Scala 类中的 val 和 object? [英] val and object inside a scala class?

问题描述

在 scala 类中将字段声明为 val、lazy val 和 object 有什么区别，如下面的代码片段所示:

What is the difference between declaring a field as val, lazy val and object inside a scala class, as in the following snippet:

class A

class B {
  val a1 = new A      { def foo = 1 }
  object a2 extends A { def foo = 1 }
  lazy val a3 = new A { def foo = 1 }
}

推荐答案

在前者中，任何包含的代码都会在创建类 B 后立即执行.然而，在后者中，在您实际使用该对象之前，它不会被实例化.

In the former, any code included is executed as soon as class B is created. In the latter, however, until you actually use the object, it won't be instantiated.

您可以在这里看到不同之处:

You can see the difference here:

class A { println("Creating a new A") }
class B {
  val a1 = new A { println("a1"); def foo = 1 }
  object a2 extends A { println("a2"); def foo = 1 }
}

scala> val b = new B
Creating a new A
a1
b: B = B@1176e8a

scala> b.a2.foo
Creating a new A
a2
res0: Int = 1

在创建的 .class 文件的名称等方面也存在隐藏的差异；当然，两者有不同的类型.

There are also hidden differences in what the created .class files are named and such; and of course the two have different types.

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