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Nested vs Inner

 
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What is the difference between Nested class and Inner class? In which circumstances can an Inner class not be a nested class?
TIA...
 
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A top-level nested class is an "inner class" that is defined as static. As such, it is not truly an inner class at all. You can look at the JLS §8.1.2 Inner Classes and Enclosing Instancesn for more details.
Hope that helps,
Corey
 
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Definition: A nested class is any class whose declaration occurs within the body of another class or interface.
Definition: A top level class is a class that is not a nested class.
Definition: An inner class is a nested class that is not explicitly or implicitly declared static.
There was a time in the history of Java when people used the word �inner� class for both the static and the non-static kind. With the second edition of the JLS, inner classes are non-static.
This is how the JLS defines these terms. Some exam prep books have different meanings for these words.
 
Marlene Miller
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The definition of inner class used to bother me. Why do they say �explicitly or implicitly� declared static? What does it mean to be implicitly declared static?
A class declared in another class either has the modifier static or it doesn�t. If there is no modifier, the class is an inner class.
A class declared in an interface is implicitly static. It is not an inner class.
 
Marlene Miller
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Sorry, I got carried away. Let�s try again.
>>What is the difference between Nested class and Inner class?
An inner class is always a nested class. Some nested classes (static member classes) are not inner classes.
>>In which circumstances can an Inner class not be a nested class?
Never.
 
Marlene Miller
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The organization is this:
A class is either (1) a top-level class or (2) a nested class.
If a class is a nested class, it is either (1) a member class, (2) a local class or (3) an anonymous class.
Member classes declared in classes are either static or inner.
Member classes declared in interfaces are static.
Local classes and anonymous classes are always inner.
 
Marlene Miller
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Some people say an instance of an inner class always has a reference to an instance of an enclosing class. That is how they define inner class.
But that is not true.
Some inner classes are declared in a static context - in a static method, static initializer or an explicit constructor invocation parameter.
To be safe, use the JLS definition. An inner class is a nested class that is not explicitly or implicitly declared static.
 
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good knowledge in JLS Marlene.
 
Marlene Miller
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Thank you Jose. I hope somewhere in that core dump Paulo can find the answer to his question.
 
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Here are related discussions:
standard terminology on inner class
Inner classes and 1.4 exam
 
Paulo Freitas
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Thanks a lot for the time spent on this question Marlene!!!
More comprehensive explanation than that would be impossible!!!
 
Marlene Miller
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Thank you Paulo. It just tumbled out, one thought after another. I was worried that I overloaded you.
The definitions of inner class and static member class taken together can drive you in circles. There is no getting around it. Each definition refers to the other.
Beware that quite a few authors use inner class when they mean nested class. Gradually everyone is learning and using the JLS definitions. And beware that some exam prep books also use the old definitions (top level nested class, static inner class � crazy).
 
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Hi Marlene,
Your explainations have helped me alot. However, I am still struggling with one of your posts. It is possible that I am 99% clear and need that last piece of puzzle. Or there is logical inconsistency in two statemets made by you:
Statement 1:


Some inner classes are declared in a static context - in a static method, static initializer or an explicit constructor invocation parameter.


Statement 2:


To be safe, use the JLS definition. An inner class is a nested class that is not explicitly or implicitly declared static.


Per my reading both statements can not be true. Statement 1 is saying that inner class can be implicitly static. Statement 2 is catagorically denying that.
Barkat
 
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Marlene is right. This just means that the body of a local method of a class defined in a static context has no access to instance variables of the enclosing class. Of course, final local variables from the surrounding method are accessible.
 
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