how can I get specific value of an array list using stream

Question

I have an ArrayList of Person ordered by ids the 0 as the id:1 the 1 as the id:2 etc and I want to get only some of them in another ArrayList using stream here's what I tried but it don't seems to work:

public ArrayList<Person> getPersonfoById(int... ids) {
    if (IntStream.of(ids).anyMatch(id -> id > totalpersonnumber)) {
        throw new IllegalArgumentException("the given person id" + Arrays.asList(IntStream.of(ids).filter(i -> i >= totalpersonnumber).boxed().toArray()) + " dosen't exist in " + fileName);
    }
    try (IntStream stream = Arrays.stream(ids)) {
        return (ArrayList<Pokemon>) stream.boxed().map(x -> myArrayOfPerson.get(x))
                                                  .collect(Collectors.toList());
    }
}

Have you tried Stream::filter in combination with Stream::collect and Collectors.toList()? — Turing85
– Turing85, Commented Dec 29, 2019 at 23:39
"I want to get only some of them in another ArrayList" what's your filter criteria? — Robert Bain
– Robert Bain, Commented Dec 29, 2019 at 23:39
I have an array list of Person a person is defined by an id, a name and his gender all my Person are in an array list I want with this method to enter ids like 1,2,18,20 and get a new array list of person with only the ids I gave — lolozen
– lolozen, Commented Dec 30, 2019 at 17:04

Nikolas · Accepted Answer · 2020-01-01 10:01:37Z

Since the input of selected ids might vary, the best way is to use List::contains in the filter's predicate:

// Get a list of ids at the beginning once and use with each iteration
List<Integer> idsList = Arrays.stream(ids).boxed().collect(Collectors.toList());

List<Person> personList myArrayOfPerson.stream()         // Stream<Person>
    .filter(person -> idsList.contains(person.getId()))  // Stream<Person> of the given ids
    .collect(Collectors.toList());                       // List<Person>

This is the solution where the id is a part of the object Person and not driven by the behavior of the List which is not a good practice.

However, if you insist to get these on the certain positions in the List, then a bit different approach is required:

List<Person> personList = Arrays.stream(ids)             // IntStream
    .boxed()                                             // Stream<Integer>
    .map(myArrayOfPerson::get)                           // Stream<Person>
    .collect(Collectors.toList());                       // List<Person>

This is a fail-fast approach which doesn't handle the ids out of the bounds. You should know that List::get might throw an IndexOutOfBoundsException, so to avoid this, filter out the ids which doesn't fit the list size:

List<Person> personList = Arrays.stream(ids)             // IntStream
    .filter(id -> id > 0 && id < myArrayOfPerson.size()) // IntStream (wanted ids only)
    .boxed()                                             // Stream<Integer>
    .map(myArrayOfPerson::get)                           // Stream<Person>
    .collect(Collectors.toList());                       // List<Person>

An alternative way would be to return null objects for unusual ids or log a message or handle it another way which is out of the scope of this answer.

I like the way @Nikolas writes the solution, he always adds up comment at each step in #stream, making it comfortable for newbies to know what is going on.

Karim SNOUSSI · Accepted Answer · 2019-12-30 00:13:52Z

0

Maybe you need something like this

Arrays.stream(ids).filter(id->id>20).toArray();

For stream uses visit this link

answered Dec 30, 2019 at 0:13

Karim SNOUSSI

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