从 java class 调用时，Kotlin 数据 class 复制功能不起作用

Question

也许我误解了 data class 的 copy 函数是如何工作的，或者可能存在错误，但以下是 copy 的示例功能未按预期工作：

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
}

Java

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 只是在创建 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。

Answer 1

好吧，我在文档中漏掉了这句话：

If any of these functions is explicitly defined in the class body or inherited from the base types, it will not be generated.

事实上，这使得 copy 并不比 Java 互操作的构造函数更好。

Answer 2

要绕过 Kotlin 的 copy() 的限制，您可以做的是在数据中创建自己的复制函数 class。示例如下：

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)
    }

}

Answer 3

对于与 java 的互操作，您可以制作使用 kotlin 生成的函数 .copy

@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
}}

Answer 4

这个问题的搜索排名很高，可能会让 kotlin 新手感到困惑，因为该问题的示例代码不是典型的 kotlin 代码或复制功能的用法。我在下面添加了一些示例代码以帮助阐明发生了什么，并且还显示了数据 class.
的典型用法简而言之，copy 函数在从 kotlin class 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);
    }
}

关于数据的官方文档 classes，包括 copy 函数：
https://kotlinlang.org/docs/reference/data-classes.html

从 java class 调用时，Kotlin 数据 class 复制功能不起作用

Kotlin data class copy function not working when called from java class

java

kotlin

data-class