Scala:参数的 (a: String) 和 (a: => String) 之间有什么区别? [英] Scala: What is the difference between (a: String) and (a: => String) for argument?

问题描述

这两种方法有什么区别?

What is the difference between these two method?

def method1(msg: => String) = print(msg)
def method2(msg: String) = print(msg)

我可以同时调用它们.

  method1("1")
  method2("2") 

然后他们打印 12

推荐答案

它被称为按名称传递参数，当参数按名称传递并在函数内部访问时，它会再次计算.您可以运行示例代码:

It's called passing argument by name, when argument passed by name and accessed inside function it evaluates again. You can run example code:

  def method1(msg: => String) = {
    println(msg)
    println(msg)
  }
  def method2(msg: String) = {
    println(msg)
    println(msg)
  }

  method1({println("Evaluate Arg1"); "Method1"})
  method2({println("Evaluate Arg2"); "Method2"})

输出:

Evaluate Arg1
Method1
Evaluate Arg1
Method1
Evaluate Arg2
Method2
Method2

正如你在 method1 中看到的，每次访问 msg 都会重新评估

As you can see in method1 every time you access msg it's re-evaluated

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