如何在Kotlin中实现Builder模式？

Question

嗨，我是Kotlin世界的新手。我喜欢到目前为止所看到的，并开始考虑将我们在应用程序中使用的一些库从Java转换为Kotlin。

第二次更新：问题是如何用Kotlin中的一些参数为一个简单的pojo编写一个构建器设计模式？下面的代码是我的尝试，方法是编写java代码，然后使用eclipse-kotlin-plugin转换为Kotlin。

class Car private constructor(builder:Car.Builder) {
    var model:String? = null
    var year:Int = 0
    init {
        this.model = builder.model
        this.year = builder.year
    }
    companion object Builder {
        var model:String? = null
        private set

        var year:Int = 0
        private set

        fun model(model:String):Builder {
            this.model = model
            return this
        }
        fun year(year:Int):Builder {
            this.year = year
            return this
        }
        fun build():Car {
            val car = Car(this)
            return car
        }
    }
}

Answer 1

首先，在大多数情况下，您不需要使用Kotlin中的构建器，因为我们有默认和命名参数。这使您能够编写

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

并按如下方式使用：

val car = Car(model = "X")

如果您绝对想要使用构建器，下面是您可以使用的方法：

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)
    }
}

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)
    }
}

如果某些值是必需的，并且没有默认值，则需要将它们放在构建器的构造函数中，还需要放在我们刚刚定义的build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”}