Kotlin---- control statement

1、 Conditional branch

(1)、 Simple branches –if…else

kotlin in if…else Statement has a return value , Be similar to Java The trinomial operation in

// Pass in two values  a  and  b ,   Show large values in TextView Control 
nameTextView.text=if(a>b) a else b

(2)、 Multiple branches

A: characteristic

kotlin There is no switch/case ,

• Use when/else replace
• when/else There are also return values
• After executing a node, the loop is terminated
• The criteria in the branch can be constant / Variable / expression ——Java Medium switch/case in case Nodes can only be constants
• To make sure everything is judged , Usually cannot omit else

B: Basic use

var count:Int = 0 btn_when_simple.setOnClickListener {
tv_answer.text = when (count) { 0 -> " The value is 0"
1 -> " The value is 1"

else -> " Other values "
}
count = (count+1) % 3
}

C: through

java in switch/case Each constant condition needs to be listed when penetration is required , but kotlin Can simplify ——

• Multiple conditions are written directly on one line , Use ”,“
• Interval if the condition is continuous , You can specify the interval range
• If the condition is a continuous number , And it needs to be judged that it is not within the range , Then use

btn_when_region.setOnClickListener { tv_answer.text = when (count) {
1,3,5,7,9 -> " The value is 13579 One of them "
in 13..19 -> " The value is 13 To 19 One of them "
!in 6..10 -> " Value no longer 6 To 10 Between " else -> " Other values "
}
count = (count+1) % 20
}

D、 Take the expression as the branching condition

kotlin in adopt is Keywords are used to judge A Is it B Example —— , Equate to Java Medium A instance B .

var countType:Number; btn_when_instance.setOnClickListener {
count = (count+1) % 3 countType = when (count) {
0	-> count.toLong();
1	-> count.toDouble() else -> count.toFloat()
}

tv_answer.text = when (countType) { is Long -> "Long type "
is Double -> "Double type " else -> " Other types "
}
}

2、 Loop processing

(1)、 Traversal cycle

namely for-in 、forEach、 The use of iterators

(2)、 condition loop

Execute when a condition is met / End cycle .

A: Use keywords

So , Added multiple keywords ：until、setp、downTo

//  Traverse 11 To 66 Value between ,until  A left closed and right open interval is declared —— It doesn't contain 66, contain 11 for (i in 11 until 66) {
...
}

//      Traverse 23-89 Value between , Step by step to 4—— Every time +4.23..89 Declare an interval that is both left and right closed 
for (i in 23..89 step 4) {
...
}

// Traversal in reverse order from 50 To 7
for (i in 50 downTo 7) {
...
}

B: Use while、do/while

Due to the limited keywords , The effect that can be achieved is limited , therefore , More complex logic can be used while or do/while Realization

btn_repeat_begin.setOnClickListener { var poem:String=""
var i:Int = 0
while (i < poemArray.size) { if (i%2 ==0) {
poem = "$poem${poemArray[i]}\n"
} else {
poem = "$poem${poemArray[i]}\n"
}
i++
}
poem = "${poem} The poem consists of ${i} sentence " tv_poem_content.text = poem
}

btn_repeat_end.setOnClickListener { var poem:String=""
var i:Int = 0 do {
if (i%2 ==0) {
poem = "$poem${poemArray[i]}\n"
} else {
poem = "$poem${poemArray[i]}\n"
}
i++
} while (i < poemArray.size)

poem = "${poem} The poem consists of ${i} sentence " tv_poem_content.text = poem
}

(3)、 Break the loop

Use break—— Break the loop 、continue—— Skip this cycle , The basic usage is the same as Java,

in addition , When nested loops , You can also use @ Loop tag name Specify the loop to break .

btn_repeat_break.setOnClickListener { var i:Int = 0
var is_found = false
// [email protected]  To label an outer loop , The name is  outside [email protected] while (i < poemArray.size) {
var j:Int = 0
var item = poemArray[i]; while ( j < item.length) { if (item[j] == ' One ') { is_found = true
// The interrupt tag is named outside The cycle of  [email protected]
}
j++
}
i++
}

3、 Empty processing

(1)、 Validity judgment of string

Method for verifying empty string ：

• isNullOrEmpty —— by null Or the length is 0 When to return to true.
• isNullOrBlank —— by null Or the length is 0 Or all the spaces , return true.
• isEmpty —— The length is 0 When to return to true, We must first judge the non - null
• isBlank —— The length is 0 Or all spaces return true, We must first judge the non - null
• isNotEmpty —— Longer than 0 When to return to true, We must first judge the non - null
• isNotBlank —— Longer than 0 And non space returns true, We must first judge the non - null

(2)、 Declare nullable variables

kotlin The default variable in is not empty , If you need to declare an nullable variable , Then a question mark is appended to the type ？

ar strCanNull:String?

(3)、 Operators for checking null values ——?、?:、!!

​ ? Can be empty , If empty, return directly null

​ ?: Indicates that once it is empty, the value after the colon is returned , Otherwise, it returns the normal value

​ !! Indicates that the assertion is not empty ( Discard non empty judgments ). however , If you make this assertion , But still null, Then null pointer will be reported .

var length_null:Int? btn_question_dot.setOnClickListener {
   //strB Followed by one ”?“ It means that you can null, If null Then return directly null,    here    length_null The value of is null
   length_null = strB?.length
   tv_check_result.text = " Additional  ?  The length obtained when calling the object property is  $length_null"
}

btn_question_colon.setOnClickListener {
	// If strB Not null, Return to normal  strB.length;  If strB by null, Then return to -1 
    length = strB?.length ?: -1
	tv_check_result.text = " Use  ?:  The length obtained when calling the object property is  $length"
}

btn_exclamation_two.setOnClickListener {
    strB = "ABCDE"
	// Only onehundredpercent are certain that it is not null when , Only use !!, Otherwise, the null pointer will still be reported  	length = strB!!.length
	tv_check_result.text   = " Use !! The length of the attribute value obtained when asserting is $length"
}

4、 Equality judgment

(1)、 Equal structure

• kotlin Use in == Judge whether two data are equal , Use != Judge whether it is not equal .
• kotlin Medium String It's the same way .Java Use in equals()
• kotlin When comparing strings in, the storage addresses in memory are no longer compared , Instead, compare the values of variables directly , This method is called Equal structure , That is, they look the same / Same appearance .

(2)、 Reference equivalence

If you need to determine whether the references are equal , That is to judge whether the memory addresses are consistent ,kotlin Use in =, If the judgment is inconsistent , Then use ！

• For basic data types ( Including strings ), Structure equality is consistent with reference equality .
• adopt clone Even if the attribute value of the object is the same , But the reference address is inconsistent .

(3)、is and in

• is and !is —— Judge A Is it B Example , Such as ： Object name is Class name
• in and !in —— Determine whether the array contains an element , Such as Variable name in Array name