Simple use and precautions of kotlin's array array and set list

Original address ：Kotlin Array of array And collection list Simple use and precautions - Simple books

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Array Array

Kotlin The array in is also a container , Different types are provided ：ByteArray, CharArray, ShortArray, IntArray, LongArray, BooleanArray, FloatArray,DoubleArray, A lot of , I won't introduce you here , The usage is almost the same , Only arrays with types can only contain specified types .

initialization

        // Initialize an empty array , In fact, its size is 0 Inherit arrayOfNulls Array of 
        var array1 = emptyArray<String>()
        // Initialize an empty array , You need to pass in the array size 
        var array2 = arrayOfNulls<String>(3)
        // Initialize an array of specified size , The first parameter is size, The second parameter is the function that generates the logic ,
        //  amount to java Traversing the array , Subscript each with k Array assignment of array2[k] = k * k;
        var array3 = Array(10) { k -> k * k}
        // Initialize an array of multiple data types 
        var array4 = arrayOf("1",2,3)
        // Initialize a specific data type - An array of character types 
        var array5 = charArrayOf('d')

Let's print the initialized array , Look at the result of initialization ：

"/Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java" -ea -Didea.test.cyclic.buffer.size=1048576 -Didea.launcher.port=49730 "-Didea.launcher.bin.path=/Applications/Android Studio.app/Contents/bin" -Dfile.encoding=UTF-8 -classpath "/Applications/Android Studio.app/Contents/lib/idea_rt.jar:/Applications/Android Studio.app/Contents/plugins/junit/lib/junit-rt.jar:/Applications/Android Studio.app/Contents/plugins/junit/lib/junit5-rt.jar:/Users/kingnewspring/Library/Android/sdk/platforms/android-27/data/res:/Users/kingnewspring/Downloads/KotlinTest/app/build/intermediates/classes/debug:/Users/kingnewspring/Downloads/KotlinTest/app/build/tmp/kotlin-classes/debug:/Users/kingnewspring/Downloads/KotlinTest/app/build/generated/res/rs/debug:/Users/kingnewspring/Downloads/KotlinTest/app/build/generated/res/resValues/debug:/Users/kingnewspring/Downloads/KotlinTest/app/build/tmp/kotlin-classes/debugUnitTest:/Users/kingnewspring/.gradle/caches/modules-2/files-2.1/android.arch.lifecycle/common/1.1.0/edf3f7bfb84a7521d0599efa3b0113a0ee90f85/common-1.1.0.jar:/Users/kingnewspring/.gradle/caches/modules-2/files-2.1/com.android.support/support-annotations/27.1.1/39ded76b5e1ce1c5b2688e1d25cdc20ecee32007/support-annotations-27.1.1.jar:/Users/kingnewspring/.gradle/caches/transforms-1/files-1.1/animated-vector-drawable-27.1.1.aar/1be6be214d4bf5183a3327597856db9a/jars/classes.jar:/Users/kingnewspring/.gradle/caches/transforms-1/files-1.1/support-compat-27.1.1.aar/5318e651055d11149bff8ecc6ee849b7/jars/classes.jar:/Users/kingnewspring/.gradle/caches/transforms-1/files-1.1/support-compat-27.1.1.aar/5318e651055d11149bff8ecc6ee849b7/res:/Users/kingnewspring/.gradle/caches/modules-2/files-2.1/org.jetbrains.kotlin/kotlin-stdlib-jdk7/1.2.50/3811b225f9a22abf4f9d8a6f81adef0ba78a3c5/kotlin-stdlib-jdk7-1.2.50.jar:/Users/kingnewspring/.gradle/caches/transforms-1/files-1.1/viewmodel-1.1.0.aar/80fe4b8d90eafb67cf217b6d35b9d09e/jars/classes.jar:/Users/kingnewspring/.gradle/caches/transforms-1/files-1.1/support-vector-drawable-27.1.1.aar/c608acaab2dec891d8b39f2483c7e51b/jars/classes.jar:/Users/kingnewspring/.gradle/caches/transforms-1/files-1.1/support-core-ui-27.1.1.aar/18f9fb381d82d8ce4a1d4bda82e765cf/jars/classes.jar:/Users/kingnewspring/.gradle/caches/transforms-1/files-1.1/support-core-ui-27.1.1.aar/18f9fb381d82d8ce4a1d4bda82e765cf/res:/Users/kingnewspring/.gradle/caches/modules-2/files-2.1/org.jetbrains.kotlin/kotlin-stdlib-common/1.2.50/6b19a2fcc29d34878b3aab33fd5fcf70458a73df/kotlin-stdlib-common-1.2.50.jar:/Users/kingnewspring/.gradle/caches/transforms-1/files-1.1/support-core-utils-27.1.1.aar/dc31deadc75ff47a72c687e48f9669d7/jars/classes.jar:/Users/kingnewspring/.gradle/caches/transforms-1/files-1.1/constraint-layout-1.1.3.aar/0b9e57814d1dfbd5924ef90766162087/res:/Users/kingnewspring/.gradle/caches/transforms-1/files-1.1/constraint-layout-1.1.3.aar/0b9e57814d1dfbd5924ef90766162087/jars/classes.jar:/Users/kingnewspring/.gradle/caches/modules-2/files-2.1/org.jetbrains.kotlin/kotlin-stdlib/1.2.50/66d47b004c5b8a1d2d1df9e463187390ed741316/kotlin-stdlib-1.2.50.jar:/Users/kingnewspring/.gradle/caches/modules-2/files-2.1/org.jetbrains/annotations/13.0/919f0dfe192fb4e063e7dacadee7f8bb9a2672a9/annotations-13.0.jar:/Users/kingnewspring/.gradle/caches/transforms-1/files-1.1/support-fragment-27.1.1.aar/52d6725357d2b27fdf8d861bfd464aa4/jars/classes.jar:/Users/kingnewspring/.gradle/caches/modules-2/files-2.1/junit/junit/4.12/2973d150c0dc1fefe998f834810d68f278ea58ec/junit-4.12.jar:/Users/kingnewspring/.gradle/caches/transforms-1/files-1.1/runtime-1.1.0.aar/6c59003f6d713b57600f30ff3d7250e3/jars/classes.jar:/Users/kingnewspring/.gradle/caches/modules-2/files-2.1/org.hamcrest/hamcrest-core/1.3/42a25dc3219429f0e5d060061f71acb49bf010a0/hamcrest-core-1.3.jar:/Users/kingnewspring/.gradle/caches/transforms-1/files-1.1/appcompat-v7-27.1.1.aar/a88341bae42c2eb22d3a232380966a18/res:/Users/kingnewspring/.gradle/caches/transforms-1/files-1.1/appcompat-v7-27.1.1.aar/a88341bae42c2eb22d3a232380966a18/jars/classes.jar:/Users/kingnewspring/.gradle/caches/transforms-1/files-1.1/livedata-core-1.1.0.aar/9683c5714c058ad2ce152c04a0e3a19d/jars/classes.jar:/Users/kingnewspring/.gradle/caches/modules-2/files-2.1/android.arch.core/common/1.1.0/8007981f7d7540d89cd18471b8e5dcd2b4f99167/common-1.1.0.jar:/Users/kingnewspring/.gradle/caches/modules-2/files-2.1/com.android.support.constraint/constraint-layout-solver/1.1.3/bde0667d7414c16ed62d3cfe993cff7f9d732373/constraint-layout-solver-1.1.3.jar:/Users/kingnewspring/.gradle/caches/transforms-1/files-1.1/runtime-1.1.0.aar/e51aad556365d772d1b760355b0ea1f6/jars/classes.jar:/Users/kingnewspring/Downloads/KotlinTest/app/build/intermediates/sourceFolderJavaResources/test/debug:/Users/kingnewspring/Downloads/KotlinTest/app/build/intermediates/sourceFolderJavaResources/debug:/Users/kingnewspring/Downloads/KotlinTest/app/build/generated/mockable-android-27.v3.jar" com.intellij.rt.execution.application.AppMainV2 com.intellij.rt.execution.junit.JUnitStarter -ideVersion5 com.example.kingnewspring.kotlintest.ExampleUnitTest
arra2:null
arra2:null
arra2:null
arra3:0
arra3:1
arra3:4
arra3:9
arra3:16
arra3:25
arra3:36
arra3:49
arra3:64
arra3:81
arra4:1
arra4:2
arra4:3
arra5:d

Process finished with exit code 0

You can see

• array1 No element , In line with expectations , It's an empty array , amount to arrayOfNulls<String>(0)
• array2 Yes 3 Elements , All are null
• array3 Yes 10 Elements , They are subscripts 0~9 Of 2 Power , Conform to the initialization logic
• array4 Yes 3 Elements , Consistent with the initialized element value
• array5 There's an element , Namely char Type of d

Element expansion of array :plus

and Java equally ,Kotlin Of array The default size is what you give , Initialization is 0 Namely 0, Initialization is 100 It is 100, But look at the source code , You'll find that kotlin It also provides a way to add elements api,plus Method

/**
 * Returns an array containing all elements of the original array and then the given [element].
 */
public actual operator fun <T> Array<T>.plus(element: T): Array<T> {
    val index = size
    val result = java.util.Arrays.copyOf(this, index + 1)
    result[index] = element
    return result
}

plus The method notes say , Returns an array containing all the elements of the original array , In fact, the original array will be copied in the future , And the length +1, Then add new elements to the array , Although the performance is not good , But you can easily add elements . Here's the thing to notice , Method returns a new array , So you need to receive it , Otherwise, the addition will not succeed , We use it array4 Under demonstration ：

// Initialize an array of multiple data types 
        var array4 = arrayOf("1",2,3)
        array4.plus("1")
        for (i in array4){
            println(i)
        }

The printout is ：

1
2
3

Process finished with exit code 0

Now let's change the code a little ：

 // Initialize an array of multiple data types 
        var array4 = arrayOf("1",2,3)
        //array4.plus("4")
        array4 = array4.plus("4")
        for (i in array4){
            println(i)
        }

The print result at this time ：

1
2
3
4

Process finished with exit code 0

Padding of array elements ：fill

array Provides fill function , Fill the original array （ If the original data will be overwritten ）, Operation demonstration ：

// Initialize an array of multiple data types 
        var array4 = arrayOf("1",2,3)
        // Fill the array with data "4",fromIndex 0 toIndex 2, From the subscript 0 To 2, It doesn't contain 2
        array4.fill("4",0,2)
        for (i in array4){
            println(i)
        }

The result can be guessed ：

4
4
3

Process finished with exit code 0

Let's see fill Method source code

/**
 * Fills original array with the provided value.
 */
public fun <T> Array<T>.fill(element: T, fromIndex: Int = 0, toIndex: Int = size): Unit {
    // Here we call Java Of fill operation 
    java.util.Arrays.fill(this, fromIndex, toIndex, element)
}

Looking directly at the above function, you can't see the principle , But we can see that , What he's calling is Java Of fill Method , Keep looking at java.util.Arrays.fill

 /**
     * Assigns the specified Object reference to each element of the specified
     * range of the specified array of Objects.  The range to be filled
     * extends from index <tt>fromIndex</tt>, inclusive, to index
     * <tt>toIndex</tt>, exclusive.  (If <tt>fromIndex==toIndex</tt>, the
     * range to be filled is empty.)
     *
     * @param a the array to be filled
     * @param fromIndex the index of the first element (inclusive) to be
     *        filled with the specified value
     * @param toIndex the index of the last element (exclusive) to be
     *        filled with the specified value
     * @param val the value to be stored in all elements of the array
     * @throws IllegalArgumentException if <tt>fromIndex &gt; toIndex</tt>
     * @throws ArrayIndexOutOfBoundsException if <tt>fromIndex &lt; 0</tt> or
     *         <tt>toIndex &gt; a.length</tt>
     * @throws ArrayStoreException if the specified value is not of a
     *         runtime type that can be stored in the specified array
     */
    public static void fill(Object[] a, int fromIndex, int toIndex, Object val) {
        rangeCheck(a.length, fromIndex, toIndex);
        for (int i = fromIndex; i < toIndex; i++)
            a[i] = val;
    }

Very simple operation , Is to traverse the assignment

 

aggregate List

Kotlin The collection and Java The set of is similar to , It's all done Collection Interface ,Kotlin The set is divided into

• Immutable set （immutable）
• Variable set （mutable）

initialization

        // Initialize an immutable set through elements 
        var list1 = listOf("aaa", "bbb","ccc","dddd")
        // Initialize an empty immutable set 
        var list2 = emptyList<String>()
        // Initialize an immutable set that filters empty elements 
        var list3 = listOfNotNull(1,null)
        // Initialize an empty immutable set , and list2 It's the same 
        val list4:List<String> = listOf()


        // Create a variable set with elements 
        var list5 = mutableListOf("22","33")
        // Create an empty arrayList, It is also a variable set 
        var list6 = arrayListOf<String>()

Here we won't print their elements one by one , It should be noted that

• list3, It has only one element 1, The set size is also 1, Because the filter is adjusted null！
• list2 and list4 The principle is the same , Let's see. list2 Source code

/**
 * Returns an empty read-only list.  The returned list is serializable (JVM).
 * @sample samples.collections.Collections.Lists.emptyReadOnlyList
 */
public fun <T> emptyList(): List<T> = EmptyList

You can see that it returns an empty read-only list, And in JVM Is serializable ( stay android In the project Kotlin At this stage, it is still parasitic JVM Platform , be-all Kotlin The source code will be compiled into class file )

• list5 and list6 It's all done MutableList Interface , The source code can see their equivalence

/**
 * Returns a new [MutableList] with the given elements.
 * @sample samples.collections.Collections.Lists.mutableList
 */
public fun <T> mutableListOf(vararg elements: T): MutableList<T> =
    if (elements.size == 0) ArrayList() else ArrayList(ArrayAsCollection(elements, isVarargs = true))

mutableListOf() It's actually calling ArrayList

Expansion of set elements

For immutable sets plus

Immutable set , Logically, elements cannot be extended , Because they read only （read—only）. But you can also add elements if you want , By calling plus Method , Its principle is the same as array , It's all about creating new collections , length +1, It's not really changeable , there plus Method is an operator overloaded function , So you can also use "+" To add

        // Initialize an immutable set through elements 
        var list1 = listOf("aaa", "bbb","ccc","dddd")
        // adopt plus add to 
        list1 = list1.plus("fff")
        // adopt + add to 
        list1 += "eeee"
      // Print 
        for (i in list1){
            println(i)
        }

The result is ：

aaa
bbb
ccc
dddd
fff
eeee

Process finished with exit code 0

Variable sets are used add

This simple , direct add Just come in , and Java Of arraylist Use the same . Some people will ask , Can I use it here plus Well , The answer is that you can use , But you can't add elements , Like immutable sets , Let's see ：

        // Create a variable set with elements 
        var list5 = mutableListOf("22","33")
        // Use plus add to 
        list5.plus("444")
        // Use add add to 
        list5.add("r3r3")
        // Print 
        for (i in list5){
            println(i)
        }

Print the results ：

22
33
r3r3

Process finished with exit code 0

there plus The effect is the same as that in immutable arrays and immutable sets , If you want to use it, you need to reassign the set , Use of reference immutable sets , It is strongly not recommended to use

Filling of set elements

Filling and Java Of List It is also very similar , The usage is the same . It's the same for both mutable and immutable sets ！

The transformation between arrays and sets

Array to set :toList()

       val array1 = arrayOf(1,2,3)
        // Array to set 
        var list7 = array1.toList()
        println("list7 = $list7")

Print the results ：

list7 = [1, 2, 3]

Process finished with exit code 0

Set array :toTypedArray()

         // Initialize an immutable set through elements 
        var list1 = listOf("aaa", "bbb","ccc","dddd")
        // Set array 
        val array1 = list1.toTypedArray()
        for (temp in array1)
            println(temp)

Print the results ：

aaa
bbb
ccc
dddd

Process finished with exit code 0