同伴对象与普通对象相比有什么优势？

Question

Kotlin代码是这样的：

class Foo {
  companion object {
     fun a() : Int = 1
  }
  fun b() = a() + 1
}

可以简单地更改为

object FooStatic {
   fun a() : Int = 1
}

class Foo {
  fun b() = FooStatic.a()
}

我知道同伴对象可以用来作为真正的java静态函数使用，但是使用同伴对象还有其他的优点吗？

Answer 1

其中一个关键区别是成员的可见性。在伴生对象中，包含类的可见性就像--如果成员是类的一部分--对于原始对象就不是这样。

下面的示例显示，您不能使用“对象”来实现类的私有静态内部。

package com.example

class Boo {

    companion object Boo_Core {
        // Public "static" call that non-Boo classes should not be able to call
        fun add_pub(a:Int) = a+1;

        // Internal "static" call that non-Boo classes should not be able to call
        private fun add_priv(a:Int) = a+1;
    }

    // OK: Functions in Boo can call the public members of the companion object
    fun blah_pub(a:Int) = add_pub(a)
    // OK: Functions in Boo can call the private members of the companion object
    fun blah_priv(a:Int) = add_priv(a)
}

//Same idea as the companion object, but as an "object" instead.
object Foo_Core {
    fun add_pub(a:Int) = a+1
    private fun add_priv(a:Int) = a+1;
}

class Foo {
    // OK Foo can get Foo_Cors add_pub
    fun blah_pub(a:Int) = Foo_Core.add_pub(a);

    // ERROR: can not get to add_priv
    // fun blah_priv(a:Int) = Foo_Core.add_priv(a);
}

class AnInterloper {

    // OK Other classes can use public entries in Foo.
    fun blah_foo_pub(a:Int) = Foo_Core.add_pub(a); 

    // ERROR Other classes can use public entries in Foo.
    // fun blah_foo_priv(a:Int) = Foo_Core.add_priv(a); 

    // OK: Other classes can use public Boo classes
    fun blah_boo_pub(a:Int) = Boo.add_pub(a);

    // ERROR: Other classes can not use private Boo classes
    // fun blah_boo_priv(a:Int) = Boo.add_priv(a);
}