如何将 Java 赋值表达式转换为 Kotlin [英] How to convert Java assignment expression to Kotlin

问题描述

Java 中的一些东西

Something in java like

int a = 1, b = 2, c = 1;
if ((a = b) !=c){
    System.out.print(true);
}

现在应该像 kotlin 一样转换成 kotlin

now it should be converted to kotlin like

var a:Int? = 1
var b:Int? = 2
var c:Int? = 1
if ( (a = b) != c)
    print(true)

但它不正确.

这是我得到的错误:

in " (a=b)" Error:(99, 9) Kotlin: Assignments are not expressions, and only expressions are allowed in this context

其实上面的代码只是为了说明问题的一个例子.这是我的原始代码:

Actually the code above is just an example to clarify the problem. Here is my original code:

fun readFile(path: String): Unit { 
    var input: InputStream = FileInputStream(path) 
    var string: String = "" 
    var tmp: Int = -1 
    var bytes: ByteArray = ByteArray(1024) 

    while((tmp=input.read(bytes))!=-1) { } 
}

推荐答案

正如@AndroidEx 正确指出的那样，赋值不是 Kotlin 中的表达式，与 Java 不同.原因是通常不鼓励具有副作用的表达式.请参阅此讨论类似的话题.

As @AndroidEx correctly stated, assignments are not expressions in Kotlin, unlike Java. The reason is that expressions with side effects are generally discouraged. See this discussion on a similar topic.

一种解决方案是拆分表达式并将赋值移出条件块:

One solution is just to split the expression and move the assignment out of condition block:

a = b
if (a != c) { ... }

另一个是使用 stdlib 中的函数，例如 let，它以接收者为参数执行 lambda 并返回 lambda 结果.apply 和 run 具有相似的语义.

Another one is to use functions from stdlib like let, which executes the lambda with the receiver as parameter and returns the lambda result. apply and run have similar semantics.

if (b.let { a = it; it != c }) { ... }

if (run { a = b; b != c }) { ... }

感谢 内联，这将与内联一样高效取自 lambda 的普通代码.

Thanks to inlining, this will be as efficient as plain code taken from the lambda.

你的 InputStream 示例看起来像

while (input.read(bytes).let { tmp = it; it != -1 }) { ... }

另外，考虑 readBytes 函数用于从 InputStream 读取 ByteArray.

Also, consider readBytes function for reading a ByteArray from an InputStream.

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