lambda - Kotlin:lambda 从不编译

Question

我刚开始学习 Kotlin，我遇到了 lambdas 语法方面的问题。有一个小示例类:

class MathFunctions {
    @FunctionalInterface
    interface Operation {
        fun calculate(a: Int, b: Int): Int
    }

    fun makeCalculations(a: Int, b: Int, operation: Operation): Int = operation.calculate(a, b)

    companion object {
        @JvmStatic
        fun main(args: Array<String>) {
            val plus = Operation { a, b -> a + b }
            val mathFunctions = MathFunctions()
            println(mathFunctions.makeCalculations(10, 20, plus))
        }
    }
}

该类永远不会编译，因为 lambda 有问题。 Java 中的相同内容如下所示:

public class MathFunctions {
    @FunctionalInterface
    interface Operation {
        int calculate(int a, int b);
    }

    public int makeCalculations(int a, int b, Operation operation) {
        return operation.calculate(a, b);
    }

    public static void main(String[] args) {
        Operation plus = (a, b) -> a + b;
        MathFunctions example = new MathFunctions();

        System.out.println(example.makeCalculations(10, 20, plus));
    }
}

应该纠正什么？我已经尝试在 Intellij Idea 中自动将 Java 转换为 Kotlin，但失败了。

Answer 1

您的代码的问题是 Kotlin does not allow SAM conversion to interfaces written in Kotlin .这个限制的原因是 Kotlin 有 functional types built into the language ，应该改用它们(或退回到 anonymous object expressions )。

您可以将代码重写为:

class MathFunctions {
    fun makeCalculations(a: Int, b: Int, operation: (Int, Int) -> Int): Int = 
            operation(a, b)

    companion object {
        @JvmStatic
        fun main(args: Array<String>) {
            val plus: (Int, Int) -> Int = { a, b -> a + b }
            val mathFunctions = MathFunctions()
            println(mathFunctions.makeCalculations(10, 20, plus))
        }
    }
}

为了使代码更具可读性，可以使用type aliases :

typealias Operation = (Int, Int) -> Int