9. 属性与字段

1. 属性的基本使用

val id: String = "123"

var name: String? = "kotlin"

var age: Int? = 22

var sex: String? = "男"

var weight: Float? = 120.5f

2. getter()和setter()

演示getter()和setter()

class Test {

    // 当用var修饰时，必须为属性赋默认值，即使在有getter()的情况下，不过这样写出来，不管如何修改，其值都为123
    var test1: String = " "
        get() = "123"
        set(value) {field = value}

    // 用val修饰时，用getter()函数时，属性可以不赋默认值，但是不能有setter()函数
    val test2: String
        get() = "123"  // 等价于 val test2 = 123

}

2.1 自定义getter()和setter()

class Mine {

    private val size: Int = 0

    val isEmpty: Boolean
        get() = this.size == 0

    val num: Int = 2
        get() = if (field > 5) 10 else 0

}

val mine = Mine()
println("isEmpty = ${mine.isEmpty}")  // isEmpty = true
println("num = ${mine.num}")  // num = 0

class Mine {

    var str1: String = "test"
        get() = field
        set(value) {field = if (value.isNotEmpty()) value else "null"}

    var str2: String = ""
        get() = "随意怎么修改都不会改变"
        set(value) {field = if (value.isNotEmpty()) value else "null"}

}

println("str1 = ${mine.str1}")  // str1 = test
mine.str1 = ""
println("str1 = ${mine.str1}") // str1 = null
mine.str1 = "kotlin"
println("str1 = ${mine.str1}") // str1 = kotlin

println("str2 = ${mine.str2}") // str2 = 随意怎么修改都不会改变
mine.str2 = ""
println("str2 = ${mine.str2}") // str2 = 随意怎么修改都不会改变
mine.str2 = "kotlin"
println("str2 = ${mine.str2}") // str2 = 随意怎么修改都不会改变