The collection interface is the main interface that represents a collection of items. The collection interface is a member of the immutable type of list and therefore provides read-only functionalities. The collection interface inherits the Iterable interface and provides its own functions:

fun collectionInterface(){
    val collectionValues : Collection <Int>    = listOf(1,2,3,4,5)
    val collectionIterator = collectionValues.iterator()
    while (collectionIterator.hasNext()) {
         print(collectionIterator.next())
     }
 }

Take a look at the following diagram, which shows the properties and functions that are provided by the collection interface:

The following is a list of the properties and functions of the collection interface:

• size: This property return the size of the collection.
• isEmpty(): This function returns true if the collection is empty. Otherwise, it returns false.
• contains(): This function returns true if the specified element is in the list. Otherwise, it returns false.
• containsAll(): This function returns true if a list is a subset of a collection. Otherwise, it returns false.

Take a look at the following example:

fun collectionFunctions(){
    val collectionValues : Collection <Int> = listOf(1,2,3,4,5)
    println("Size ${collectionValues.size}")
    println("is collection empty: ${collectionValues.isEmpty()}")
    println("collection contains element 3: ${collectionValues.contains(3)}")
 
    var mini = listOf(2,3,4)
    var answer = collectionValues.containsAll(mini)
 
    println("Does collection contain mini collection: $answer")
}

The output of the preceding code is as follows:

Size 5
is collection empty: false
contains contains element 3: true
Collection contains mini collection true