Confuse point in Chapter 1 (Java OCA 8 Programmer I Study Guide, Sybex)

[Saumyaraj Zala]

In the end of page 6, chapter 1 the points are mentioned as :

Each file can contain only one class

I was a bit confused as I was pretty sure I had seen a file before having more than one class. It should have been "Each file can contain only one public class".

Same applies for

The filename must match the class name, including case, and have a .java extension

It should have been "The filename must match the public class name, including case, and have a .java extension".

Moreover, out of curiosity I was thinking whether it is possible to escape multiline line comment inside a multiline comment. I know it wont make any sense but was just thinking how compiler checks or can skip a "*/".

[Jeanne Boyarsky]

Saumyaraj Zala wrote:In the end of page 6, chapter 1 the points are mentioned as :

Each file can contain only one class

I was a bit confused as I was pretty sure I had seen a file before having more than one class. It should have been "Each file can contain only one public class".

Same applies for

The filename must match the class name, including case, and have a .java extension

It should have been "The filename must match the public class name, including case, and have a .java extension".

You are correct on both of these. At this point, the reader could easily not know there is such a thing as a non-public class. So we don't confuse the issue. You are going to see some simplifications both in the OCA book and on the OCA exam since it is covering a subset of concepts in the language.

Saumyaraj Zala wrote:Moreover, out of curiosity I was thinking whether it is possible to escape multiline line comment inside a multiline comment. I know it wont make any sense but was just thinking how compiler checks or can skip a "*/".

No. Once you see the */, the comment is done.

[Roel De Nijs]

Saumyaraj Zala wrote:Moreover, out of curiosity I was thinking whether it is possible to escape multiline line comment inside a multiline comment. I know it wont make any sense but was just thinking how compiler checks or can skip a "*/".

No, you can't have nested multi-line comments. I also can't think of a use case where having nested multi-line comments would be useful. So this will compile

/**
 * line1
 * /* line2 
 * line3
 */
class MultiLineComment {
}

But this will procude a compiler error

/**
 * line1
 * /* line2 */
 * line3
 */
class MultiLineComment {
}

This is (far) beyond the scope of the OCAJP8 certification exam! Even if you would replace the first occurence of */ with the appropriate Unicode values (\u002A\u002F), the code will not compile

/**
 * line1
 * /* line2 \u002A\u002F
 * line3
 */
class MultiLineComment {
}

And here is the reason why it doesn't compile. Unicode escape sequences (like \u002A) are replaced by the actual characters they represent before the Java compiler does anything else with the source code. So \u002A\u002F has 2 Unicode escape sequences and thus are replaced internally with */ (and that's exactly as the second code snippet which didn't compile). And you can have some fun with Unicode escape sequences and create real Java puzzlers (as in this topic). But once again this is outside the scope of the OCAJP certification exams.

Hope it helps!
Kind regards,
Roel

[Jeanne Boyarsky]

Roel De Nijs wrote: I also can't think of a use case where having nested multi-line comments would be useful.

I can. Suppose you have code like this:

public class Comments {
/**
  * JavaDoc here
  */
public void method1() {}

/**
  * JavaDoc here
  */
public void method2() {}


/**
  * JavaDoc here
  */
public void method3() {}


/**
  * JavaDoc here
  */
public void method4() {}


/**
  * JavaDoc here
  */
public void method5() {}


}

Now imagine I want to comment out all the methods. I'd want to put a /* */ around the whole thing. But I can't because it "ends" before method 1. The same use case comes up in HTML/XML comments.

[Roel De Nijs]

Jeanne Boyarsky wrote:Now imagine I want to comment out all the methods. I'd want to put a /* */ around the whole thing. But I can't because it "ends" before method 1.

If you have to do it manually, I definitely agree. As you could comment out all these methods with typing only /* at the beginning and */ at the end. But if you are using an IDE, you simply use a single-line comment

class Comments {
//    /**
//     * JavaDoc here
//     */
//    public void method1() {
//    }
//
//    /**
//     * JavaDoc here
//     */
//    public void method2() {
//    }
//
//    /**
//     * JavaDoc here
//     */
//    public void method3() {
//    }
//
//    /**
//     * JavaDoc here
//     */
//    public void method4() {
//    }
//
//    /**
//     * JavaDoc here
//     */
//    public void method5() {
//    }
}

Using the single-line comment makes it also pretty easy, to uncomment for example only method3. Or you could even decide to add a block comment (which looks a bit messy)

class Comments {
    /**
     * JavaDoc here
     *//*
    public void method1() {
    }

    *//**
     * JavaDoc here
     *//*
    public void method2() {
    }

    *//**
     * JavaDoc here
     *//*
    public void method3() {
    }

    *//**
     * JavaDoc here
     *//*
    public void method4() {
    }

    *//**
     * JavaDoc here
     *//*
    public void method5() {
    }*/
}

Although this works, it might be a little bit harder to uncomment just one method. If I select all methods again and click "Remove block comment", all added block comments are removed again. Yay! Both code snippets were created using Eclipse.

[Jeanne Boyarsky]

Ok. So my use case at the command line .