Build java classes having circular reference / import

[JimmyD]

I am working on some codes that involved a circular reference which gives me a headache to compile. I googled circular reference / import java, the information I found does not really help me to solve the problem. I understand it might be poor design but I am working with codes not written by me or my colleague. Changing the code requires greater effort.

I wrote a simple test code that isolates the problem. The problem is funny: if I keep class file within the same directory as its source file, it will compile. If I attempt to isolate class file from the .java file, it won't compile. Let me put up my code:

The directory I am working with is named: C:\java\Testing\testcircular.
C:\java is added to my CLASSPATH environmental variable.

I make 2 folders in testcircular called src and com. That's attempting to seperate source code from class files.

I write 2 simple test classes: called testClass1.java, and testClass2.java . I package testClass1.java into packageA, testClas2 to packageB.

================================================== ====
The code:

package Testing.testcircular.src.packageA;

import Testing.testcircular.src.packageB.testClass2;

public class testClass1 {
	
	public int a;

	public testClass1()
	{
		a = 0;
	}
	
	public void add(testClass2 tc2)
	{
		a += tc2.a;
	}

}

and

package Testing.testcircular.src.packageB;

import Testing.testcircular.src.packageA.testClass1;

public class testClass2 {
	
	public int a;

	public testClass2()
	{
		a = 1;
	}
	
	public void add(testClass1 tc1)
	{
		a += tc1.a;
	}

}

I have a simple make.bat file located in C:\java\Testing\testcircular :

@echo off

echo compiling packageA
echo.
javac src\packageA\*.java
if not exist com\packageA\nul mkdir com\packageA > nul
::move src\packageA\*.class com\packageA

echo.
echo compiling packageB
echo.
javac src\packageB\*.java
if not exist com\packageB\nul mkdir com\packageB > nul
::move src\packageB\*.class com\packageB

javac *.java >nul

And I have a simple test client that tests whether the classes are working correctly:

import Testing.testcircular.src.packageA.testClass1;
import Testing.testcircular.src.packageB.testClass2;

public class test {

	public static void main(String[] args)
	{
		testClass1 tc1 = new testClass1();
		testClass2 tc2 = new testClass2();
		
		System.out.println("Initial tc1 = " + tc1.a);
		System.out.println("Initial tc2 = " + tc2.a);
		
		for (int i = 0; i < 50; i++)
		{
			tc1.add(tc2);
			System.out.println("tc1 = " + tc1.a);
			tc2.add(tc1);
			System.out.println("tc2 = " + tc2.a);
		
		}
	
	}

}

(Note if working properly, the test client will print the Fibonacci sequence.

================================================== ==========


Now here is the problem: note that in make.bat, I commented out the move statement. So the .class file is left in the src folder. Accordingly, the package statement in the codes bears "package ***.src.packageA" . Now that compilation works, and the test client works fine.

HOWEVER, if I un-comment the move statement in make file, and change every occurrence of "src" into "com" in the codes, the compilation doesn't work. Could someone help to explain why and offer a solution?

[kjkrum]

Lots of things wrong here.

First, your classes are in Testing.testcircular.src.* packages, but then you move the class files to testing/testcircular/com. No wonder the compiler can't find them there.

Say your directory structure looks like this:

java
   testcircular
      src
      bin

src and bin should should both have directory structures inside them corresponding to your package names. You should only be running javac once, and when you do, src should be in your classpath and bin should not (or else javac may find classes in bin and not recompile your changes). If you simply compile the main class of your program, javac will descend into the src tree and compile everything needed, automatically resolving circular references. Then you can copy all the class files to the bin tree, and run your main class with bin in your classpath.

Also, see the link in my sig for naming conventions. Naming things the way you do makes your code hard to follow.

[JimmyD]

Thanks kjkrum for the reply! I wasn't being accurate when I describe my directory structure. I didn't mess it up though. I did have folders in src.

The structure looks like this:

C:\java\Testing
     testcircular
            com
                  packageA
                          testClass1.class
                  packageB
                          testClass1.class

            src
                  packageA
                          testClass1.java
                  packageB
                          testClass1.java

When you say "You should only be running javac once", could you please show me how to run javac in this case?


P.S. Thank you so much for the awesome link. I am reading it.

[kjkrum]

That would be the correct directory structure if testClass1 was in the package packageA, not Testing.testcircular.src.packageA. You could run javac like this:

cd c:\java\Testing\testcircular\src
javac -cp . packageA\testClass1.java

This would compile both testClass1 and testClass2, because the compiler would see the reference to packageB.testClass2 and automatically look for $CLASSPATH/packageB/testClass2.java.

[JimmyD]

I see. However, the code I put up there compiles correctly. I am able to run my make file cleanly. I think the make file is essentially in the same spirit as your 2 step compilation above. because I have C:\java in my CLASSPATH env variable.

What doesn't work, is if I change every src to com. the error I receive is:

compiling packageA

src\packageA\testClass1.java:3: package Testing.testcircular.com.packageB does not exist
import Testing.testcircular.com.packageB.testClass2;
^
src\packageA\testClass1.java:14: cannot find symbol
symbol : class testClass2
location: class Testing.testcircular.com.packageA.testClass1
public void add(testClass2 tc2)
^
src\packageA\testClass1.java:16: inconvertible types
found : <nulltype>
required: int
a += tc2.a;
^
3 errors

[kjkrum]

What doesn't work, is if I change every src to com.

And I explained why in my first comment. Your directory structure, relative to your classpath, must correspond to your package names.

[JimmyD]

I perhaps only get half of what you mean in your first comment. Let me rephrase my question so you can check my understanding:

Suppose I want to separate the source code from the class files, how should I do it?

I can think of 2 naive ways right now:

1: i code the target direction in source .java files (in my example it would be the "com"), and I compile and move the .class files to those directories.

2: I leave my source files as the way they are, compile, and then remove the .java files.

Based on reading your first comment, you seem to imply the 2nd way.

And I explained why in my first comment. Your directory structure, relative to your classpath, must correspond to your package names.

[kjkrum]

1: If you put the classes in a com.* package but they're not in the com/ directory relative to the classpath, they won't compile.

Classpath trips a lot of people up, but it's not really that complicated. Classpath is simply a directory (or a list of directories) relative to which javac and java look for classes. If your classpath is c:\java, your package name is mypackage, and a class in that package is called MyClass, then the source file must be c:\java\mypackage\MyClass.java and the class file must be c:\java\mypackage\MyClass.class.

What I'm suggesting is that you can change the classpath between compiling and running. You can make the classpath be c:\java\src when you compile, then move your class files to c:\java\bin and make that the classpath when you run the program.

[JimmyD]

Thank you kjkrum