我在Kotlin数据类中声明了10+变量，我想为它创建一个空构造函数，就像我们通常在Java中做的那样。
数据类:
data class Activity(
var updated_on: String,
var tags: List<String>,
var description: String,
var user_id: List<Int>,
var status_id: Int,
var title: String,
var created_at: String,
var data: HashMap<*, *>,
var id: Int,
var counts: LinkedTreeMap<*, *>,
)
预期用途:
val activity = Activity();
activity.title = "New Computer"
sendToServer(activity)
但是data类要求在创建构造函数时传递所有参数。我们如何像Java POJO类构造函数那样简化它?
val activity = Activity(null,null,null,null,null,"New Computer",null,null,null,null)
sendToServer(activity)
如果你给每个主构造函数参数一个默认值:
data class Item(var id: String = "",
var title: String = "",
var condition: String = "",
var price: String = "",
var categoryId: String = "",
var make: String = "",
var model: String = "",
var year: String = "",
var bodyStyle: String = "",
var detail: String = "",
var latitude: Double = 0.0,
var longitude: Double = 0.0,
var listImages: List<String> = emptyList(),
var idSeller: String = "")
从课堂上的实例中你可以不带参数地调用它或者用你当时的参数
var newItem = Item()
var newItem2 = Item(title = "exampleTitle",
condition = "exampleCondition",
price = "examplePrice",
categoryId = "exampleCategoryId")
这个问题的现代答案应该是使用Kotlin的无参数编译器插件，它为经典的apies创建了一个无参数的构造代码
简单地说，你必须在build中添加插件类路径。Gradle项目级别
dependencies {
....
classpath "org.jetbrains.kotlin:kotlin-noarg:1.4.10"
....
}
然后配置注释以生成无参数构造函数
apply plugin: "kotlin-noarg"
noArg {
annotation("your.path.to.annotaion.NoArg")
invokeInitializers = true
}
然后定义注释文件noargt .kt
@Target(AnnotationTarget.CLASS)
@Retention(AnnotationRetention.SOURCE)
annotation class NoArg
最后，在任何数据类中，您都可以简单地使用自己的注释
@NoArg
data class SomeClass( val datafield:Type , ... )
我曾经创建自己的无参数构造函数作为接受的答案，这是我通过搜索得到的，但后来这个插件发布了什么的，我发现它更干净。
Kotlin中数据类的非空辅助构造函数:
data class ChemicalElement(var name: String,
var symbol: String,
var atomicNumber: Int,
var atomicWeight: Double,
var nobleMetal: Boolean?) {
constructor(): this("Silver", "Ag", 47, 107.8682, true)
}
fun main() {
var chemicalElement = ChemicalElement()
println("RESULT: ${chemicalElement.symbol} means ${chemicalElement.name}")
println(chemicalElement)
}
// RESULT: Ag means Silver
// ChemicalElement(name=Silver, symbol=Ag, atomicNumber=47,
// atomicWeight=107.8682, nobleMetal=true)
Kotlin中数据类的空辅助构造函数:
data class ChemicalElement(var name: String,
var symbol: String,
var atomicNumber: Int,
var atomicWeight: Double,
var nobleMetal: Boolean?) {
constructor(): this("", "", -1, 0.0, null)
}
fun main() {
var chemicalElement = ChemicalElement()
println(chemicalElement)
}
// ChemicalElement(name=, symbol=, atomicNumber=-1,
// atomicWeight=0.0, nobleMetal=null)
