首先，在大多数情况下，您不需要在 Kotlin 中使用构建器，因为我们有默认和命名参数。这使您可以编写

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

并像这样使用它：

val car = Car(model = "X")

如果您绝对想使用构建器，可以这样做：

使 Builder acompanion object没有意义，因为objects 是单例。而是将其声明为嵌套类（在 Kotlin 中默认为静态）。

将属性移动到构造函数，以便对象也可以以常规方式实例化（如果不应该将构造函数设为私有）并使用辅助构造函数，该构造函数接受构建器并委托给主构造函数。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()

可以通过使用构建器 DSL来缩短此代码：

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.build { model = "X" }

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以及build我们刚刚定义的方法中：

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" }