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Kingsta1993
Kingsta1993

Reputation: 25

What does List<Set<Integer>> mean?

One of the questions I have been given asks:

All the lines should be stored in an object of type List<Set<Integer>>.

How do you write this in Java, as in how do you initialise this list? I've never seen this before.

Please provide a link to an explanation as i'm not sure what this is called in Java so have no idea about how to learn about it. Thank You.

Upvotes: 1

Views: 12813

Answers (5)

Jerome Anthony
Jerome Anthony

Reputation: 8021

Its a List of Sets where each Set can hold only Integers.

Set<Integer> singlesSet = new HashSet<>();
singlesSet.add(1);
singlesSet.add(2);

Set<Integer> tensSet = new HashSet<>();
tensSet.add(10);
tensSet.add(20);

List<Set<Integer>> list = new ArrayList<>();
list.add(singlesSet);
list.add(tensSet);

System.out.println(list);

Upvotes: 5

yossarian
yossarian

Reputation: 1677

In Java, the List interface represents an abstract list of things. Any class the implements List (for example, LinkedList) must implement its methods and behave according to its contract.

You can essentially think of it as an array, but keep in mind that arrays are only one kind of list, and that implementations of List do no have to use arrays internally.

The Set also represents a collection of elements, but with no relationship or connection between them. Visually, you can think of a set as a sort of bag of things. You can add and remove things from the bag, but none of the items need to be related.

An Integer, of course, is just an object wrapper around Java's int primitive.

As such, a List<Set<Integer>> object would be similar to a two-dimensional array, only without a defined order in the second dimension.

You would initialize a List<Set<Integer>> as follows:

    List<Set<Integer>> myList = new ArrayList<HashSet<Integer>>();

Where ArrayList and HashSet can be any classes that implement List and Set, respectively.

Upvotes: 0

Gren
Gren

Reputation: 1854

The short way:

    List<Set<Integer>> list = new ArrayList<Set<Integer>>();
    Set<Integer> set = new HashSet<Integer>();
    list.add(set);
    set.add(1);
    set.add(2);
   ....

What is the difference between Set and List?

Upvotes: 0

MeetTitan
MeetTitan

Reputation: 3568

Like this List<Set<Integer>> yourList = new ArrayList<Set<Integer>>();?

You may want to take a look at https://docs.oracle.com/javase/7/docs/api/java/util/List.html

Upvotes: 0

martijnn2008
martijnn2008

Reputation: 3640

Example of usages of Set and List. Note that elements in a TreeSet are always sorted. 

List<Set<Integer>> listofsets = new ArrayList<Set<Integer>>();
Set<Integer> set1 = new TreeSet<Integer>();
set1.add(1);
set1.add(2);

Set<Integer> set2 = new TreeSet<Integer>();
set2.add(6);
set2.add(4);

listofsets.add(set);

// listofsets = {{1,2}, {4,6}}

Upvotes: 3

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