Android Programming – Part 8 Kotlin Fundamentals Ⅷ: Collection Filters

Kotlin’s filtering functions are quite unique and very clean when compared to other legacy languages’ syntax, such as Java or C/C++. This is some way of eliminating the repetitive if-statement, isn’t it? Here’s a description from Kotlin’s official page.

Filtering is one of the most popular tasks in collection processing. In Kotlin, filtering conditions are defined by predicates – lambda functions that take a collection element and return a boolean value: true means that the given element matches the predicate, false means the opposite.

The standard library contains a group of extension functions that let you filter collections in a single call. These functions leave the original collection unchanged, so they are available for both mutable and read-only collections. To operate the filtering result, you should assign it to a variable or chain the functions after filtering.

kotlinlang.org

And here are some of the codes I coded on my Linux environment.

filer {} by length

CollectionFilter2.kt

fun listOfMyMutableSet(myList: MutableList<String>) {
    val found = myList.filter {
        it.length > 4
    }
    println(found)
}

fun main() {
    val mylist = mutableListOf("James", "Sam", "Foxx")
    listOfMyMutableSet(mylist)
}

$java -jar CollectionFilter2.jar
[James]

filer {} by ‘==’ match-comparison

CollectionFilter3.kt

fun listOfMyMutableSet(myList: MutableList<String>) {
    val found = myList.filter {
        it == "James"
    }
    println(found)
}

fun main() {
    val mylist = mutableListOf("James", "Sam", "Foxx")
    listOfMyMutableSet(mylist)
}

$java -jar CollectionFilter3.jar
[James]

filer {} with endsWith() function

CollectionFilter4.kt

fun listOfMyMutableSet(myList: MutableList<String>) {
    val found = myList.filter {
        it.endsWith("x")
    }
    println(found)
}

fun main() {
    val mylist = mutableListOf("James", "Sam", "Foxx")
    listOfMyMutableSet(mylist)
}

johnito@skynewubuntuserver:testdir$ java -jar CollectionFilter4.jar
[Foxx]

filer {} with startsWith() function

CollectionFilter5.kt

fun listOfMyMutableSet(myList: MutableList<String>) {
    val found = myList.filter {
        it.startsWith("J", ignoreCase = true)
    }
    println(found)
}

fun main() {
    val mylist = mutableListOf("James", "Sam", "Foxx")
    listOfMyMutableSet(mylist)
}

johnito@skynewubuntuserver:testdir$ java -jar CollectionFilter5.jar
[James]

via:

kotlinlang.org