Java: Static vs inner class

Question

What is the difference between static and non-static nested class?

Solution 1

An inner class, by definition, cannot be static, so I am going to recast your question as "What is the difference between static and non-static nested classes?"

A non-static nested class has full access to the members of the class within which it is nested. A static nested class does not have a reference to a nesting instance, so a static nested class cannot invoke non-static methods or access non-static fields of an instance of the class within which it is nested.

Solution 2

Let's look in the source of wisdom for such questions: Joshua Bloch's Effective Java:

Technically, there is no such thing as a static inner class. According to Effective Java, the correct terminology is a static nested class. A non-static nested class is indeed an inner class, along with anonymous classes and local classes.

And now to quote:

Each instance of a non-static nested class is implicitly associated with an enclosing instance of its containing class... It is possible to invoke methods on the enclosing instance.

A static nested class does not have access to the enclosing instance. It uses less space too.

Solution 3

1. Static inner class cannot access non-static members of enclosing class. It can directly access static members (instance field and methods) of enclosing class same like the procedural style of getting value without creating object.

2. Static inner class can declare both static and non-static members. The static methods have access to static members of main class. However, it cannot access non-static inner class members. To access members of non-static inner class, it has to create object of non-static inner class.

3. Non-static inner class cannot declare static field and static methods. It has to be declared in either static or top level types. You will get this error on doing so saying "static fields only be declared in static or top level types".

4. Non-static inner class can access both static and non-static members of enclosing class in procedural style of getting value, but it cannot access members of static inner class.

5. The enclosing class cannot access members of inner classes until it creates an object of inner classes. IF main class in accessing members of non-static class it can create object of non-static inner class.

6. If main class in accessing members of static inner class it has two cases:

   • Case 1: For static members, it can use class name of static inner class
   • Case 2: For non-static members, it can create instance of static inner class.

Solution 4

Discussing nested classes...

The difference is that a nested class declaration that is also static can be instantiated outside of the enclosing class.

When you have a nested class declaration that is not static, Java won't let you instantiate it except via the enclosing class. The object created out of the inner class is linked to the object created from the outer class, so the inner class can reference the fields of the outer.

But if it's static, then the link does not exist, the outer fields cannot be accessed (except via an ordinary reference like any other object) and you can therefore instantiate the nested class by itself.

Solution 5

static inner class: can declare static & non static members but can only access static members of its parents class.

non static inner class: can declare only non static members but can access static and non static member of its parent class.

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