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question about inner class instantiation

ronnir paterl
Greenhorn

Joined: Oct 18, 2007
Posts: 11
Hi I had a doubt about the instantiation of inner class�
From what I know instantiation of an inner class requires an object of outer class, except when the inner class is instantiated within the outer class itself.

public class outer{
class inner{
}

inner in = new inner();// (1) valid

public static void main(String []args){

inner in = new inner();// (2) not valid outer class object reqd.
}

}


On similar lines consider this example�


interface outer{
class inner{
}
}


public class outer2 implements outer{

public static void main(String args[]){

inner in = new inner(); // (3) valid

}
}

So my question is
1.Why cant we instantiate an inner class in a static method of outer class.
2.And why can we do the same in a class which implements the interface of the outer class.


Thanks in advance�
marc weber
Sheriff

Joined: Aug 31, 2004
Posts: 11343

Good question!

In an interface, a nested class definition is implicitly static. (See JLS 9.5.) Therefore, although it is a "nested" class, it is not an "inner" class. (See JLS 8, JLS 8.1.3, and JLS 8.5.2.)

Because it is static, a static nested class can be instantiated without any enclosing instance. This makes it possible to instantiate from a static context, like main, where there is no implicit "this" referencing the current instance.

(Edit: Added JLS references.)
[ November 01, 2008: Message edited by: marc weber ]

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