What is the effect of the dot (.) in the Java classpath?

Question

This is an example question from "SCJP mock exam":

Given the default classpath:

/foo

And this directory structure:

foo
  |
 test
    |
   xcom
     |--A.class
     |--B.java

And these two files:

package xcom;
public class A { }
package xcom;
public class B extends A { }

Which allows B.java to compile? (Choose all that apply.)

A. Set the current directory to xcom then invoke

javac B.java

B. Set the current directory to xcom then invoke

javac -classpath . B.java

C. Set the current directory to test then invoke

javac -classpath . xcom/B.java

D. Set the current directory to test then invoke

javac -classpath xcom B.java

E. Set the current directory to test then invoke

javac -classpath xcom:. B.java

The answer is C, I don't understand the use of the operator . there. Please explain.

The book says:

In order for B.java to compile, the compiler first needs to be able to find B.java. Once it's found B.java, it needs to find A.class. Because A.class is in the xcom package the compiler won't find A.class if it's invoked from the xcom directory. Remember that the -classpath isn't looking for B.java, it's looking for whatever classes B.java needs (in this case A.class).

I don't get this, if both files are on the same package, why wouldn't the compiler find A?

Answer 1

the dot means 'the current directory'. If you call javac from within xcom, then it will look for A.class in xcom/xcom/A.class, and won't find it.

Answer 2

In order for B.java to compile, the compiler first needs to be able to find B.java.

This is why D. and E. are wrong.

Once it's found B.java, it needs to find A.class. Because A.class is in the xcom package the compiler won't find A.class if it's invoked from the xcom directory. (...)

You missed the important part here which is if it's invoked from the xcom directory. A.class is in the xcom package so it is expected to be found in xcom/A.class (relatively to where you run javac).

This is why A. and B. are wrong. And this leave C. as the right answer.