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", "Series: Game of Thrones: The Graphic  Novel (Book 1)", 2012, "978-0440423218"), 
      Book("J.K. Rowling", "Harry Potter And The Goblet Of Fire (Book  4)...