Collection Operations: Predicates

An introduction to the most important functions for predicates: all, any, none, isEmpty, isNotEmpty, ifEmpty, contains, containsAll, and their variants.

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THE CURRENT VERSION OF THIS ARTICLE IS PUBLISHED HERE.

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Tags: #FYI++

This article is part of the Kotlin Primer, an opinionated guide to the Kotlin language, which is indented to help facilitate Kotlin adoption inside Java-centric organizations. It was originally written as an organizational learning resource for Etnetera a.s. and I would like to express my sincere gratitude for their support.

It is recommended to read the Introduction before moving on. Check out the Table of Contents for all articles.

all, any, none

inline fun <T> Iterable<T>.all(predicate: (T) -> Boolean): Boolean

inline fun <T> Iterable<T>.any(predicate: (T) -> Boolean): Boolean

The all method accepts a predicate and returns true if every element satisfies the predicate, and false otherwise. The some method has the same input, but returns true if at least one element satisfies the predicate, and false otherwise.

Example

There are also variations of any and none that take no arguments, and return true if the collection has at least one element and no elements, respectively.

Example

isEmpty, isNotEmpty, ifEmpty

fun isEmpty(): Boolean

inline fun <T> Collection<T>.isNotEmpty(): Boolean

inline fun <C : Collection<*>, R> C.ifEmpty(
    defaultValue: () -> R
): R where C : R

The isEmpty method is not an extension, but defined directly on the Collection interface. Predictably, it returns true if the collection contains no elements. For List, none is an alias for isEmpty. The opposite of isEmpty is isNotEmpty. There is also isNullOrEmpty, which is defined on a nullable receiver:

inline fun <T> Collection<T>?.isNullOrEmpty(): Boolean

Of interest is the ifEmpty method, which allows us to substitute a different Collection if the receiver is empty. Notice that the type of the return value is always the type of what is returned by defaultValue, which can be a super-type of the receiver.

Example

contains, containsAll

operator fun <T> Iterable<T>.contains(element: T): Boolean

inline fun <T> Collection<T>.containsAll(
    elements: Collection<T>
): Boolean

We already know contains from the chapter on operators. It can be invoked using the in keyword. The containsAll expands on this functionality by returning true only if the receiver contains all the elements of the argument, and false otherwise. Naturally, it is not an operator.

Example

Go back to Collection Operations: Single Element Access, jump to the Table of Contents, or continue to Collection Operations: Aggregators.