Possible errata on abstract classes, page 263 (Java OCA 8 Programmer I Study Guide, Sybex)

[Bram Vonk]

IN chapter 5, on page 263 of the OCA Java 8 Study guide, in the second-to-last sentence of the second paragraph, it says:
"The key point is that the first class to extend the nonabstract class must implement all inherited abstract methods."
Shouldn't this be:
"The key point is that the first nonabstract class to extend an the nonabstract class must implement all inherited abstract methods."

Also: at the top of the errata table on http://www.selikoff.net/java-oca-8-programmer-i-study-guide/ there seems to be a dangling table row.

[Roel De Nijs]

Hi Bram Vonk,

First of all, a warm welcome to CodeRanch!

Bram Vonk wrote:IN chapter 5, on page 263 of the OCA Java 8 Study guide, in the second-to-last sentence of the second paragraph, it says:
"The key point is that the first class to extend the nonabstract class must implement all inherited abstract methods."
Shouldn't this be:
"The key point is that the first nonabstract class to extend an the nonabstract class must implement all inherited abstract methods."

You are spot-on! From the moment you have a concrete class (nonabstract) you must implement all inherited abstract methods, otherwise your class won't compile. So the sentence should be updated to: "The key point is that the first concrete class to extend the abstract class must implement all inherited abstract methods." (or something similar of course).

Bram Vonk wrote:Also: at the top of the errata table on http://www.selikoff.net/java-oca-8-programmer-i-study-guide/ there seems to be a dangling table row.

Yeah, that's probably not how it's supposed to look. And it's weird because I looked at the source code of that page and all errata items are actually a table row with table data elements, but that one is just a paragraph. So something went wrong when one of the authors edited that page. But it will probably be rectified in the next few days.

Hope it helps!
Kind regards,
Roel

[Bram Vonk]

Roel De Nijs wrote:From the moment you have a concrete class (nonabstract) you must implement all inherited abstract methods (...)

Roel De Nijs wrote:So the sentence should be updated to: "The key point is that the first concrete class to extend the abstract class (...)

Bram Vonk wrote:Shouldn't this be:
"The key point is that the first nonabstract class (...)

That's something else that irked me: I wrote "nonabstract", because the book said on page 262:

book wrote:A concrete class is the first nonabstract subclass that extends an abstract class (...)

(emphasis mine)

Not only the first nonabstract class that extends an abstract class is concrete: all classes that extend a concrete are also concrete, right?

Then it should have been something like:

The first nonabstract subclass that extends an abstract class is called a concrete class (...).

[Paul Anilprem]

Bram Vonk wrote:Then it should have been something like:

The first nonabstract subclass that extends an abstract class is called a concrete class (...).

Seem like it is getting needlessly convoluted.
Simply put:
A class that inherits an abstract method but does not implement it, must be declared abstract. Any class that is not declared abstract is a concrete class.

[Roel De Nijs]

Bram Vonk wrote:Not only the first nonabstract class that extends an abstract class is concrete: all classes that extend a concrete are also concrete, right?

No, definitely not! An abstract class can easily extend from a concrete class which extends from another abstract class. Illustrated in this code snippet

abstract class Abstract {}
class Concrete extends Abstract {}
abstract class BackAbstract extends Concrete {}
class AndConcreteAgain extends BackAbstract {}
class TestAbstractConcrete {
    public static void main(String[] args) {
        Abstract a = new Abstract(); // compiler error
        Concrete c = new Concrete();
        BackAbstract ba = new BackAbstract(); // compiler error
        AndConcreteAgain aca = new AndConcreteAgain();
    }
}

Any class which is not marked as abstract is a nonabstract or concrete class. And if it extends an abstract class (and/or implements an interface with abstract methods), it must provide an implementation for all abstract methods; otherwise this nonasbtract (or concrete) class will not compile.

Hope it helps!
Kind regards,
Roel

[Scott Selikoff]

Fixed dangling row.

[Jeanne Boyarsky]

And noted the errata.