我是 Kotlin 世界的新手。我喜欢我目前所看到的,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。
这些库充满了带有 setter、getter 和 Builder 类的 Pojo。现在我用谷歌搜索找到在 Kotlin 中实现构建器的最佳方法,但没有成功。
第二次更新:问题是如何在 Kotlin 中为带有一些参数的简单 pojo 编写 Builder 设计模式?下面的代码是我尝试编写 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 } } }
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)。
companion object
object
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()
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" }
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:
build
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" }
val car = Car.build(required = "requiredValue") { model = "X" }