tagIds.stream()
      .filter(tagId -> !tags.stream().map(Tag::getId).collect(Collectors.toList()).contains(tagId))
      .forEach(tagId -> {
            Tag tag = new Tag();
            tag.setId(tagId);
            tags.add(tag);
       });

This modification has disadvantages like the complexity of reading and debugging code

Fritz Duchardt :

List<Tag> combinedTags = Stream
        .concat( // combine streams
                tags.stream(),
                tagIds.stream().map(Tag::new) // assuming constructor with id parameter
        )
        .distinct() // get rid of duplicates assuming correctly implemented equals method in Tag
        .collect(Collectors.toList());