How can I filter an ArrayList in Kotlin so I only have elements which match my condition?

I have an array:

var month: List<String> = arrayListOf("January", "February", "March")

I have to filter the list so I am left with only `"January"`.

You want to filter this list of Strings containing months.

<!-- language-all: kotlin -->

var month : List<String> = arrayListOf("January", "February", "March")

You can use [`filterNot()`](

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) method of list. It returns a list containing all elements except the given predicate.

var filteredMonthList : List<String> = month.filterNot { s -> s == "January" }
// results: ["February", "March"]

You can use [`filter()`](

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) method of list. It returns a list containing all elements matching the given predicate.

var filteredMonthList : List<String> = month.filter { s -> s == "January" }
// results: ["January"]

After `filter()` if we use [`single()`](

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) method then it will return a single value and throw an exception if more than one value is in the list.

var filteredMonth : String = month.filter { s -> s == "January" }.single()
// result: "January"

There are a number of functions for filtering collections, if you want to keep only values matching `"January"`, you can use the simple [`filter()`](

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):

<!-- language-all: kotlin -->

val months = listOf("January", "February", "March")

months.filter { month -> month == "January" } // with explicit parameter name
months.filter { it == "January" } // with implicit parameter name "it"

These will give you a list containing only `"January"`.

If you want all months that are *not* `"January"`, you can either reverse the condition using `!=`, or use [`filterNot()`](

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):

months.filter { it != "January" }
months.filterNot { it == "January" }

These will give you a list containing `"February"` and `"March"`.

Note that unlike Java, using the `==` and `!=` operators in Kotlin is actually the same as calling the `equals` function on the objects. For more, see the docs about [equality](

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).

For the complete list of collection functions in the standard library, see the [API reference](

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).

I am just sharing that if you have **custom list** and check whether it is **null or blank** you can check in Kotlin in single line
Just do it like that

fun filterList(listCutom: List<Custom>?) {
var fiterList = listCutom!!.filter { it.label != "" }
//Here you can get the list which is not having any kind of lable blank
}

You can check multiple conditions also

fun filterList(listCutom: List<Custom>?) {
var fiterList = listCutom!!.filter { it.label != "" && it.value != ""}
//Here you can get the list which is not having any kind of lable or value blank
}

Note : I am assuming that **label & value** are the variables of **Custom** Model class.

You can also use [find][1] or `findLast`. This is specifically meant to return only one value instead of a list of `String` returned in case of `filter`.

var month = arrayListOf("January", "February", "March")
var result = month.find { s -> s == "January" }


[1]:

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You can use this code to filter out January from array, by using this code

<!-- language-all: kotlin -->

var month: List<String> = arrayListOf("January", "February", "March")
// to get the result as list
var monthList: List<String> = month.filter { s -> s == "January" }

// to get a string
var selectedMonth: String = month.filter { s -> s == "January" }.single()

**Filtering by predicate**

val numbers = listOf("one", "two", "three", "four")
var items: List<String> = numbers.filter { s -> s == "one" }

var item = numbers.singleOrNull { it == "one" }

if (item != null) {
print("FOUND:$item")
} else {
print("Not FOUND!")
}