Thursday, May 6, 2010

philosophy of java nested classes

"inner classes" = non-static nested classes. For beginners, let's focus on typical scenarios:
1) C.java encloses N as a *private* inner class.
2) C.java has a field n1 of type N

Q: how would you use an instance of N? What does this object in memory represent?
A: a component of an object c (of type C).

Internalize -- this.n1 is very similar to ... a regular field, whereas N.class is more like a regular class than a C field. Allow me to repeat -- Whenever you look at an inner class N, it's very similar to a regular class.
Q: what does this.n1 resemble most? A field in an instance (c) of C?
A: No. Suppose a field j (of type J) in C has method j.m1(). m1() can't (N can) access C's private members
A: i think this.n1 most resembles a sophisticated and "trusted field", a regular filed like j + the additional trust. The trust means this.n1 can have methods to access C's private members.

Q: is such a construct never necessary and can be achieved using regular OO constructs?
A: No. The "trust" is hard to achieve otherwise.

Q: where do you instantiate N? How do you pass the instance around? How do you call N's instance methods?
A: all inside C. Outside C, no other objects can see N

Q: how about a public (instead of private) inner class N2? What's the use case or justification?
A: I would make N2's constructor private, so outer class C.java is the only access point. So an instance (n2) of N2 becomes a slave object dedicated to the outer object. Note trust still applies.
-------
Now for static nested class S declared in C.java
Q: how would you use an instance of S? What does this object in memory represent?
A: not part of a C instance.
Q: what does this instance resemble most? A static field in an instance (c) of C?
A: No. I think in some usages this instance most resembles a better static method wrapper. You can group static methods in C and move them into S and make them non-static [1] inside S. An alternative design is the System.out pattern --. put (converting to non-static) C's static methods [2] into a regular class A, and create an A instance as C's static field. However, static nested class S (not A) can access C's private static members.
A: in the case of AbstractMap.java, static nested class resembles....?


[2] they lose access to C's private static members.
[1] I think most methods in S should be non-static. Static methods in a static nested class is a waste of time.

Q: where do you instantiate S? How do you pass the instance around? How do you call S's instance methods?
A: instantiate in a static method in C. You can also do so in a static initializer or a non-static method.

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