Why can't sub-packages see package private classes?

Question

Okay so, I have this project structure:

package A.B

class SuperClass (this class is marked package private)

package A.B.C

class SubClass (inherits from super class)

I'd rather not make SuperClass publicly visible... It is really just a utility class for this specific project (A.B).

It seems to me that SubClass should be able to see SuperClass, because package A.B.C is a subpackage of A.B... but this is not the case.

What would be the best way to resolve this issue? I don't think it makes sense to move everything in A.B.C up to A.B or move A.B down to A.B.C... mainly because there will probably be an A.B.D which inherits from stuff in A.B as well...

I'm a bit new to Java, so be nice :D (I'm a C++ and .NET guy)

score 12 · Accepted Answer · answered Mar 21 '10 at 22:58

12

Packages are unique identifiers. You cannot make them follow the inheritance rules. Package and SubPackages are not analogical to Super and Sub classes.

I dont see any flaws in making the class that you wanted to use in the sub package to be visible to the outside world. I would be interested to know how this criterion is handled in C++/.net (as I am a java guy :) )

answered Mar 21 '10 at 22:58

bragboy

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Yeah I kind of figured I was just misunderstanding packages – Polaris878 Mar 21 '10 at 23:18
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In .NET I would've just put the classes in folders and have them marked internal :) – Polaris878 Mar 21 '10 at 23:55
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I don't get the inheritance-related analogy. Packages are namespaces. Namespaces are hierarchical by definition. That Java's package private classes are not visible in sub-packages is simply wrong. – Alexandru Nedelcu Dec 27 '17 at 10:47
@AlexandruNedelcu - "Namespaces are hierarchical by definition" - Not according to java. – bragboy Dec 27 '17 at 12:47

score 8 · Answer 2 · answered Mar 21 '10 at 22:59

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Your best bet is to declare the (default) constructor(s) of SuperClass as protected. This way only classes in the same package and subclasses regardless of the package can instantiate it and extend from it.

answered Mar 21 '10 at 22:59

BalusC

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score 0 · Answer 3 · answered Mar 21 '10 at 23:04

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Why not put them in the same package level?

Could you somehow use composition instead of inheritance? I've been trying to do more of this myself after reading Effective Java. Not sure if this is possible given your needs, but it might be worth considering.

Good luck.

answered Mar 21 '10 at 23:04

Marshall Alsup

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I don't want them in the same package level because I'll have around 20-30 classes that will rely on the super class, and I'm trying to keep things organized lol. – Polaris878 Mar 21 '10 at 23:56

Why can't sub-packages see package private classes?

3 Answers3

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