{"id":197,"date":"2023-08-18T21:14:00","date_gmt":"2023-08-18T13:14:00","guid":{"rendered":"https:\/\/blog.bexonbai.com\/?p=197"},"modified":"2026-03-02T21:15:49","modified_gmt":"2026-03-02T13:15:49","slug":"closure-in-kotlin","status":"publish","type":"post","link":"https:\/\/blog.bexonbai.com\/?p=197","title":{"rendered":"Closure in Kotlin"},"content":{"rendered":"\n

In Kotlin, a lambda expression or anonymous function (as well as a local function<\/a> and an object expression<\/a>) can access its closure<\/em><\/strong>, which includes the variables declared in the outer scope. The variables captured in the closure can be modified within the lambda.<\/p>\n\n\n\n

Here is an example from Kotlin\u2019s website:<\/p>\n\n\n\n

var<\/strong> sum = 0\nints.filter<\/strong> { it > 0 }.forEach<\/strong> {\n    sum += it\n}\nprint<\/strong>(sum)\n<\/code><\/pre>\n\n\n\n
Java 8 introduced lambdas, which can access external variables but cannot modify them. This limitation restricts the ability to perform functional programming in Java 8.<\/p>\n\n\n\n
This example will result in a compilation error:<\/p>\n\n\n\n
List<\/strong><<\/strong>String<\/strong>><\/strong> strings =<\/strong> Arrays<\/strong>.<\/strong>asList(<\/strong>\"sup1\",<\/strong> \"sup2\",<\/strong> \"sup3\");<\/strong>\nList<\/strong><<\/strong>String<\/strong>><\/strong> transformers =<\/strong> Arrays<\/strong>.<\/strong>asList(<\/strong>\"trans1\",<\/strong> \"trans2\",<\/strong> \"trans3\");<\/strong>\n\n@Test\npublic<\/strong> void<\/strong> demo2<\/strong>()<\/strong> {<\/strong>\n    strings.<\/strong>stream().<\/strong>map(<\/strong>s -><\/strong> transformers.<\/strong>stream().<\/strong>forEach(<\/strong>t -><\/strong> {<\/strong>  \/\/ There will be error here.<\/em>\n        s =<\/strong> (<\/strong>t +<\/strong> \", \" +<\/strong> s);<\/strong>\n    })).<\/strong>forEach(<\/strong>System<\/strong>.<\/strong>out::<\/strong>println);<\/strong>\n}<\/strong>\n<\/code><\/pre>\n\n\n\n
error: incompatible types: inference variable R has incompatible bounds\n        strings.stream()<\/strong>.map(<\/strong>s -> transformers.stream()<\/strong>.forEach(<\/strong>t -> {<\/strong>\n                            ^\n    lower bounds: Object\n    lower bounds: void\n  where R,T are type-variables:\n    R extends Object declared in <\/strong>method <R>map(<\/strong>Function<? super T,? extends R>)<\/strong>\n    T extends Object declared in <\/strong>interface Stream\n<\/code><\/pre>\n\n\n\n
but not in Kotlin.<\/p>\n\n\n\n
var<\/strong> strings: List<\/strong><String<\/strong>> = mutableListOf<\/strong>(\"sup1\", \"sup2\", \"sup3\")\nvar<\/strong> transformers: List<\/strong><String<\/strong>> = mutableListOf<\/strong>(\"trans1\", \"trans2\", \"trans3\")\n\n@Test\nfun<\/strong> demo2<\/strong>() {\n    strings.stream<\/strong>().map<Any<\/strong>> { s: String<\/strong> ->\n        var<\/strong> s = s\n        transformers.stream<\/strong>().forEach<\/strong> { t: String<\/strong> -> s = \"$t, $s\" }\n    }.forEach<\/strong> { o: Any<\/strong>? -> println<\/strong>(o) }\n}\n<\/code><\/pre>\n\n\n\n
Extended link:<\/em><\/p>\n\n\n\n
Closure in Swift<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"
In Kotlin, a lambda expression or anonymous function (as well as a local function and an object expression) can access its closure, which includes the variables declared in the outer scope. The variables captured in the closure can be modified within the lambda.<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[],"class_list":["post-197","post","type-post","status-publish","format-standard","hentry","category-develop"],"_links":{"self":[{"href":"https:\/\/blog.bexonbai.com\/index.php?rest_route=\/wp\/v2\/posts\/197","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blog.bexonbai.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blog.bexonbai.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blog.bexonbai.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/blog.bexonbai.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=197"}],"version-history":[{"count":1,"href":"https:\/\/blog.bexonbai.com\/index.php?rest_route=\/wp\/v2\/posts\/197\/revisions"}],"predecessor-version":[{"id":198,"href":"https:\/\/blog.bexonbai.com\/index.php?rest_route=\/wp\/v2\/posts\/197\/revisions\/198"}],"wp:attachment":[{"href":"https:\/\/blog.bexonbai.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=197"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blog.bexonbai.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=197"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blog.bexonbai.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=197"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}