chapter 10 question number 11 k&b book (on jar files)

[Ash Gill]

Given the following directory structure:

test-|
      |- GetJar.java
      |
      |- myApp-|
               |-Foo.java

And given the contents of GetJar.java and Foo.java:

3. public class GetJar {
4. public static void main(String[] args) {
5. System.out.println(myApp.Foo.d);
6. }
7. }

3. package myApp;
4. public class Foo { public static int d = 8; }

If the current directory is "test", and myApp/Foo.class is placed in a JAR file called MyJar.jar
located in test, which set(s) of commands will compile GetJar.java and produce the output 8?
(Choose all that apply.)

A. javac -classpath MyJar.jar GetJar.java
java GetJar

B. javac MyJar.jar GetJar.java
java GetJar

C. javac -classpath MyJar.jar GetJar.java
java -classpath MyJar.jar GetJar

D. javac MyJar.jar GetJar.java
java -classpath MyJar.jar GetJar

the answer is option A, i have tried and it works perfectly fine if the myapp has got a class Foo.class. can anyone please confirm if there has been some mistake found in the question.

[Ogeh Ikem]

The book says that option A is the correct answer. Are you saying that the question is wrong?

[Gaurav Sagar]

All the answers are wrong. It should be:-

javac -classpath MyJar.jar GetJar.java
java -classpath MyJar.jar;. GetJar

Regards,
Gaurav

[Ash Gill]

ogeh ikem wrote:The book says that option A is the correct answer. Are you saying that the question is wrong?

yes, i have tried it out. for option A to be correct, the "myapp" in the given directory structure should have a "Foo.class". OR as gaurav sagar said there should be an option

javac -classpath MyJar.jar GetJar.java
java -classpath MyJar.jar;. GetJar

thanks gaurav & ogeh

[Sagar Shroff]

Gaurav Sagar wrote:All the answers are wrong. It should be:-

javac -classpath MyJar.jar GetJar.java
java -classpath MyJar.jar;. GetJar

Regards,
Gaurav

Why is the specification of current directory a necessary???

[Ash Gill]

Sagar Shroff wrote:
Why is the specification of current directory a necessary???

Hi sagar, after compiling GetJar.java using:
javac -classpath MyJar.jar GetJar.java
we get a GetJar.class file in the current directory. So, we need to specify the current directory for java to look for GetJar.class.

I would like to add that from my experience of trying numerous examples, ".class" files are searched in the current directory by default if we are not using the "-classpath" option. however, if we use the "classpath" option then the current directory is not searched by default for .class files.

[Gaurav Sagar]

Sagar Shroff wrote:
.....

Why is the specification of current directory a necessary???

When you specify a classpath, it overrides the current classpath (this even includes the one you might have set using an environmental variable as in Windows), so you explicitly need to specify the classpath of the class that you want to run, which in the question above was the current directory.

Hope you got the point, Sagar.

Regards,
Gaurav

[fadi aboona]

Gaurav Sagar wrote:All the answers are wrong. It should be:-

javac -classpath MyJar.jar GetJar.java
java -classpath MyJar.jar;. GetJar

Regards,
Gaurav

What i don't understand is how come we didn't use the "." in the javac command but java command wouldn't work without it?

[Norbert Muench]

Ash Gill wrote:I would like to add that from my experience of trying numerous examples, ".class" files are searched in the current directory by default if we are not using the "-classpath" option. however, if we use the "classpath" option then the current directory is not searched by default for .class files.

That is correct. See How classes are found.
This can be easily verified by running the following code. If no class path is specified on the command line and none is set in the CLASSPATH env variable, then this will output "Class Path: ."

public class ShowClassPath {
    public static void main(String[] args) {
        System.out.println("Class Path: " + System.getProperty("java.class.path"));
    }
}

fadi aboona wrote:What i don't understand is how come we didn't use the "." in the javac command but java command wouldn't work without it?

Simple: The javac command wasn't looking for a GetJar.class file, it was creating it. The java command on the other hand needs to find the GetJar.class and it will do so by searching the class path.

[fadi aboona]

Norbert Muench wrote:

Ash Gill wrote:I would like to add that from my experience of trying numerous examples, ".class" files are searched in the current directory by default if we are not using the "-classpath" option. however, if we use the "classpath" option then the current directory is not searched by default for .class files.

That is correct. See How classes are found.
This can be easily verified by running the following code. If no class path is specified on the command line and none is set in the CLASSPATH env variable, then this will output "Class Path: ."

public class ShowClassPath {
    public static void main(String[] args) {
        System.out.println("Class Path: " + System.getProperty("java.class.path"));
    }
}

fadi aboona wrote:What i don't understand is how come we didn't use the "." in the javac command but java command wouldn't work without it?

Simple: The javac command wasn't looking for a GetJar.class file, it was creating it. The java command on the other hand needs to find the GetJar.class and it will do so by searching the class path.

Thanks Norbert, i guess i have to read more about classpath.