What exactly is a classpath in java? Head First Java 2nd Edition

[John Drulo]

I was reading the chapter on deploying the code and came across this thing called class path. I googled it and found many resources , but found the resources to be arcane and difficult to understand.

Can somebody please give me a Head First version of:
1) What a class path is exactly?
2) Given that I am nearing the end of the Head First Book, how do I google and figure out such arcane stuff on my own, without having to pester you guys all the time. Like , how would you guys go about finding what a class path is if you hadn't a clue about it?

Yours Gratefully,
John

[Jacob Gmur]

Classpath is a parameter—set either on the command-line, or through an environment variable—that tells the Java Virtual Machine or the Java compiler where to look for user-defined classes and packages.

http://en.wikipedia.org/wiki/Classpath_%28Java%29

If you're using Windows you can go trough the Control Panel to find Enviroment Variables and set CLASSPATH
For example - mine CLASSPATH is:
.;C:\Java Projects\LIB

The CLASSPATH variable is one way to tell applications, including the JDK tools, where to look for user classes. (Classes that are part of the JRE, JDK platform, and extensions should be defined through other means, such as the bootstrap class path or the extensions directory.)

The default value of the class path is ".", meaning that only the current directory is searched. Specifying either the CLASSPATH variable or the -cp command line switch overrides this value.

To check whether CLASSPATH is set on Microsoft Windows execute the following:

C:> echo %CLASSPATH%

http://docs.oracle.com/javase/tutorial/essential/environment/paths.html

Do you know what is package?

EXAMPLE
I've got a package:
pl\corso\simple
Inside path:
C:\Java Projects\Lib\pl\corso\simple
Inside directory simple I've got public class called List.java which belongs to pl.corso.simple package

package pl.corso.simple;

public class List
{
	public List()
	{
		System.out.println("Package: ||||| pl.corso.simple |||||");
		System.out.println("||||| List object created. |||||");
	}
}

and I've got a public class outside the package which uses pl.corso.simple package (exacly uses classt List from package)

import pl.corso.simple.*;

public class LibTest
{

	public static void main(String[] args)
	{
		System.out.println("CLASSPATH: ||||| c:\\Java Projects\\LIB |||||");
	
		List l = new List();
	
	}
}

//**the output is
//    CLASSPATH: ||||| c:\\Java Projects\\LIB |||||
//    Package: ||||| pl.corso.simple |||||
//    ||||| List object created. |||||

JAVAC knows that he have to search for package inside C:\Java Projects\Lib <-- because i allready set the classpath
import pl.corso.simple.*; <- it is a place where javac have to search for the List class, if i do not set the classpath or do not import the package how the compiler could know where to search for class that I want to use

You can make lots of packages with lots of classes and use them.
You should read some about Access Control.

Correct me someone if im wrong somewhere

[fred rosenberger]

John Drulo wrote:I was reading the chapter on deploying the code and came across this thing called class path.

Are you sure? Programming requires you to be VERY precise. You should get in the habit now of always saying exactly what you mean. Did the book refer to "class path" or "classpath"?

John Drulo wrote:1) What a class path is exactly?

The "classpath" is an environment variable that programs can use. It basically consists of a bunch of directories. The programs then search each of those directories when looking for stuff - and as the name implies, Java uses it to find class files.

John Drulo wrote:2) Given that I am nearing the end of the Head First Book, how do I google and figure out such arcane stuff on my own, without having to pester you guys all the time. Like , how would you guys go about finding what a class path is if you hadn't a clue about it?

I would google "classpath". The very first hit I see is the Wikipedia page titled Classpath (Java)