CODEWITHSUNDEEP
javastatic
Dmitry Berdnikov
Dmitry Berdnikov

Reputation: 415

Why would i want to create an instance of a static class in java?

The Java Tutorial says that the static nested classes are accessed by using the name of the enclosing class like new EnclosingClassNameHere.StaticNestedClassNameHere()

Why would i want to create an instance of a static class at all? Can somebody please explain?

Upvotes: 3

Views: 524

Answers (3)

josefx
josefx

Reputation: 15656

In java inner classes have an implicit reference to an instance of the outer class. This way you can access members of the outer class directly, which is usefull in annonymous classes used for callbacks.

class A{
   private int a = 3;
   class Inner{
      Inner(){

         System.out.println(a);//also A.this.a
      }
   }
   static class StaticInner{
       StaticInner(){
          System.out.println(a);//FAILS
       }

   }
}

Declaring an inner class static simply removes this implicit reference and that is the only difference between static and non static inner classes.

Upvotes: 1

Kylar
Kylar

Reputation: 9344

"static" in this case can be misleading. What it really means is that the class can exist independently. Non-static inner classes can't exist without an instance of the enclosing class.

IMO, when you start using an inner class outside the class that it's in, you need to consider moving it and making it its own top level class. There are very few cases where the relationship between the classes is so tightly coupled that you need to keep it as an inner class.

In your code example:

OuterClass.StaticNestedClass nestedObject = new OuterClass.StaticNestedClass();

You're creating a stand-alone instance of StaticNestedClass.

If it wasn't static, you couldn't do that. You could only create instances of StaticNestedClass() from an instance of OuterClass.

If you moved it to its own .java file, you could treat it nearly identically:

StaticNestedClass notNestedAnymore = new StaticNestedClass();

As to your real question: Why would you want to create an instance of it? For the same reason that you create instances of any class - it does some piece of work that you need.

Upvotes: 6

Tomasz Nurkiewicz
Tomasz Nurkiewicz

Reputation: 340963

There is nothing confusing with this code. Static nested class is just a way to introduce yet another namespace.

By creating a static nested class you express very strong relationship between outer and inner class. Typically nested class is a helper or a part of the outer class. For instance when you create a Tree class, Node class is a good candidate for a nested static class. The Tree.Node clearly explains the purpose of the Node class.

In fact, static keyword usage is consistent with static fields. In both cases you can access static entity without an instance of enclosing class. When it comes to static classes it basically means: "I can create an instance of this static nested class without having an instance of outer class". By default (when static keyword is not used) the nested class becomes inner class. In this case you cannot simply write:

new OuterClass.StaticNestedClass();

Instead you are required to pass OuterClass instance with a bit obscure syntax:

OuterClass outerClassInstance = new OuterClass();
outerClassInstance.new InnerClass();

Fortunately when new InnerClass() is executed inside an OuterClass body, this is implictly used as enclosing instance.

Upvotes: 5

Related Questions