kotlin-fork/libraries/stdlib/src/kotlin

Collections API

There are a number of extension functions on Collection, Iterator, Map and arrays to allow easy composition collections using familiar combinators like

• filter()
• flatMap()
• map()
• fold()

Functions on Collection, Map and arrays are all eager, in that methods like map() will create a complete result List when the method completes.

Functions on Iterator are lazy so that they try to return new iterators that lazily evaluate things. For example map() returns a new Iterator that lazily maps the values in the original iterator.

Preconditions

When writing code it is recommended you add checks early in a function to ensure parameters are valid. There are a number of helper methods for this:

• assert(Boolean) and assert(Boolean) { lazyMessage } which use the JDK's assertion feature so they can be disabled using the -ea option
• check(Boolean) and check(Boolean) { lazyMessage } for checking something to be true and throwing IllegalStateException if not
• require(Boolean) and require(Boolean) { lazyMessage } for requiring something to be true and throwing IllegalArgumentException if not