Question

I am trying to make a project with packages of classes with different access modifiers to see all the characteristics of classes with different access modifiers. I tried having a package with a public class, one with a default class and then tried to have 2 other for protected and private classes but it said “Illegal modifier for the class privatez; only public, abstract & final are permitted” for them. Why is this so. And secondly, is testing stuff for each class a good way to understand it. I am trying to give a better question than the previous post. I also want to learn application of each access modifier.

Answer 1

I tried having a package with a public class, one with a default class and then tried to have 2 other for protected and private classes but it said “Illegal modifier for the class privatez; only public, abstract & final are permitted” for them. Why is this so.

The simple answer is that the JLS says you can’t declare a top-level class as private.

The reason is that it doesn’t make sense to have a class that is only visible to itself. There’s no way that any other class would ever be able to use it (apart, hypothetically, from using dirty tricks with reflection). So the JLS is just saying: “It doesn’t make sense, and I’m not allowing it.”

(Now a nested class can have access private … but that’s because the private means private to all classes in the outermost enclosing classes …)

And secondly, is testing stuff for each class a good way to understand it.

I think it is better to read and try to understand the text book / tutorial. The problem is that if you try to learn by writing your own examples, you can easily draw the wrong conclusions from them.

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For this example:

package privatez; protected class privatez { }

A hypothetical protected class would/might only be visible to its subclasses. But classes are always visible to their subclasses, so this doesn’t make any sense. (Besides, the way to prevent subclasses is to declare a class final, and you can restrict subclassing using “package private”; i.e. no access modifier.)

The protected access modifier only makes any sense for stuff inside a class. In a top level class, the normal meaning of protected doesn’t make any sense.

(This is one of the problem of trying to learn by examples. You end up struggling with understanding why the compiler rejects the examples. Don’t expect the compiler to “explain” why something is wrong. That’s not its role.)