CODEWITHSUNDEEP
javalistcollections
OutOfMemoryError
OutOfMemoryError

Reputation: 421

From List of Set of String to a plain list

Is there a way to convert a List<Set<String>> mainList to a plain List, without iterating over elements?

For example this one has value:

mainList = {ArrayList@705}  size = 2
 0 = {HashSet@708}  size = 3
  0 = "A2"
  1 = "A1"
  2 = "A3"
 1 = {HashSet@709}  size = 3
  0 = "A6"
  1 = "A5"
  2 = "A7"

I would like to have a new list like so:

list = A2,A1,A3, A6, A5, A7

Upvotes: 0

Views: 312

Answers (3)

Ravi
Ravi

Reputation: 729

If you are only curious about not using iterator, you can use simple for each loop to solve the purpose

    List<Set<String>> hs = null ; // Actual given List<Set<String>
    ArrayList<String> arrayList = new ArrayList<String>(); // Plain List that will contain all the strings
    for(Set<String> set :hs) {
        arrayList.addAll(new ArrayList<String>(set)); // adding all the elements in list from hashset
    }

and with using streams(java 1.8 and above) in this way

    List<Set<String>> list = null ;
    List<String> al =  hs.stream().flatMap(Set::stream).collect(Collectors.toList());

but can you please explain why you don't want to use iterators?

Upvotes: 2

Aniket Sahrawat
Aniket Sahrawat

Reputation: 12937

If you are using java 1.8 and above, then you can use streams but it will still use an internal iterator. Here is an example:

List<String> list = mainList.stream() // create stream
    .flatMap(Set::stream)             // convert Set<String> to Stream
    .collect(Collectors.toList());    // collect to new ArrayList

Upvotes: 1

Michael
Michael

Reputation: 44240

You can't. Ordinarily, the only way to copy n things is to iterate over each of them.

The only way to avoid iterating over elements would be a lower-level operation like an array copy.

An ArrayList would do this (others like LinkedList would not) but no Set implementation in the JDK provides its own toArray implementation. They all use AbstractCollection.toArray which internally iterates over all of the elements.

If you implemented or found an array-based Set implementation (which would almost certainly not be an optimal Set, however) then you could flatten an ArrayList<ArraySet<String>> by using a series of array copies without iterating over the elements.

Upvotes: 1

Related Questions