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Kotlin可以有mutable和ready-only的field，同时它的getters和setters方法默认是自动生成的，你也可以自定义。

class User {
        val id: String = "" //immutable. just getter
        var name: String = "" //default getter and setter
        var surName: String = "" //custom getter, default setter
            get() = surName.toUpperCase() //custom getter declaration

        var email: String = "" //default getter, customer setter
            set(value) {     // custom setter declaration
                //"value" custom setter's parameter
                //"field" = property's backing field;
                if (isEmailValid()) field = value 
            }

Properties and Fields: Getters, Setters, const, lateinit – Kotlin Programming Languagekotlinlang.org

使用data class当你想要保存一些数据的时候，它默认实现了equals，hashcode，toString以及copy

data class User(val name: String, val email: String)

    class UserListDiffCallback {
        fun areContentsTheSame(oldPosition: Int, newPosition: Int): Boolean {
            // use the generated equals method
            return newList[oldPosition] == oldList[newPosition]
        }
    }

Equality – Kotlin Programming Languagekotlinlang.org

在Kotlin里，默认的修饰符都是public，但它还是提供了一些其他的修饰符：private，protected，internal

// public by default
var isVisible = true

// only in the same file
private var isHidden = false

internal val almostVisible = true

class Foo {
    // public by default
    var isVisible = true

    // visible to my subclasses
    protected val isInheritable = true

    // only in the same class
    private val isHidden = true
}

Visibility Modifiers – Kotlin Programming Languagekotlinlang.org

如果过度重载导致函数的数量急剧增加，我们可以指定Kotlin中参数的default值，还可以通过在call函数时候，指定参数值，增加code的可读性

// parameters with default values
class BulletPointSpan(private val bulletRadius: Float = 1.2f,
                      private val gapWidth: Int = 5,
                      private val color: Int = 255)

// using only default values
val bulletPointSpan = BulletPointSpan()
// passing a value to the first argument and using default values for the other two
val bulletPointSpan2 = BulletPointSpan(1.6f)
// using a named parameter for the last argument and default values for the other two
val bulletPointSpan3 = BulletPointSpan(color = 0)

Functions: infix, vararg, tailrec – Kotlin Programming Languagekotlinlang.org

密封类用来限制类的继承关系，这意味着密封类的子类数量是固定的。看起来就像是枚举那样，当你想在一个密封类的子类中寻找一个指定的类的时候，你可以事先知道所有的子类。不同之处在于枚举的实例是唯一的，而密封类可以有很多实例，它们可以有不同的状态。

它和Enum class的区别：sealed class子类可以有multi instances，但是enum constant只有一个instance实例，如果需要constant behavior，选择enum，否则选择sealed；sealed类本身是抽象的，不能实例化，可以添加abstract member

sealed class Intention {
    // 在kotlin1.1之前，必须放到Intention之中
    class Refresh: Intention()
    class LoadMore: Intention()
}

fun main(args: Array<String>) {
    val intention: Intention = Intention.LoadMore()
    // else statement is unnecessary when using sealed class
    val output = when (intention) {
        is Intention.Refresh -> "refresh"
        is Intention.LoadMore -> "loadMore"
    }
    println(output)
}

//另外一种方式定义
sealed class NetworkResult
data class Success(val result: String): NetworkResult()
data class Failure(val error: String): NetworkResult()

Sealed Classes – Kotlin Programming Languagekotlinlang.org