A set is a collection that contains no duplicate items. This means you can't have i1 and i2 in the collection if i1==i2 (which translates to i1.equals(i2) == true). The same reasoning applies for a null reference - you can't have more than one null item stored in your set.

To create instances of sets, you can use any of the methods in the following code example:

 data class Book(val author: String, val title: String, val year: Int, val isbn: String) val intSet: Set<Int> = setOf(1, 21, 21, 2, 6, 3, 2) //1,21,2,6,3 println("Set of integers[${intSet.javaClass.canonicalName}]:$intSet") val hashSet: java.util.HashSet<Book> = hashSetOf( Book("Jules Verne", "Around the World in 80 Days Paperback", 2014, "978-1503215153"), Book("George R.R. Martin", ...

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