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Control statements in Kotlin differ from Java in many ways, such as when statements, ternary operators, etc.

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The if statement

The if statement in Kotlin is somewhat different from Java in that it can be more flexible in Java, implementing expressions (ternary operators) and using them as a block in addition to Java writing.

1.1. Traditional writing (the sameJavaThe same way)

Ex. :

 var numA = 2

 if (numA == 2) {

     println("numA == $numA => true")

 }else{

     println("numA == $numA => false")

 }

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The output is:

numA= =2= >true

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1.2,KotlinThe ternary operator in

  • There are no ternary operators in Kotlin (condition ? then : else) this kind of operation.
  • That’s because of the nature of the if statement (ifThe expression returns a value), so no ternary operators are needed.

Ex. :

 // In Java, you can write this, but Kotlin will directly report an error.

 // var numB: Int = (numA > 2) ? 3:5



 // Kotlin uses if.. The else instead. Ex. :

 var numB: Int = if ( numA > 2 ) 3 else 5  NumB is set to 3 if numA is greater than 2; numB is set to 5 otherwise

 println("numB = > $numB")

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The output is:

 numB = > 3

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As you can see above, if in Kotlin can be used as an expression and return a value.

As a block structure, and the last expression is the value of the block

Ex. :

 var numA: Int = 2

 var numC: Int = if (numA > 2) {

     numA++

     numA = 10

     println("numA > 2 => true")

     numA

 }else if (numA == 2) {

     numA++

     numA = 20

     println("numA == 2 => true")

     numA

 }else{

     numA++

     numA = 30

     println("numA < 2 => true")

     numA

 }



 // Each if branch is a block of code that returns a value. The value of numC should be 20 depending on the condition

 println("numC => $numC")

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The output is:

 numA == 2= >true

 numC => 20

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The for statement

  • KotlinThe abolition of theJavaIn thefor(Initial value; Conditions; Increase or decrease step size). butKotlinFor in theforThe loop statement adds additional rules to satisfy the rules just mentioned.
  • forLoops provide iterators to iterate over anything
  • forThe loop array is compiled as an index-based loop that does not create an iterator object

2.1. New rules should be metfor(Initial value; Conditions; Increase or decrease step size)

I have something to say about the for loop!

2.1.1, incremental

Keyword: until Range: until[N,m) => That is, the value is greater than or equal to N and less than m

Ex. :

// Loop 5 times, incrementing by 1

 for (i in 0 until 5){

     print("i => $i \t")

 }

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The output is

 i => 0    i => 1    i => 2    i => 3    i => 4

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2.1.2, decreasing

  • Key words:downTo
  • Scope:downTo[n,m]= > theLess than or equal to n, greater than or equal to m,n >; m

Ex. :

// Loop 5 times and decrement step 1

 for (i in 15 downTo 11){

     print("i => $i \t")

 }

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The output is:

 i => 15    i => 14    i => 13    i => 12    i => 11     

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2.1.3 Symbols ('.. ') represents another operation of an incrementing loop

  • Use symbols ('.. ').
  • Scope:..[n,m]= > theGreater than or equal to n, less than or equal to m
  • anduntilOne is simplicity. The second is the difference in scope.

Ex. :

 print("Use the symbol '.. 'print result \n")

 for (i in 20 .. 25){

     print("i => $i \t")

 }



 println()



 print("Use until print \n")

 for (i in 20 until 25){

     print("i => $i \t")

 }

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The output is:

Use the symbol '.. 'to print the result

 i => 20     i => 21     i => 22     i => 23     i => 24     i => 25     



Use the print result from until

 i => 20     i => 21     i => 22     i => 23     i => 24 

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2.1.4. Set the step size

Key words: step

Ex. :

 for (i in 10 until 16 step 2){

     print("i => $i \t")

 }

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The output is:

 i => 10     i => 12     i => 14 

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2, iterations,

  • forLoops provide an iterator to iterate over anything.
  • forThe loop array is compiled as an index-based loop that does not create an iterator object

2.2. Iterate over the string

Ex. :

 for (i in "abcdefg") {

     print("i => $i \t")

 }

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The output is:

 i => a    i => b    i => c    i => d    i => e    i => f    i => g     

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2.3 Go through the number group

Ex. :

Var arrayListOne = arrayOf,20,30,40,50 (10)



 for (i in arrayListOne){

     print("i => $i \t")

 }

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The output is:

 i => 10     i => 20     i => 30     i => 40     i => 50     

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2.4. Use arraysindicesAttribute traversal

Ex. :

 var arrayListTwo = arrayOf(1.3.5.7.9)

 for (i in arrayListTwo.indices){

     println("arrayListTwo[$i] => " + arrayListTwo[i])

 }

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The output is:

 arrayListTwo[0] = >1

 arrayListTwo[1] = >3

 arrayListTwo[2] = >5

 arrayListTwo[3] = >7

 arrayListTwo[4] = >9

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2.5. Use arrayswithIndex()Methods through

Ex. :

 var arrayListTwo = arrayOf(1.3.5.7.9)

 for ((index,value) in arrayListTwo.withIndex()){

     println("index => $index \t value => $value")

 }

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The output is:

 index => 0      value => 1

 index => 1      value => 3

 index => 2      value => 5

 index => 3      value => 7

 index => 4      value => 9

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2.7 Iterator traversal

It is usually used with the while loop

Ex. :

 var arrayListThree = arrayOf(2.'a'.3.false.9)

 var iterator: Iterator<Any> = arrayListThree.iterator()



 while (iterator.hasNext()){

     println(iterator.next())

 }

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The output is:

2

a

3

false

9

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2.8 foreach traversal

Ex. :

 val array = arrayOf(2.'a'.3.false.9)

 array.forEach { println(it) }

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The output is:

2

a

3

false

9

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The when statement

  • inKotlinChinese has been abolishedJavaIn theswitchStatements. And a newwhen(exp){}Statements.
  • whenStatements can not only be replacedswitchStatement, and more thanswitchMore powerful statements

3.1. When statement realizes the function of switch statement

Ex. :

 when(5) {

     1 -> println("1")

     2 -> println("2")

     3 -> println("3")

     5 -> println("5")

     else- > {

         println("error!")

         println("0")

     }

 }

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The output is:

 5

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3.2 when used together with a comma, it is equivalent to not using the break jump statement in the switch statement

Ex. :

 when(1) {

     // if x = 1,2, and 3, output 1.

     1 , 2 , 3 -> println("1")

     5 -> println("5")

     else- > {

         println("error!")

         println("0")

     }

    }

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The output is:

 1

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3.3. Conditions can use any expression, not just constants

Equivalent to the use of an if expression.

Ex. :

 var num:Int = 5

 when(num > 5) {

     true -> println("num > 5") 

     false -> println("num < 5")

     else- > {

         println("error!")

         println("num = 5")

     }

 }

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The output is:

 num < 5

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3.4. Check whether the value exists in a collection or array

  1. (in) 在
  2. (! in)Not in

PS: Applies only to numeric types

Ex. :

 var arrayList = arrayOf(1.2.3.4.5)

 when(1) {

     in arrayList.toIntArray() -> println("1 exists in an arrayList")

     in 0.10 -> println("1 belongs to 0 to 10.")

! in5.10 -> println("One does not belong to the five to ten")

     else- > {

         println("error!")

         println("All wrong haha!")

     }

 }

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The output is:

The element1 ` `Exists in an arrayList array

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Where, the symbol (..) Indicates the meaning of to. As in the example 0.. 10 means 0 to 10 or 0 to 10.

3.5. Check whether the value is of the specified type

  1. is(is)
  2. not(! Is)

Ex. :

 when("abc") {

     is String -> println("ABC is a string")

     else- > {

         println("ABC is not a string.")

     }

 }

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The output is:

ABC is a string

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3.6. When statements that do not use expressions

Is expressed as the simplest Boolean expression

Ex. :

 var array = arrayOfNulls<String>(3)

 when{

     true- > {

         for (i in array) {

             print(" $i \t")

         }

     }

     else- > {}

 }

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The output is:

 null    null    null 

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In summary, this is a common use of the WHEN control statement in Kotlin. You can see how powerful it is. And convenience. Not only can it replace swICTH statements in Java statements. You can even replace the if statement.

While statement

This is the same as the while loop in Java. No elaboration will be made here.

Definition format:

 while(expOf) {expFor the expression

.

 }

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Ex. :

 var num = 5

 var count = 1

 while (num < 10) {

     println("num => $num")

     println("Loop $count times")

     count++

     num++

 }

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The output is:

 num= >5

The loop1 次

 num= >6

The loop2 次

 num= >7

The loop3 次

 num= >8

The loop4 次

 num= >9

The loop5 次

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Fifth, the do… While statement

This is the same as Java do… Same as the while loop. No elaboration will be made here.

Definition format:

 do{ 

.

 }while(exp)

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Ex. :

 var num = 5

 var count = 1

 do {

     println("num => $num")

     println("Loop $count times")

     count++

     num++

 }while (num < 10)

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The output is:

 num= >5

The loop1 次

 num= >6

The loop2 次

 num= >7

The loop3 次

 num= >8

The loop4 次

 num= >9

The loop5 次

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PS: do{… }while is executed at least once,while(exp) may not be executed at all, as in Java

Ex. :

 var num = 5

 var count = 1

 do {

     println("num => $num")

     println("Loop $count times")

     count++

     num++

 }while (num < 5)

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The output is:

 num= >5

The loop1 次

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Jump statements (return, break, continue)

1. Return statement

By default, returns from the nearest closed or anonymous function.

Ex. :

 fun returnExample(a){

     var str: String = ""

     if (str.isBlank()){

         println("I quit the method.")

         return

     }

 }

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2. Break statement

Function: Terminates the most recent closed loop.

Ex. :

 var count: Int = 1

 for (i in 1 until 10) {

     if (i == 5) {

         println("I'm the first$iI'm out of the loop.")

         break

     }

     count++

 }



 println("How many times do I loop: count =>$count")

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The output is:

I was in the first5The second time exits the loop

How many times do I loop: count =>5

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The continue statement

Progress to the next step (iteration) of the nearest closed loop.

Ex. :

 for (i in 1 until 10) {

     if (i == 5) {

         println("I skipped number one$iLoops")

         continue

     }

     println("i => $i")

 }

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The output is:

 i => 1

 i => 2

 i => 3 

 i => 4

I skipped number one5loops

 i => 6

 i => 7

 i => 8

 i => 9

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Seven,

No matter for any programming language, there are the most basic conditional logic control statements, and their statements are the foundation of learning a programming language. So please be careful and follow the code to type again. Specifically, the for statement and the when statement. Because both of these points change a lot from the Java language. Of course, the ternary operators in the if statement are also worth noting

This article has been collected on GitHub: Jetictors/KotlinLearn, welcome star articles continue to update, can be wechat search “J guy talk” first time read, everyone’s three-in-one is the best power for the old J, you are sure not to wave? If you have any mistakes or suggestions on this blog, please leave comments


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