如何在 Kotlin Android 中为数据类创建空构造函数

发布于06月17日

我在Kotlin数据类中声明了10多个变量，我想为它创建一个空构造函数，就像我们在Java中通常做的那样.

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<*, *>,
)

预期用法:

val activity =  Activity();
activity.title = "New Computer"
sendToServer(activity)

但是数据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)

推荐答案

这里有两个选项:

for each primary constructor参数指定一个默认值:

data class Activity(
    var updated_on: String = "",
    var tags: List<String> = emptyList(),
    var description: String = "",
    var user_id: List<Int> = emptyList(),
    var status_id: Int = -1,
    var title: String = "",
    var created_at: String = "",
    var data: HashMap<*, *> = hashMapOf<Any, Any>(),
    var id: Int = -1,
    var counts: LinkedTreeMap<*, *> = LinkedTreeMap<Any, Any>()
)

声明没有参数的a secondary constructor:

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<*, *>
) {
    constructor() : this("", emptyList(), 
                         "", emptyList(), -1, 
                         "", "", hashMapOf<Any, Any>(), 
                         -1, LinkedTreeMap<Any, Any>()
                         )
}

如果您不依赖Activity类中的copy或equals，或者根本不使用自动生成的data class方法，那么您可以像这样使用常规类:

class ActivityDto {
    var updated_on: String = "",
    var tags: List<String> = emptyList(),
    var description: String = "",
    var user_id: List<Int> = emptyList(),
    var status_id: Int = -1,
    var title: String = "",
    var created_at: String = "",
    var data: HashMap<*, *> = hashMapOf<Any, Any>(),
    var id: Int = -1,
    var counts: LinkedTreeMap<*, *> = LinkedTreeMap<Any, Any>()
}

不是每DTO个都需要data class个，反之亦然.事实上，根据我的经验，我发现数据类在涉及一些复杂业务逻辑的领域特别有用.