Combine collect of list with single value using streams

Question

I have the following data:

 CustomClass first = new CustomClass(1, Arrays.asList(1, 2, 3));
 CustomClass second  = new CustomClass(5, Arrays.asList(6,7,8));
 List<CustomClass> list = Arrays.asList(first, second);

Now I want to collect this data into a list that contains the value from CustomClass and values from its list.

I want to obtain this: 1, 1, 2, 3, 5, 6, 7, 8

This is what I implemented:

list.stream().forEach(item -> {
        result.add(item.value);
        item.values.stream().forEach(seconditem -> result.add(seconditem));
    });

I know there's a collect method for stream, but I don't understand how to use it to collect both one value and the list of values from the same class.

If nothing else, you can shorten it to `item.values.stream().forEach(result::add)` or just `result.addAll(item.values())` — tobias_k, Aug 23 '17 at 08:11
@tobias_k but I want also to add that one value from class to the list — , Aug 23 '17 at 08:18
@sweepserene Sure, I meant you can just shorten that one very long line to that somewhat shorter line. The rest remains the same. Or use `Stream.concat`... — tobias_k, Aug 23 '17 at 08:21
yes, they have to be in the order i mentioned, value than the values from list — , Aug 23 '17 at 08:27

score 22 · Accepted Answer · answered Aug 23 '17 at 08:17

22

You can use Stream.concat to combine a Stream.of the single value and the stream of the other values:

List<Integer> result = list.stream()
        .flatMap(x -> Stream.concat(Stream.of(x.value), x.values.stream()))
        .collect(Collectors.toList());

answered Aug 23 '17 at 08:17

tobias_k

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Combine collect of list with single value using streams

1 Answers1