Kotlin learning 1: variables, functions, conditional statements and loop statements

For a long time Android Development , see kotlin Code , Probably understand , Look carefully but don't understand . So learn about it systematically , Enter a door .

Create the first project

Created the first kotlin project , It only needs java Change it to kotlin that will do ：

Compiler error ：

Could not resolve org.jetbrains.kotlin:kotlin-gradle-plugin:1.3.72.
...

Modify and delete jcenter() ：

repositories {
  mavenCentral()
  google()
}

Can run .

Variable

val ： Immutable variable , Corresponding Java Of final .

var ： Volatile variables , Corresponding Java Non - final.

Define a variable , No semicolon at the end ：

val a = 10
 or 
val a: Int = 10

Java How to write it ：

private int a = 10;

It's really simpler . it is to be noted that Java Medium int, stay kotlin Turn into Int, All initials of other data types are capitalized ;

take a Do calculations , Now a Use var Statement ：

        var a = 10
        a = a * 5

stay kotlin in ,val and var It must be added , This is different from Java,Java Of final Is additive or non additive .

When using , Always give priority to val, When val Not enough to use var.

function

That is, the method often said . Define one pass in two parameters , And the return value is int Function of type ：

    fun setName(val1: Int, val2: String): Int {
        return 100;
    }

fun Fix the writing method for the function ,setName Is the function name ; The first parameter is int type , Parameter name val1, The second is String type , Parameter name val2; Change the function to return a int Type value .

Conditional statements ：if and when

if sentence , And Java It can't be as like as two peas. , It can only be said that there is no difference ：

        val a = 10
        val b = 20
        if (a > b) {
            Log.w(" Maximum a", "" + a)
        } else {
            Log.w(" Maximum b", "" + b)
        }

When there are many conditional statements , have access to when, Call method transmission B You can get it 9527 了 ：

    fun getInt(name: String):Int {
        return when (name) {
            "A" -> 9526
            "B" -> 9527
            "c" -> 9528
            else -> 0
        }
    }

    // The following is the abbreviation ：
    fun getInt(name: String) =
        when (name) {
            "A" -> 9526
            "B" -> 9527
            "c" -> 9528
            else -> 0
        }

Abbreviation can save a lot of code for a large amount of code .

The above code can also be written like this ：

    fun getInt(name: String) =
        when {
            name == "A" -> 9526
            name == "B" -> 9527
            name == "c" -> 9528
            else -> 0
        }

In this way, you can write some complex logic , such as ：

    fun getInt(name: String) =
        when {
            name.contains("A") -> 9526
            name == "B" -> 9527
            name == "c" -> 9528
            else -> 0
        }

The actual needs are much more complex than the above .

Loop statement while and for

do while and Java similar ：

        var b = 0
        do {
            b++
            Log.w("b The value of is ", "" + b)
        } while (b < 12)

for Recycling changes slightly ,for-in Cycle through each element of the interval and print ：

        for (i in 0..10){
            Log.w("i The value of is ", "" + i)
        }

A slightly more complex common usage , Data in the loop list ：

        var myList = arrayListOf<String>() // Define an array 
        myList.add("A")
        myList.add("B")
        myList.add("C")
        myList.add("D")
        for (i in 0..myList.size) {
            Log.w(" The value in the list is ", "" + myList[i])
        }

Combined with some keywords, you can realize a more complex cycle ：

        for (i in 0 until 10){
            Log.w("i The value of is ", "" + i)
        }

until It means that the right side is the open section ,i The range of values is 0-9, That is, mathematical expression ：[0,10).

More complex cycles ：

        for (i in 0 until 10 step 2){
            Log.w("i The value of is ", "" + i)
        }

Print the results ：

i The value of is : 0
i The value of is : 2
i The value of is : 4
i The value of is : 6
i The value of is : 8

step 2 It means i Increase by two each time , So with the print result above .

Take another look at the descending order ,downTo Representation of descending order ：

        for (i in 0 downTo -10 step 2){
            Log.w("i The value of is ", "" + i)
        }

First create a [0,-10] In descending order , And then let i Every reduction 2, The result is ：

i The value of is : 0
i The value of is : -2
i The value of is : -4
i The value of is : -6
i The value of is : -8
i The value of is : -10

In this way, you can write some simple logic .