如何处理从Java迁移到Kotlin的泛型边界？ [英] How to deal with generic bounds migrating from Java to Kotlin?

问题描述

我的目标是让接口方法接受实现的类类型。这里是我写的代码：

My goal is to have the interface methods accept the class type of the implementation. Here's the code I have written so far:

internal interface Diff<Diff> {   //in Java I was using <? extends Diff>
    fun lessThan(other: Diff): Boolean
}

private class ADiff(private val value: Int) : Diff<ADiff> {

    override fun lessThan(other: ADiff): Boolean {
        return value < other.value
    }

}

//B can now accept even int types which is not desired
private class BDiff(private val value: Int) : Diff<Int> {

    override fun lessThan(other: Int): Boolean {
        return value < other
    }

}

推荐答案

在Java中工作的原因是因为< T extends Diff> 使用原始类型 差异。不要这么做！

The reason why this "worked" in Java was because <T extends Diff> uses the raw type Diff. Don't do this!

最接近的是使用递归类型绑定：

The closest you can get is to use a recursive type bound:

interface Diff<T : Diff<T>> {
    fun lessThan(other: T): Boolean
}

是，您可以替换 Diff 的任何其他子类型。

The problem is, you can substitute any other subtype of Diff.

但是，当使用 Diff ，使用泛型类型约束 T：Diff< T> ：

However, when using the Diff, use the generic type constraint T : Diff<T>:

fun <T : Diff<T>> diffUser(diff1: T, diff2: T) {
    println(diff1.lessThan(diff2))
}

和任何 Diff 不会执行 Diff< SameType> 不会被接受。

and any Diff that doesn't implement Diff<SameType> will not be accepted.

示例：

Example:

class CDiff(private val value: Int) : Diff<DDiff> { // <-- Wrong type!
    override fun lessThan(other: DDiff) = value < other.value
}

class DDiff(val value: Int) : Diff<DDiff> {
    override fun lessThan(other: DDiff) = value < other.value
}

fun test() {
    diffUser(CDiff(3), CDiff(4)) // <-- Doesn't compile due to the generic constraint
    diffUser(DDiff(3), DDiff(4))
}

同样的方法被可比较的类。

虽然这个方法可行，但 want是一个自我类型，虽然它在路线图上，但不受支持在某个时候。我相信JetBrains拒绝了这个请求，但是我找不到错误报告。

Although this works, what you really want is a "self type" and this is not supported, although it was on the roadmap at some point. I believe JetBrains denied the request for this, though I cannot find the bug report.

这个答案详细介绍了Java中使用 CRT模式的解决方法，尽管它不一定是安全的。

This answer details a workaround in Java using the CRT pattern, though it is not necessarily type safe.

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