数组

数组

数组是一种保存固定数量相同类型或其子类型的值的数据结构。 Kotlin 中最常见的数组类型是对象类型数组，由 Array 类表示。

如果在对象类型数组中使用原生类型，那么会对性能产生影响，因为原生值都装箱成了对象。为了避免装箱开销，请改用原生类型数组。

何时使用数组

当需要满足特殊的底层需求时，可以在 Kotlin 中使用数组。例如，如果性能要求超出了常规应用程序的需要，或者需要构建自定义的数据结构。如果没有这些限制，那么请改用集合。

与数组相比，集合具有以下优点：

集合可以是只读的，这提供了更多的控制权而支持编写具有明确意图的健壮代码。

易于对集合增删元素。相比之下，数组大小是固定的。对数组增删元素的唯一方式是每次创建一个新数组，效率很低：

fun main() {
//sampleStart
    var riversArray = arrayOf("Nile", "Amazon", "Yangtze")
    // 使用 += 赋值操作创建了一个新的 riversArray，
    // 复制了原始元素并添加了“Mississippi”
    riversArray += "Mississippi"
    println(riversArray.joinToString())
    // Nile, Amazon, Yangtze, Mississippi
//sampleEnd
}

可以使用相等操作符（==）来检验两个集合是否在结构上相等。但不能对数组使用这个操作符。相反，必须使用一个特殊函数，关于这点可以参阅比较数组了解更多信息。

关于集合的更多信息请参见集合概述。

创建数组

To create arrays in Kotlin, you can use:

functions, such as arrayOf(), arrayOfNulls()) or emptyArray().
the Array constructor.

This example uses the arrayOf() function and passes item values to it:

fun main() {
//sampleStart
    // Creates an array with values [1, 2, 3]
    val simpleArray = arrayOf(1, 2, 3)
    println(simpleArray.joinToString())
    // 1, 2, 3
//sampleEnd
}

This example uses the arrayOfNulls()) function to create an array of a given size filled with null elements:

fun main() {
//sampleStart
    // Creates an array with values [null, null, null]
    val nullArray: Array<Int?> = arrayOfNulls(3)
    println(nullArray.joinToString())
    // null, null, null
//sampleEnd
}

This example uses the emptyArray() function to create an empty array :

    var exampleArray = emptyArray<String>()

You can specify the type of the empty array on the left-hand or right-hand side of the assignment due to Kotlin’s type inference.

For example:
var exampleArray = emptyArray<String>()
var exampleArray: Array<String> = emptyArray()

The Array constructor takes the array size and a function that returns values for array elements given its index:

fun main() {
//sampleStart
    // Creates an Array<Int> that initializes with zeros [0, 0, 0]
    val initArray = Array<Int>(3) { 0 }
    println(initArray.joinToString())
    // 0, 0, 0
    // 创建一个 Array<String> 初始化为 ["0", "1", "4", "9", "16"]
    val asc = Array(5) { i -> (i * i).toString() }
    asc.forEach { print(it) }
    // 014916
//sampleEnd
}

Like in most programming languages, indices start from 0 in Kotlin.

嵌套数组

Arrays can be nested within each other to create multidimensional arrays:

fun main() {
//sampleStart
    // Creates a two-dimensional array
    val twoDArray = Array(2) { Array<Int>(2) { 0 } }
    println(twoDArray.contentDeepToString())
    // [[0, 0], [0, 0]]
    // Creates a three-dimensional array
    val threeDArray = Array(3) { Array(3) { Array<Int>(3) { 0 } } }
    println(threeDArray.contentDeepToString())
    // [[[0, 0, 0], [0, 0, 0], [0, 0, 0]], [[0, 0, 0], [0, 0, 0], [0, 0, 0]], [[0, 0, 0], [0, 0, 0], [0, 0, 0]]]
//sampleEnd
}

Nested arrays don’t have to be the same type or the same size.

访问与修改元素

Arrays are always mutable. To access and modify elements in an array, use the indexed access operator[]:

fun main() {
//sampleStart
    val simpleArray = arrayOf(1, 2, 3)
    val twoDArray = Array(2) { Array<Int>(2) { 0 } }
    // Accesses the element and modifies it
    simpleArray[0] = 10
    twoDArray[0][0] = 2
    // Prints the modified element
    println(simpleArray[0].toString()) // 10
    println(twoDArray[0][0].toString()) // 2
//sampleEnd
}

Arrays in Kotlin are invariant. This means that Kotlin doesn’t allow you to assign an Array<String> to an Array<Any> to prevent a possible runtime failure. Instead, you can use Array<out Any>. For more information, see Type Projections.

使用数组

In Kotlin, you can work with arrays by using them to pass a variable number of arguments to a function or perform operations on the arrays themselves. For example, comparing arrays, transforming their contents or converting them to collections.

向函数传入可变数量的实参

In Kotlin, you can pass a variable number of arguments to a function via the vararg parameter. This is useful when you don’t know the number of arguments in advance, like when formatting a message or creating an SQL query.

To pass an array containing a variable number of arguments to a function, use the spread operator (*). The spread operator passes each element of the array as individual arguments to your chosen function:

fun main() {
    val lettersArray = arrayOf("c", "d")
    printAllStrings("a", "b", *lettersArray)
    // abcd
}
fun printAllStrings(vararg strings: String) {
    for (string in strings) {
        print(string)
    }
}

For more information, see Variable number of arguments (varargs).

比较数组

To compare whether two arrays have the same elements in the same order, use the .contentEquals() and .contentDeepEquals() functions:

fun main() {
//sampleStart
    val simpleArray = arrayOf(1, 2, 3)
    val anotherArray = arrayOf(1, 2, 3)
    // Compares contents of arrays
    println(simpleArray.contentEquals(anotherArray))
    // true
    // Using infix notation, compares contents of arrays after an element 
    // is changed
    simpleArray[0] = 10
    println(simpleArray contentEquals anotherArray)
    // false
//sampleEnd
}

Don’t use equality (==) and inequality (!=) operators to compare the contents of arrays. These operators check whether the assigned variables point to the same object.

To learn more about why arrays in Kotlin behave this way, see our blog post.

转换数组

Kotlin has many useful functions to transform arrays. This document highlights a few but this isn’t an exhaustive list. For the full list of functions, see our API reference.

Sum

To return the sum of all elements in an array, use the .sum() function:

fun main() {
//sampleStart
    val sumArray = arrayOf(1, 2, 3)
    // Sums array elements
    println(sumArray.sum())
    // 6
//sampleEnd
}

The .sum() function can only be used with arrays of numeric data types, such as Int.

Shuffle

To randomly shuffle the elements in an array, use the .shuffle() function:

fun main() {
//sampleStart
    val simpleArray = arrayOf(1, 2, 3)
    // Shuffles elements [3, 2, 1]
    simpleArray.shuffle()
    println(simpleArray.joinToString())
    // Shuffles elements again [2, 3, 1]
    simpleArray.shuffle()
    println(simpleArray.joinToString())
//sampleEnd
}

将数组转换为集合

If you work with different APIs where some use arrays and some use collections, then you can convert your arrays to collections and vice versa.

转换为 List 或 Set

To convert an array to a List or Set, use the .toList() and .toSet() functions.

fun main() {
//sampleStart
    val simpleArray = arrayOf("a", "b", "c", "c")
    // Converts to a Set
    println(simpleArray.toSet())
    // [a, b, c]
    // Converts to a List
    println(simpleArray.toList())
    // [a, b, c, c]
//sampleEnd
}

转换为 Map

To convert an array to a map, use the .toMap() function.

Only an array of Pair can be converted to a map. The first value of a Pair instance becomes a key, and the second becomes a value. This example uses the infix notation to call the to function to create tuples of Pair:

fun main() {
//sampleStart
    val pairArray = arrayOf("apple" to 120, "banana" to 150, "cherry" to 90, "apple" to 140)
    // Converts to a Map
    // The keys are fruits and the values are their number of calories
    // Note how keys must be unique, so the latest value of "apple"
    // overwrites the first
    println(pairArray.toMap())
    // {apple=140, banana=150, cherry=90}
//sampleEnd
}

原生类型数组

If you use the Array class with primitive values, these values are boxed into objects. As an alternative, you can use primitive-type arrays, which allow you to store primitives in an array without the side effect of boxing overhead:

Primitive-type array	Equivalent in Java
BooleanArray	`boolean[]`
ByteArray	`byte[]`
CharArray	`char[]`
DoubleArray	`double[]`
FloatArray	`float[]`
IntArray	`int[]`
LongArray	`long[]`
ShortArray	`short[]`

These classes have no inheritance relation to the Array class, but they have the same set of functions and properties.

This example creates an instance of the IntArray class:

fun main() {
//sampleStart
    // Creates an array of Int of size 5 with values
    val exampleArray = IntArray(5)
    println(exampleArray.joinToString())
    // 0, 0, 0, 0, 0
//sampleEnd
}

To convert primitive-type arrays to object-type arrays, use the .toTypedArray() function.

To convert object-type arrays to primitive-type arrays, use .toBooleanArray(), .toByteArray(), .toCharArray(), and so on.

下一步做什么？

To learn more about why we recommend using collections for most use cases, read our Collections overview.
Learn about other basic types.
If you are a Java developer, read our Java to Kotlin migration guide for Collections.