如何在Kotlin中过滤ArrayList,以便我只有符合条件的元素？

Question

我有一个数组:

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

我必须过滤列表,所以我只留下"January".

Answer 1

您可以使用此代码从数组中过滤掉January

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()

Answer 2

有许多用于过滤集合的函数,如果只想保持值匹配"January",可以使用简单的filter():

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

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

这些将为您提供仅包含的列表"January".

如果您希望所有月份都没有 "January",您可以使用反转条件!=,或使用filterNot():

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

这些将为您提供包含"February"和的列表"March".

请注意,与Java不同,在Kotlin中使用==和!=运算符实际上与equals在对象上调用函数相同.有关更多信息,请参阅有关相等性的文档.

有关标准库中的集合函数的完整列表,请参阅API参考.

Answer 3

您想要过滤包含月份的字符串列表.

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

您可以使用filterNot()列表的方法.它返回一个包含除给定谓词之外的所有元素的列表.

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

您可以使用filter()列表的方法.它返回一个列表,其中包含与给定谓词匹配的所有元素.

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

后filter()如果我们使用single()方法,那么它会返回一个值,如果超过一个值列表抛出异常.

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

Answer 4

您也可以使用find或findLast。这特别意味着仅返回一个值，而不是String在filter.

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

Answer 5

我只是分享,如果你有自定义列表并检查它是否为空或空白你可以在单行检查Kotlin只是这样做

  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
  }

您也可以检查多个条件

 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
  }

注意:我假设label和value是Custom Model类的变量.

Answer 6

按谓词过滤

    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!")
    }