Access control modifiers in Java

Just a small blog post.

Some time ago I came across a basic fact in the core Java programming language, that I have interpreted too narrowly for a very long time.

I always have interpreted the "protected" keyword as making the properties or methods of a class available in its subclasses.

Turns out, according to [1] this is not entirely accurate. That's not all it does.

The following table is lifted from the docs in [1]:

Modifier	Class	Package	Subclass	World
public	Y	Y	Y	Y
protected	Y	Y	Y	N
no modifier	Y	Y	N	N
private	Y	N	N	N

If you look at the table above, you'll notice that protected is really only one level more secure than public, which was an eye opener for me.

It turns out "protected", isn't really that protected really.

No modifier

If you omit any kind of modifier, you will automatically get the behaviour that the method/property can be accessed only by other classes in the same package.

There are several sources declaring it "no modifier" or "default modifier" or "default protected modifier", but I prefer the "package private modifier" (or just "package-private" for short). The latter is the official Java convention as described in [1].

To my mind it is the best clear short precise definition of what you can do with it.

The JLS² strictly only mentions "package access".

Conclusion

Apparently there is no Access Modifier in Java that makes methods or fields only be inherited from subclasses³.

However we can achieve the same goal, by putting the subclasses of a class into the same package, and keep this package devoid of all others.

Sealing JARs

Of course, it means that if I create a package with the same name as a package in a JAR file included in my project, I will gain access to all the package private classes in the same package in the JAR.

This problem can apparently be solved by sealing⁴ the JAR file.

References

[1] Oracle - The Java^tm Tutorials - Controlling Access to Members of a Class

https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html

[2] Oracle - Java Language Spec - JDK 8

https://docs.oracle.com/javase/specs/jls/se8/jls8.pdf

[3] StackOverflow - Why is there no subclasses only access modifier in Java?

https://softwareengineering.stackexchange.com/questions/238581/why-is-there-no-subclasses-only-access-modifier-in-java

[4] Oracle - The Java^tm Tutorials - Sealing Packages within a JAR File

https://docs.oracle.com/javase/tutorial/deployment/jar/sealman.html