Why instantiation of static nested class object is allowed?

问题

I have started learning Java language for Android Application developement.

As per my understanding based on static class, we cannot instantiate object of static class.

But why instantiation of static nested class object is allowed in following situaltion?

class EnclosingClass 
{     
      //...     
      class static StaticInnerClass 
      {         
          //...     
      } 
}

Why we can create object of inner class if it is marked as static?

EnclosingClass.StaticInnerClass s = new EnclosingClass.StaticInnerClass()

回答1:

As per my understanding based on static class, we cannot instantiate object of static class.

Your understanding of the meaning of "static class" is incorrect. Basically a "static class" in Java is a nested class which doesn't have an implicit reference to an instance of the containing class. See section 8.5.1 of the JLS for more information, in particular:

The static keyword may modify the declaration of a member type C within the body of a non-inner class or interface T. Its effect is to declare that C is not an inner class. Just as a static method of T has no current instance of T in its body, C also has no current instance of T, nor does it have any lexically enclosing instances.

Perhaps you were thinking of static classes in C#, which are completely different?

回答2:

Why we can create object of inner class if it is marked as static?

You may need to use a nested class in a static context, for example:

public class Test {

    public static void main(String args[]) {
        InnerClass innerClass = new InnerClass();
    }

    class InnerClass {

    }
}

In this case, when you try to instantiate the innerClass you get the error:

No enclosing instance of type Test is accessible. Must qualify the allocation with an enclosing instance of type Test (e.g. x.new A() where x is an instance of Test).

To avoid this, you could instantiate an object of type Test and create an instance of innerClass from it:

Test test = new Test();
InnerClass innerClass = test.new InnerClass();

or better, declare also the innerClass as static and instantiate it in a static context:

public class Test {

    public static void main(String args[]) {
        InnerClass innerClass = new InnerClass();
    }

    static class InnerClass {

    }
}

回答3:

check it, maybe it can help you Nested Classes

来源：https://stackoverflow.com/questions/12836506/why-instantiation-of-static-nested-class-object-is-allowed

标签

java

oop

inner-classes

nested-class