① var (variable variable)
Used to declare a variable, indicating that the variable can be modified, reassigned, and referenced;
var is a mutable variable, which is a variable that can be changed to another value by reassignment;
this way of declaring variables Same way as declaring variables in Java.
grammar
Define variables: Variables are readable and writable, and can be assigned repeatedly after definition, which is equivalent to ordinary variables in Java:
var <variable identifier>: <data type> = <initialization value>
var name: String = "kotlin" //声明时直接赋值
var age: Int //无初始化值时不能省略数据类型
age = 100 //先声明后赋值
var sex = 1 //自动推断数据类型 Int
- For variable variables, you should generally pay attention to using the safe operator?, and you must add it when using it.
For example: var name: String? = null
② val (value) :
It is used to declare a constant, indicating that the variable cannot be modified once it is assigned, and the reference is immutable;
val is a read-only variable, and this way of declaring a variable is equivalent to a Final variable in Java;
a val must be initialized when it is created, because later cannot be changed.
grammar
Define constants: Constants are readable but not writable. Once defined, their values cannot be modified, which is equivalent to final-modified constants in Java
val <constant identifier>: <data type> = <initialization value>
val PI: Double = 3.1415 //声明常量直接赋值
// PI = 3.20 此时会报语法错误,val修饰的常量不能重新赋值
val title: String //声明常量,暂不赋值
title = "kotlin Hello" //未赋值的常量可以赋值一次
- An important concept: use val whenever possible, because val variables are more safe and reliable in complex logic.
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