我想说的是，Kotlin中的模式和实现基本保持不变。由于默认值，您有时可以跳过它，但对于更复杂的对象创建，构建器仍然是一个不可省略的有用工具。

我个人从来没有在Kotlin见过一个建筑工人，但也许只有我这样。

所有需要的验证都发生在init块中:

class Car(val model: String,
          val year: Int = 2000) {

    init {
        if(year < 1900) throw Exception("...")
    }
}

在这里，我冒昧地猜测，您并不真的希望型号和年份可以更改。而且，这些默认值似乎没有任何意义，(特别是null作为name)，但为了演示目的，我留下了一个。

一个意见: 在Java中使用的构建器模式意味着不需要命名参数。在带有命名参数的语言(如Kotlin或Python)中，使用带有长串(可能是可选的)参数的构造函数是一个很好的实践。

首先，在大多数情况下，您不需要在Kotlin中使用构建器，因为我们有默认参数和命名参数。这使你能够写作

class Car(val model: String? = null, val year: Int = 0)

像这样使用它:

val car = Car(model = "X")

如果你绝对想要使用构建器，你可以这样做:

将Builder作为伴生对象没有意义，因为对象是单例对象。相反，将其声明为一个嵌套类(在Kotlin中默认是静态的)。

将属性移动到构造函数中，这样对象也可以以常规方式实例化(如果不应该，则将构造函数设置为私有)，并使用接受构造函数并将其委托给主构造函数的辅助构造函数。代码如下所示:

class Car( //add private constructor if necessary
        val model: String?,
        val year: Int
) {

    private constructor(builder: Builder) : this(builder.model, builder.year)

    class Builder {
        var model: String? = null
            private set

        var year: Int = 0
            private set

        fun model(model: String) = apply { this.model = model }

        fun year(year: Int) = apply { this.year = year }

        fun build() = Car(this)
    }
}

用法:val car = car . builder ().model("X").build()

这段代码可以通过使用构建器DSL进一步缩短:

class Car (
        val model: String?,
        val year: Int
) {

    private constructor(builder: Builder) : this(builder.model, builder.year)

    companion object {
        inline fun build(block: Builder.() -> Unit) = Builder().apply(block).build()
    }

    class Builder {
        var model: String? = null
        var year: Int = 0

        fun build() = Car(this)
    }
}

用法:val car = car。建立{模型= "X"}

如果某些值是必需的，并且没有默认值，你需要将它们放在构建器的构造函数中，也需要放在我们刚刚定义的构建方法中:

class Car (
        val model: String?,
        val year: Int,
        val required: String
) {

    private constructor(builder: Builder) : this(builder.model, builder.year, builder.required)

    companion object {
        inline fun build(required: String, block: Builder.() -> Unit) = Builder(required).apply(block).build()
    }

    class Builder(
            val required: String
    ) {
        var model: String? = null
        var year: Int = 0

        fun build() = Car(this)
    }
}

用法:val car = car。build(required = "requiredValue") {model = "X"}

因为我使用Jackson库从JSON解析对象，我需要一个空构造函数，我不能有可选字段。而且所有字段都必须是可变的。然后我可以使用这个漂亮的语法，它做的事情与生成器模式相同:

val car = Car().apply{ model = "Ford"; year = 2000 }

对于简单类，不需要单独的构建器。您可以像Kirill Rakhman描述的那样使用可选的构造函数参数。

如果你有更复杂的类，那么Kotlin提供了一种方法来创建Groovy风格的Builders/DSL:

类型安全的建筑商

这里有一个例子:

Github示例-构建器/汇编器

在kotlin中可以使用可选参数 例子:

fun myFunc(p1: String, p2: Int = -1, p3: Long = -1, p4: String = "default") {
    System.out.printf("parameter %s %d %d %s\n", p1, p2, p3, p4)
}

然后

myFunc("a")
myFunc("a", 1)
myFunc("a", 1, 2)
myFunc("a", 1, 2, "b")

我看到过许多将额外乐趣作为构建者的例子。我个人喜欢这种方法。节省编写构建程序的精力。

package android.zeroarst.lab.koltinlab

import kotlin.properties.Delegates

class Lab {
    companion object {
        @JvmStatic fun main(args: Array<String>) {

            val roy = Person {
                name = "Roy"
                age = 33
                height = 173
                single = true
                car {
                    brand = "Tesla"
                    model = "Model X"
                    year = 2017
                }
                car {
                    brand = "Tesla"
                    model = "Model S"
                    year = 2018
                }
            }

            println(roy)
        }

        class Person() {
            constructor(init: Person.() -> Unit) : this() {
                this.init()
            }

            var name: String by Delegates.notNull()
            var age: Int by Delegates.notNull()
            var height: Int by Delegates.notNull()
            var single: Boolean by Delegates.notNull()
            val cars: MutableList<Car> by lazy { arrayListOf<Car>() }

            override fun toString(): String {
                return "name=$name, age=$age, " +
                        "height=$height, " +
                        "single=${when (single) {
                            true -> "looking for a girl friend T___T"
                            false -> "Happy!!"
                        }}\nCars: $cars"
            }
        }

        class Car() {

            var brand: String by Delegates.notNull()
            var model: String by Delegates.notNull()
            var year: Int by Delegates.notNull()

            override fun toString(): String {
                return "(brand=$brand, model=$model, year=$year)"
            }
        }

        fun Person.car(init: Car.() -> Unit): Unit {
            cars.add(Car().apply(init))
        }

    }
}

我还没有找到一种方法，可以强制在DSL中初始化一些字段，比如显示错误而不是抛出异常。如果有人知道就告诉我。

class Foo private constructor(@DrawableRes requiredImageRes: Int, optionalTitle: String?) {

    @DrawableRes
    @get:DrawableRes
    val requiredImageRes: Int

    val optionalTitle: String?

    init {
        this.requiredImageRes = requiredImageRes
        this.requiredImageRes = optionalTitle
    }

    class Builder {

        @DrawableRes
        private var requiredImageRes: Int = -1

        private var optionalTitle: String? = null

        fun requiredImageRes(@DrawableRes imageRes: Int): Builder {
            this.intent = intent
            return this
        } 

        fun optionalTitle(title: String): Builder {
            this.optionalTitle = title
            return this
        }

        fun build(): Foo {
            if(requiredImageRes == -1) {
                throw IllegalStateException("No image res provided")
            }
            return Foo(this.requiredImageRes, this.optionalTitle)
        }

    }

}

现在的人们应该检查Kotlin的类型安全构建器。

使用上述方法创建对象将看起来像这样:

html {
    head {
        title {+"XML encoding with Kotlin"}
    }
    // ...
}

vaadin-on-kotlin框架是一个很好的“实际”使用示例，它利用类型安全构建器来组装视图和组件。

我在Kotlin中实现了一个基本的Builder模式，代码如下:

data class DialogMessage(
        var title: String = "",
        var message: String = ""
) {


    class Builder( context: Context){


        private var context: Context = context
        private var title: String = ""
        private var message: String = ""

        fun title( title : String) = apply { this.title = title }

        fun message( message : String ) = apply { this.message = message  }    

        fun build() = KeyoDialogMessage(
                title,
                message
        )

    }

    private lateinit var  dialog : Dialog

    fun show(){
        this.dialog= Dialog(context)
        .
        .
        .
        dialog.show()

    }

    fun hide(){
        if( this.dialog != null){
            this.dialog.dismiss()
        }
    }
}

最后

Java:

new DialogMessage.Builder( context )
       .title("Title")
       .message("Message")
       .build()
       .show();

科特林：

DialogMessage.Builder( context )
       .title("Title")
       .message("")
       .build()
       .show()

我在一个Kotlin项目中工作，该项目公开了一个由Java客户机使用的API (Java客户机不能利用Kotlin语言结构)。我们必须添加构建器以使它们在Java中可用，因此我创建了@Builder注释:https://github.com/ThinkingLogic/kotlin-builder-annotation 它基本上是Kotlin的Lombok @Builder注释的替代品。

一种方法是做以下事情:

class Car(
  val model: String?,
  val color: String?,
  val type: String?) {

    data class Builder(
      var model: String? = null,
      var color: String? = null,
      var type: String? = null) {

        fun model(model: String) = apply { this.model = model }
        fun color(color: String) = apply { this.color = color }
        fun type(type: String) = apply { this.type = type }
        fun build() = Car(model, color, type)
    }
}

使用示例:

val car = Car.Builder()
  .model("Ford Focus")
  .color("Black")
  .type("Type")
  .build()

我迟到了。如果我必须在项目中使用Builder模式，我也遇到了同样的困境。后来，经过研究，我意识到这是完全不必要的，因为Kotlin已经提供了命名参数和默认参数。

如果你真的需要实施，Kirill Rakhman的答案是关于如何以最有效的方式实施的可靠答案。您可能会发现另一件有用的事情是https://www.baeldung.com/kotlin-builder-pattern，您可以将其与Java和Kotlin的实现进行比较和对比

class Person(
    val name:String,
    val family:String,
    val age:Int,
    val nationalCode: String?,
    val email: String?,
    val phoneNumber: String?
) {

    // Private constructor
    private constructor(builder: Builder) : this (
        builder.name,
        builder.family,
        builder.age,
        builder.nationalCode,
        builder.email,
        builder.phoneNumber
    )

    // Builder class

    // 1 Necessary parameters in Builder class : name , family
    class Builder(val name :String,val family :String) {

        // 2 Optional parameters in Builder class :
        var age: Int = 0
            private set
        var nationalCode: String? = null
            private set
        var email: String? = null
            private set
        var phoneNumber: String? = null
            private set

        fun age(age: Int) = apply { this.age = age }
        fun nationalCode(nationalCode: String) =
            apply { this.nationalCode = nationalCode }
        fun email(email: String) = apply { this.email = email }
        fun phoneNumber(phoneNumber: String) =
            apply { this.phoneNumber = phoneNumber }

        // 3 Create
        fun create() = Person(this)

    }
}

存取资料:

val firstPerson = Person.Builder(
    name = "Adnan",
    family = "Abdollah Zaki")
    .age(32)
    .email("Adnan9011@gmail.com")
    .phoneNumber("+989333030XXX")
    .nationalCode("04400XXXXX")
    .create()

val secondPerson = Person.Builder(
    name = "Foroogh",
    family = "Varmazyar")
    .create()

我刚刚发现了一个有趣的方法来创建kotlin构建器:

如您所见，moduleBuilder可以用于其他grafana构建。

代码如下:

class Grafana(
    private val module: String,
    private val scene: String,
    private val action: String,
    private val metric: String
) {
    companion object {
        fun build(module: String, scene: String, action: String, metric: String) =
            Grafana(module, scene, action, metric)

        val builder = ::build.curriedBuilder()

        private fun <P1, P2, P3, P4, R> Function4<P1, P2, P3, P4, R>.curriedBuilder() =
            fun(p1: P1) = fun(p2: P2) = fun(p3: P3) = fun(p4: P4) = this(p1, p2, p3, p4)
    }

    fun report() = Unit
}


val moduleBuilder = Grafana.builder("module")
val scene = moduleBuilder("scene")
val gfA = scene("action")("metric")
gfA.report()

val sceneB = moduleBuilder("sceneB")
val gfB = sceneB("action")("metric")
gfB.report()

val gfC = Grafana.builder("xx")("xxx")("xxxx")("xxxx")
gfC.report()

上面的答案有一点改变和改进

class MyDialog {
  private var title: String? = null
  private var content: String? = null
  private var confirmButtonTitle: String? = null
  private var rejectButtonTitle: String? = null

  @DrawableRes
  private var icon: Int? = null


  fun show() {
    // set dialog content here and show at the end
  }

  class Builder {
      private var dialog: MyDialog = MyDialog()

      fun title(title: String) = apply { dialog.title = title }

      fun icon(@DrawableRes icon: Int) = apply { dialog.icon = icon }

      fun content(content: String) = apply { dialog.content = content }

      fun confirmTitle(confirmTitle: String) = apply { dialog.confirmButtonTitle = confirmTitle }

      fun rejectButtonTitle(rejectButtonTitle: String) = apply { dialog.rejectButtonTitle = rejectButtonTitle }

      fun build() = dialog
  }
}

和使用

MyDialog.Builder()
        .title("My Title")
        .content("My content here")
        .icon(R.drawable.bg_edittext)
        .confirmTitle("Accept")
        .rejectButtonTitle("Cancel")
        .build()
        .show()