在Mac上运行Java的问题

Question

I am using a Macbook Pro, Maverics OS, 64 bit machine.
Actually I am facing a very itchy problem while running java programs on my computer.

For example, this is my code:

package com.gui.helloworld;



import javax.swing.JOptionPane;



/**
 * <p>Hello World popup</p>
 * This class simply pops up a "Hello, World!"
 * message graphically.
 *
 * @author Aditya R.Singh
 * @version 1.0
 * @since 2014-06-22
 */
public class GUIHelloWorld {

    /**
     * This is the method that will popup the 
     * "Hello, World!" message.
     * @param args Unused
     */
    public static void main(String[] args) {

        JOptionPane.showMessageDialog(null, "Hello, World!");
    }
}

When I compile my code like this,

javac com/gui/helloworld/GUIHelloWorld.java  

a GUIHelloWorld.class is produced.
Upto this place, it is fine.

BUT THE PROBLEM ARISES HERE

When I try to run the code like this:

java com.gui.helloworld.GUIHelloWorld    

It gives me an error like

Error: Could not find or load main class com.gui.helloworld.GUIHelloWorld 

So I have to compile the program like this:

java -classpath . com.gui.helloworld.GUIHelloWorld  

Now, the output appears.
That's fine, but still, why is the need to put that "classpath" when the .class file is in
the same directory?
I am not liking this style.
This wasn't a problem when I used to use a windows 7 machine two months ago.
Any help? Or is it so that it is necessary for mac OS?
Please help.

Thanks, in advance ;)

Answer 1

javac or java without a classpath option have the working directory as the default classpath.

References to classes must include package names. So if you compile and execute

javac One.java
java One

class One should be in the default package (no package statement). If you compile

javac a/b/Two.java

class Two should be in the package ab Then, execution of the .class file a/b/Two.class is possible using

java a.b.Two

Other "effects" are possible by setting the environment variable CLASSPATH, which can be overridden by the -cp option. (Possibly this is what happened in your case - check by running echo $CLASSPATH .)