The first day of a solid foundation for Kotlin

The first day of a solid kotlin foundation

Quickly query data in database classes, such as finding the largest number
First define a data class

data class Person(val name:String, val age:Int? =null) where?Indicates that this data can be empty, this is an important knowledge point that must be kept in mind!
Now use this database class to put
fun main(args:Array{
fun person = listOf(Person(“Zhang”),Person(“Zhao”,15))
}

Among them, we created a list collection, which stores two data classes. Note that the constructor of one of the data classes has only one name, because the second function we set when we created the data class can be empty., so if you don't enter a value, the system will create a variable with an age of null for us by default

Now compare the size of the two data classes age

val oldest =person.maxBy(it.age ?: 0) where ?: is the Elvis operator, which means that if there is no value, it will be replaced with 0, so Zhao is the maximum age
oldest is 15

Simple writing of functions, such as passing in two values ​​to compare the maximum value
fun max(a : Int , b : Int) :Int {
return if(a>b) a else b
}
The above function is to compare the size of an Int value, but the function title of this function is composed of a single expressionIf so, you can use this expression as the complete function body, without the curly braces and return statement

fun max (a :Int , b : Int ) :Int = if (a > b) a else b If the function body is written inside curly braces, then the function has a code block, if it directly returns an expressionformula, then it is given to the body of the expression

About the difference between variable modifiers val var

Methods of declaring variables in java
String name = "Zhang San" It is necessary to declare that this variable is of type String, and if the variable needs to be immutable, a final constant needs to be added, but it is completely omitted in kotlin, just write
val name = "Zhang San" There is no need to apply for a variable type, because the kotlin compiler will deduce that this is a String type and add the val modifier, then this variable is immutable.However, one concept to understand is that the reference itself (name) is immutable, but the object it points to is mutable, such as pointing to a collection type, then the collection type is mutable, such as:
val list =arrayListOf("1")
list.add("5") This code is perfectly valid
var age = "5" A variable decorated with var is mutable, allowing to change its amount, butIt is not allowed to change its type; for example: var age = "5" | var age = "Zhangsan" This approach is wrong because its type is immutable.

Simpler string template

The output string in kotlin is println("Hello Kotlin") You can also use $(variable) in println, for example:
val name = "Kotlin" println("Hello ${name}") NoteThe point is that if there is no more character after $, you can remove the curly braces

Classes and Properties

To create a data class in java, we often need to create get and set methods for variables.But in Kotlin it is not needed.
class Person(val name:String, var age: Int) Kotlin will automatically create a get method for the name variable, because the name is val modified, then it has only one get method, but will create a get method for the var modified age variableand set method.