Get All object from a list <object> which id contains in List<long>

Question

How can i get List<Object> filteredList whose id (Object.id) contains in a given List<id> idsList from another List<Object>allObjects. what will be the effective and efficient way in terms of time to solve this considering a moderate data volume.

I am using java6

Answer 1

I don't want to iterate both the list so many times

Why? Premature optimisation is a bad thing. Test it first, measure whether it is efficient enough, and address the problem if it exists.

You can accomplish this with streams using a simple filter:

class Student
{
    long id;
}

final List<Student> students = /*something*/;
final List<Long> rollNoList = /*something*/;

List<Student> newStudents = students.stream()
                                    .filter(student -> rollNoList.contains(student.id))
                                    .collect(Collectors.toList());

The advantage of doing it with streams is that you may be able to parallelise it later.

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An additional optimisation would be to examine your use of data structures. As Seelenvirtuose points out, using something like a HashSet will reduce the complexity of contains from O(n) to O(1):

final Set<Long> rollNoList = new HashSet<>();

If you can't do this, you may also see some performance gain, at the cost of increased memory usage, by copying the List into a HashSet before filtering:

final Set<Long> rollNumbers = new HashSet<>(rollNoList);

but if you have control over the data structure, just use HashSet from the beginning.