Java – how do I combine two streams?

I'm trying to learn / understand the flow in Java and have this 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 hint on how to combine two streams into one stream?

——-Added 23.08 2018 – if we get rid of the IDS variable, it will improve the performance and the following code execution. Because we use set < integer > tagsids, there are no duplicates (for example, if tagids contains values (5,6,7,8,5,7), it can only be used for (5,8)) The modified code is as follows:

List <Tag> tags = (classA.getTags() != null ? classA.getTags() : new ArrayList<>());
List <Integer> tagIds = new ArrayList<>(Arrays.asList(5,7));
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 such as the complexity of reading and debugging code

Solution

List<Tag> combinedTags = Stream

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());