在scala中声明变量时什么时候需要显式类型? [英] when does it need explicit type when declare variable in scala?

问题描述

这是我的问题的简单示例代码.

Here is a simple sample code for my question.

var a:Int=1;//第1行

var a:Int=1; //line 1

var a=1;//第2行

var a=1; //line 2

是否需要第 1 行中的 Int?还是必须?如果没有，我可以像第 2 行那样删除它吗?

Is Int in line 1 needed? or must? if not ,can I delete it like in line 2?

推荐答案

由于 1 是 Int 类型，编译器知道 a 是也是 Int 类型.这称为类型推断.

Since 1 is of type Int, compiler knows that a is of type Int too. This is called type inference.

您应该显式指定一个类型，这样代码可读性更好.

You should specify a type explicitly when this is better for code readability.

当编译器无法推断类型或这有助于推断其他类型时，您必须指定类型.

You must specify a type when compiler can't infer the type or when this helps to infer other types.

在 Scala 中，类型推断可以双向进行，从右到左，反之亦然.例如在 val a = 1 中 a 的类型是从 1 的类型推断出来的，所以类型推断是从右到左的.在

In Scala type inference can go in both directions, from right to left and vice versa. For example in val a = 1 type of a is inferred from type of 1, so type inference went from right to left. In

def myMethod[T](): T = ???
val n: Int = myMethod()

因为 n 应该是 Int，编译器推断 myMethod() 中的 T 应该是Int 也是，所以类型推断是从左到右的.

since n is expected to be an Int, compiler infers that T in myMethod() should be Int too, so type inference went from left to right.

https://twitter.github.io/scala_school/type-basics.html#inference

http://www.scala-lang.org/old/node/127

http://allaboutscala.com/tutorials/chapter-2-learning-basics-scala-programming/scala-tutorial-overview-scala-type-in​​ference/

scala 如何推断变量的类型?

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