inner class instantiation

[Marlene Miller]

This is catalogue of inner class instantiations using new without a preceding object reference. (Not a question.)
Example 1.

class C {
    class Z {}
    //
    static void m1() {
        Z a1 = new Z();      //compiler error, static context
        C.Z a2 = new C.Z();  //compiler error, static context
    }
    //
    void m2() {
        Z b1 = new Z();
        C.Z b2 = new C.Z();  //ok, Z is a member of C
    }
}
//
: non-static variable this cannot be referenced from a static context
        Z a1 = new Z();
               ^
: non-static variable this cannot be referenced from a static context
        C.Z a2 = new C.Z();
                 ^
2 errors

The fourth one (at b2) surprised me.

[Marlene Miller]

Example 2.

class C {
    static void m1() {
        class A {}
        A a1 = new A();
        C.A a2 = new C.A();  //compiler error, C.A is a member name
    }
    //
    void m2() {
        class B {}
        B b1 = new B();
        C.B b2 = new C.B();  //compiler error, C.B is a member name
    }
}
//  
: cannot resolve symbol
symbol  : class A
location: class C
        C.A a2 = new C.A();
         ^
: cannot resolve symbol
symbol  : class A
location: class C
        C.A a2 = new C.A();
                      ^
: cannot resolve symbol
symbol  : class B
location: class C
        C.B b2 = new C.B();
         ^
: cannot resolve symbol
symbol  : class B
location: class C
        C.B b2 = new C.B();
                      ^
4 errors

[Marlene Miller]

Example 3.

class C {
    static void m1() {
        class A {}
        A a1 = new A();      //ok, static context
    }
    //
    static void m2() {
        static class B {}   //compiler error, static modifier
        B b1 = new B();
    }
}
//
: illegal start of expression
        static class B {}
        ^
: ';' expected
        static class B {}
               ^
2 errors

This one is not intuitive. An inner class declared in a static context is okay. A static nested class declared in a static context is not okay.

[Andres Gonzalez]

From example 3, it gives compiler error because only final or abstract can be declare in a method-local inner class. Is that the reason?

[Toby Davis]

It seems you can also use strictfp as a modifier as well in method-local inner classes... by itself or combined with either final or abstract....
Surprised this wasn't in the book, though it compiles fine with Sun's JDK.

[Marlene Miller]

Good points. In example 3, only abstract, final and strictfp are valid modifiers. Thank you Andres and Toby.
(I wonder why static classes are not allowed. Some other day...)

[Krishna Srinivasan]

we cannot declare static class inside a method.
the same rule as local variables cannot be static and local variables also
cannot use the access modifiers.
this rule also appplied for local classes.

[Alton Hernandez]

Hi Marlene,
Those are good examples.
EXAMPLE 1:

static void m1() {
      Z a1 = new Z();      //compiler error, static context
      C.Z a2 = new C.Z();  //compiler error, static context
 }

Z() is an inner class so it requires an instance of the enclosing class and static methods does not have a reference to that instance
However, if the instance of the enclosing class is supplied, then the call would work:

static void m1(C c) {
//        Z a1 = new Z();      //compiler error, static context
       C.Z a2 = [B]c[/B].new Z();      //OK
 }

EXAMPLE 2:
The problem with this code is that C.A() is the FQN of a member class which does not exists.
EXAMPLE 3
I always see static nested class as if it is inside a package with the outer class as its package. So that is why it can even be imported by other class. So static nested class are really like any top-level class.
I think it is not allowed to define static nested class inside a method because if it does, then it will loose some of the properties of a top-level class.
Just my $0.02.

[Marlene Miller]

Thank you Krishna and Alton for your comments. Thank you to everyone for taking the time to read through the examples and say something.
Here is what I was trying to show with these examples: On the exam, we might see �new Z()� declared anywhere, and quick as a flash, we have to know whether it is valid or not. Will there be a compiler-error?
So I decided to list places where I might see new Z(). I also wanted to know if new C.Z() is valid where new Z() is valid.
What I wanted was to be able to say � if I know all this (the valid cases and the compiler errors), there is nothing more to know about instantiating named inner classes.
[ August 07, 2003: Message edited by: Marlene Miller ]