[英]Kotlin data class copy function not working when called from java class
也许我误解了data
类的copy
功能是如何工作的,或者可能存在错误,但以下是copy
功能未按预期工作的示例:
科特林:
data class A {
public var x: String? = null
public var y: String? = null
public var z: B = B.ONE
}
enum class B {
ONE
TWO
THREE
}
爪哇
A a1 = new A()
a1.setX("Hello")
a1.setY("World")
a1.setZ(B.TWO)
A a2 = a1.copy()
// a2.x is null
// a2.y is null
// a2.z is B.ONE
似乎copy
只是制作A
的新实例而不是复制值。 如果我将变量放在构造函数中,则会分配值,但这与构造新实例没有什么不同。
好的,我在文档中遗漏了这句话:
如果这些函数中的任何一个在类主体中显式定义或从基类型继承,则不会生成它。
事实上,这使得copy
并不比 Java 互操作的构造函数更好。
为了绕过 Kotlin 的 copy() 的限制,您可以做的是在您的数据类中创建您自己的复制函数。 下面的例子:
data class User(val name : String, val property: String) {
fun copy() : User {
//uses the fields name and property defined in the constructor
return User(name,property)
}
//or if you need a copy with a changed field
fun copy(changedProperty : String) : User {
return User(name, changedProperty)
}
}
对于与 java 的互操作,您可以创建使用 kotlin 生成的 .copy 的函数
@Entity
data class User(@PrimaryKey var id: Int = 0,
var firstName: String? = null,
var lastName: String? = null,
var phone: String? = null,
var email: String? = null,
var phoneCode: String? = null,
var tokenId: String? = null,
var provider: SocialProvider? = null) : Serializable {
var countryCodeIso: String? = null
set(countryCodeIso) {
if (countryCodeIso != null) {
field = countryCodeIso.toLowerCase()
}
}
fun javaCopy(): User {
val user = copy()
user.countryCodeIso = countryCodeIso
return user
}}
这个问题在搜索排名中很高,对于那些刚接触 kotlin 的人来说可能会造成混淆,因为问题的示例代码不是典型的 kotlin 代码或复制函数的用法。 我在下面添加了一些示例代码来帮助阐明发生了什么,并展示了数据类的典型用法。
简而言之, copy
函数在从 kotlin 类调用时最有用。 我同意从 java 代码调用时它的行为并不明显。
//
// A.kt
//
// this is an idiomatic kotlin data class. note the parens around the properties, not braces.
data class A(
val x: String? = null,
val y: String? = null,
val z: B = B.ONE
) {
// this javaCopy function is completely unnecessary when being called from kotlin; it's only here to show a fairly simple way to make kotlin-java interop a little easier (like what Nokuap showed).
fun javaCopy(): A {
return this.copy()
}
}
enum class B {
ONE,
TWO,
THREE
}
fun main() {
val a1 = A("Hello", "World", B.TWO)
// here's what copy usage looks like for idiomatic kotlin code.
val a2 = a1.copy()
assert(a2.x == "Hello")
assert(a2.y == "World")
assert(a2.z == B.TWO)
// more typical is to `copy` the object and modify one or more properties during the copy. e.g.:
val a3 = a1.copy(y = "Friend")
assert(a2.x == "Hello")
assert(a3.y == "Friend")
}
public class App {
public static void main(String[] args) {
A a1 = new A("Hello", "World", B.TWO);
// the kotlin copy function is primarily meant for kotlin <-> kotlin interop
// copy works when called from java, but it requires all the args.
// calling the `javaCopy` function gives the expected behavior.
A a2 = a1.javaCopy();
assert a2.getX().equals("Hello");
assert a2.getY().equals("World");
assert a2.getZ().equals(B.TWO);
}
}
关于数据类的官方文档,包括copy
功能:
https://kotlinlang.org/docs/reference/data-classes.html
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