Question about how javac finds referenced files.

Question

Hi
I am just beginning to learn Java and have got some questions related to packages.
I have two classes:
MyClass.java
package pkg;
public class MyClass {
     public void prn(int x){
     System.out.println("Integer: "+x);
Test.java
import pkg.*;
public class Test{
     public static void main(String[] args){
          MyClass obj=new MyClass();
          obj.prn(2);
Directory structure
/root/ jprog/MyClass.java
/root/ jprog/Test.java
Executing the following sequence of commands gives me:
# ls
MyClass.java Test.java
# javac -d . MyClass.java
# ls
MyClass.java Test.java pkg/
# cd pkg
# ls
MyClass.class -----compiles.
# cd ..
# javac Test.java
Test.java:5: cannot access MyClass
bad class file: ./MyClass.java
file does not contain class MyClass
Please remove or make sure it appears in the correct subdirectory of the classpath.
MyClass obj=new MyClass();
^
1 error
Now after reading another thread on this forum I came to know that the compiler finds the
MyClass.java in the wrong package. Using verbose option with javac shows this.
So I move it to some other directory and everything works.
# mv MyClass.java /root
# ls
Test.java pkg/
# javac Test.java
# java Test
Integer: 2
Now the question begins:
Suppose we go back to how the files were arranged before I moved the MyClass.java file and
I change the Test.java file by adding
package pkg2;
statement to it.
Now everything compiles
# ls
MyClass.java Test.java ---I have deleted pkg and .class files
# cat Test.java
package pkg2;
import pkg.*;
public class Test{
public static void main(String[] args){
MyClass obj=new MyClass();
obj.prn(2);
# cat MyClass.java
package pkg;
public class MyClass {
public void prn(int x){
System.out.println("Integer: "+x);
# javac -d . MyClass.java
# javac -d . Test.java
# ls
MyClass.java Test.java pkg/ pkg2/
# java pkg2.Test
Integer: 2
Why isn't the MyClass.java file causing any problems now?
Using the verbose option with javac shows that the compiler goes straight for the pkg/MyClass.class.
My readings of Core Java Horstmann tell me that the compiler always looks in the current directory. javac documentation also doesn't help. I know that the documentation recommends that the source file locations should match the package statements.
I'm just trying to learn how javac finds referenced files.
Any help??
Thanks a load in advance.

Answer 1

I know that the
documentation recommends that the source file
locations should match the package statements.This is not merely a recommendation, it is a requirement.
Both javac and java find classes by traversing the classpath. The classpath should contain directories (or jar files) that correspond to the top of a (part of the) package hierarchy.
For example, if the Java code for your class is in a file C:\src\pkg\MyClass.java, and MyClass.java declares the class to be in package pkg, the classpath should contain C:\src. Classes are then found by transforming their package declaration into a directory structure.
Please note that javac works on files, not classes. Therefore it is necessary to specify the full filenames of the files you want to compile, even if you have specified the classpath. By default, javac will put compiled class files in the same location as their sources, so the classpath for the java command should also contain C:\src.
The exception to the rule is when all classes for the program are in the default package (i.e., they do NOT contain a package statement). In that case, it is possible for javac (and I believe java as well) to find all classes in the current directory, so it will "just work".
You can specify the classpath using the -cp parameter to both javac and java.
Hope this helps.