Kotlin introductory notes (III) kotlin program logic control (if, when)

Preface : This tutorial is best done with JAVA Study on the basis of

One 、if Use of statements

kotlin There are two main ways to implement conditional statements in ：if and when.

Same usage ：

fun largerNumber(num1 : Int , num2 : Int) : Int{
    var value = 0
    if(num1 > num2) {
        value = num1
    } else { 
        value = num2
    }
    return value
}

Different usage ：

fun largerNumber(num1 : Int , num2 : Int) : Int {
    val value = if (num1 > num2){
        num1
    }else{
        num2
    }
    return value
}

kotlin And java Of if The statements are roughly the same , but kotlin Of if Statement can have a return value ！！！

But there is a problem with the above code ,kotlin It is the most concise language , The above code has too many unnecessary parts . stay kotlin in , We can use the following expression instead （ If you have unclear friends, you can read my first two blogs ）：

fun largerNumber(num1 : Int,num2 : Int) = if(num1 > num2) num1 else num2

Does it look much more comfortable ？ Hem ...,Kotlin The charm of is still behind ！

Two 、When sentence

use when And those still in use switch...case Compared with the language of , It's so happy ...（ Roast about what anti human can't get rid of break） Don't talk much , Here comes the example ...... Ah , This example is complete , Why don't you see it yet ....

Suppose you compile a function to query exam scores ( if sentence )：

fun getScore(name : String) = if (name == "Tom") {
    86
} else if (name == "Jim") {
    77
} else if (name == "Jack") {
    95
} else if (name == "Lily") {
    100
} else {
    0
}

But when we use when when , You can compare ：

fun getScore(name : String) = when (name) {
    "Tom" -> 86
    "Jim" -> 77
    "Jack" -> 95
    "Lily" -> 100
    else -> 0
}

Is it refreshing ？

The format is as follows ：

when( Any type of parameter ) {

         Match the value -> { Perform logical }

}

When the execution logic has only one sentence , { } It can also be omitted . Of course , It's an exact match . In addition to precise matching , We also have type matching ... Examples are as follows ：

fun checkNumber(num : Number) {
    when (num) {
        is Int -> println("num is Int")
        is Double -> println("num is Double")
        else -> println("num  neither  Int, Neither  Double")
    }
}

when There is another way to write a statement without parameters

Take the example just now ：

fun getScore(name : String) = when {
    name == "Tom" -> 86
    name == "Jim" -> 77
    name == "Jack" -> 95
    name == "Jack" -> 100
    else -> 0
}

Be careful ,Kotlin When judging whether strings or objects are equal, you can directly use == .（ And Java Different ）

In addition to this precise match ,when It can also cooperate with startsWith() Realization Fuzzy matching （ With Tom Everyone at the beginning 86 branch ）：

fun getScore(name : String) = when {
    //  With Tom Everyone at the beginning 86 branch 
    name.startsWith("Tom") -> 86 
    name == "Jim" -> 77
    name == "Jack" -> 95
    name == "Jack" -> 100
}

Okay , That's all for this issue , Remember to pay attention if you like ！！！ RI Geng Zhong ！！！

Kotlin The small white , I took notes while studying teacher Guolin's works . If there are any mistakes, please point out ！ Thank you for reading ！

Reference resources :

        《 First line of code Android ( The third edition )》   --- Guo Lin