super newbie, running from terminal

[pbpersson]

I have been using NetBeans in Linux up until now without a problem, but now I want to run my app from the terminal. When I CD to the directory where my NamesLoop.class file is and type "java NamesLoop" I get all these errors and I am not sure how to fix this:

phil@phil-desktop-kubuntu710:~/NetBeansProjects/NamesLoop/build/classes/namesloop$ java NamesLoop
Exception in thread "main" java.lang.NoClassDefFoundError: NamesLoop (wrong name: namesloop/NamesLoop)
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:637)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:277)
        at java.net.URLClassLoader.access$000(URLClassLoader.java:73)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:212)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:323)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:268)
        at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:336)

[jklsemicolon]

Is it an applet?

if not you could try typing "dir" and check if the file is actually in the directory you are in

[Norm]

java NamesLoop
Exception in thread "main" java.lang.NoClassDefFoundError: NamesLoop (wrong name: namesloop/NamesLoop)
at java.lang.ClassLoader.defineClass1(Native Method)

The message says the class file is in the package namesloop.
You need to set the classpath to the beginning of the package and use the package.classname with java:
java -cp <classpath-to_namesloop>;. namesloop.NamesLoop

[pbpersson]

OKAY, I tried the following and now it is totally confused:

java -cp /home/phil//NetBeansProjects/NamesLoop/build/classes/namesloop;. namesloop.NamesLoop

[Norm]

totally confused:

Explain please
The classpath should stop before the package path. Your -cp looks like it includes the package path.
Path to class file = classpath + package path

[pbpersson]

Umm....Package path? Class Path? I have no idea what in the world you are talking about. I have never done this before. I don't know why this needs to be so complicated.

Here is the program code:

package namesloop;

/**
 *
 * @author phil
 */
public class NamesLoop {

String[] firstNames = { "Dennis", "Grace", "Bjarne", "James"};
String[] lastNames = new String[firstNames.length];

void printNames() {
    for (int i = 0; i < firstNames.length; i++)
        System.out.println(firstNames[i] + " " + lastNames[i]);
}
    public static void main(String[] args) {
        // TODO code application logic here
        NamesLoop a = new NamesLoop();
        a.printNames();
        System.out.println("-----");
        a.lastNames[0] = "Ritchie";
        a.lastNames[1] = "Hopper";
        a.lastNames[2] = "Stroustrup";
        a.lastNames[3] = "Gosling";
         a.printNames();        
    }
}

The NamesLoop.class file is in the following path:

/home/phil//NetBeansProjects/NamesLoop/build/classes/namesloop

Therefore, the entire path including the filename would be:

/home/phil//NetBeansProjects/NamesLoop/build/classes/namesloop/NamesLoop.class

Based upon that information, can you give me the command I would type at the console prompt to get the program to run?

I really appreciate your help with this. :)

[Norm]

Remove the package statement from your program and you will be able to enter: java NamesLoop as the command to execute your program.
If you put your program in a class, then you need to understand packages and classpath.
The sooner you understand classpath, the easier it will be.

[pbpersson]

I typed just java NamesLoop and here is what I received:

phil@phil-desktop-kubuntu710:~/NetBeansProjects/NamesLoop/build/classes/namesloop$ java NamesLoop
Exception in thread "main" java.lang.NoClassDefFoundError: NamesLoop (wrong name: namesloop/NamesLoop)
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:637)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:277)
        at java.net.URLClassLoader.access$000(URLClassLoader.java:73)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:212)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:323)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:268)
        at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:336)

Where can I read about classpath and why was this not setup correctly on my machine when the JRE/JDK or NetBeans was installed?

If the classpath is "broken" on my machine, why can NetBeans run everything without a problem?

Why did NetBeans add the "package" line if this keeps the programs from running via a command line prompt?

Would it be helpful to refer to some ant script down inside the NetBeans directory to see how it runs the code?

[Norm]

Remove the package statement from your program

Did you try what I suggested above?
I don't like IDEs for beginners because of this problem. They do things that are beyond your knowledge.
When you understand classpath and packages, you'll know how to execute your program. In the meantime you'll have to stay inside the IDE.

[Alexander Montero]

Hi everybody:

I was wondering if someone could give me a little tip about the process of learning java. I have several doubts regarding how long would it take me to learn java? What could be the so called learning curve and how many hours a day is it necessary to spend studying and practicing in order to obtain a certain amount of knowledge that will enable me to "feel" like a Java programmer in some 2-3 years maximum to be a Jr Programmer at least, even without a college degree that is (I want to study Java very hard at the same time with my college education).
When I learn something new, there's seems to be like a trillion other things that I need to know before I make it work. If I improved my knowledge at math could this make it easier??

I would appreciate any comments or suggestions. I hope I'm clear enough.

[pbpersson]

Did you try what I suggested above?
I don't like IDEs for beginners because of this problem. They do things that are beyond your knowledge.
When you understand classpath and packages, you'll know how to execute your program. In the meantime you'll have to stay inside the IDE.

yes....sorry......I commented it out. This is how it appears now and it still does not work:

//package namesloop;

/**
 *
 * @author phil
 */
public class NamesLoop {

String[] firstNames = { "Dennis", "Grace", "Bjarne", "James"};
String[] lastNames = new String[firstNames.length];

void printNames() {
    for (int i = 0; i < firstNames.length; i++)
        System.out.println(firstNames[i] + " " + lastNames[i]);
}
    public static void main(String[] args) {
        // TODO code application logic here
        NamesLoop a = new NamesLoop();
        a.printNames();
        System.out.println("-----");
        a.lastNames[0] = "Ritchie";
        a.lastNames[1] = "Hopper";
        a.lastNames[2] = "Stroustrup";
        a.lastNames[3] = "Gosling";
         a.printNames();        
    }
}

Regarding IDEs, they are supposed to make your life easier, not break your code so it will not work from the command line. :(

The only time I ever coded in notepad was when I was learning PHP and to this day I tell everyone it is the most useless language I have ever encountered, most likely because I was not using an IDE. Coding in PHP was like pulling my fingernails out using pliers.

If I am creating any app in an IDE, running it outside the IDE should be a piece of cake. I worked in Visual Studio for years - I just ran the .exe file from the command line or double-clicked on the file and Voila! it would run.

Now I am learning Java, creating console apps.....and there is no way to run them from the console? That seems VERY ironic. ;)

My book said the .class file was bytecode and the JRE would know how to interpret and run it. I guess my textbook just does not know the subtle complexities of the real world and the mysterious classpath that keeps programs from executing.

Fortunately, I have moved onto other chapters - that chapter was showing me how "easy" it was to pass parameters into my program from a command line. Oh.....the NamesLoop program I just chose at random to run - that was not the program for the chapter I was in, but if I can get SOMETHING to run then they will all run.

After we get this mess sorted out, I want to run the programs on Windows machine and Solaris machines I have here. I can hardly wait for that excitement based upon how difficult this has been thus far.

Again.....is there a web site that will tell me what I need to know so I can understand what is going on here?

[pbpersson]

OKAY - I have been looking at web sites that went on and on about classpath but gave no real examples on how to set this, or how to view the current setting in a Linux system.

However, in fooling with the IDE and deleting the class file and re-building without the package statement in the code, it did spit out the following help.....FINALLY:

To run this application from the command line without Ant, try:
java -jar "/home/phil/NetBeansProjects/NamesLoop/dist/NamesLoop.jar"

I don't know why it finally decided to help me, but that worked - and now that I have posted it here, I will always know where to find it. I did not even know I was creating jar files or what they are.....but for now I am just trying to get through this book.

thank you :)

[pbpersson]

I was wondering if someone could give me a little tip about the process of learning java. I have several doubts regarding how long would it take me to learn java?

You might want to start a new thread rather than embedding your question within a detailed discussion of the classpath environment variable and how it is used to locate custom-created classes in your directory structure.

How long it will take you to learn Java depends upon the quality of your teaching materials, the quality of the computer you are using, the amount of software development experience you have had in the past, the environment in which you will be studying (screaming kids and barking dogs), your concentration level, the path you will take (desktop apps, web apps, embedded apps), etc.

In my case I am using a "Learn Java in 21 Days" book and want to be a Junior software developer by the middle of September. Well....maybe a junior/junior developer. ;) However, I have been creating applications for 25 years in BASIC, Pascal, C, COBOL, and VB.NET - Java is just a new syntax for me.

Even for me, Java represents some new concepts - they talked about truncating attributes when casting between classes and that concept blew my mind.

[Norm]

it still does not work:

Can you show the console when you tried to execute the java program after removing the package statement?
IDEs will make it easier as long as you keep using them. If you try to go outside on your own, you need to understand the environment and how to use the commands that were buried inside the IDE.
Quick tutorial -
The classpath contains the path(s) where the compiler and java program will search for class files and other resources. The classpath often contains jar files containing class files and folders with class files.
The package path is the path represented by the package statement. The full path to a class in a package will be the classpath + the package path. For example a class file in package abc would be located in folder abc. To have java find that class file, you need a classpath that leads to the folder with the abc folder in it.

[Alexander Montero]

Thanks a lot for replying. those things you mention give me an idea as to what I'm up against.

So, let's suppose you've had 0 experience in the past making any kind of applications like me, what would you do to learn java in 2-3 years maximum? perhaps the solution to my problem is keep studying and practicing at the same time and researching on all topics and the problems other programmers encounter that's gotta help somehow.
Thanks again and good luck in your goal.

Sorry about the confusion, I don't know how the site works.

[pbpersson]

Can you show the console when you tried to execute the java program after removing the package statement?

phil@phil-desktop-kubuntu710:~/NetBeansProjects/NamesLoop/build/classes$ ls
namesloop  NamesLoop.class
phil@phil-desktop-kubuntu710:~/NetBeansProjects/NamesLoop/build/classes$ java NamesLoop.class
Exception in thread "main" java.lang.NoClassDefFoundError: NamesLoop/class
Caused by: java.lang.ClassNotFoundException: NamesLoop.class
        at java.net.URLClassLoader$1.run(URLClassLoader.java:217)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:323)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:268)
        at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:336)

[pbpersson]

what would you do to learn java in 2-3 years maximum?

Well.....um.....what do you mean by "learn Java"?

I spent two years studying VB.NET and I was able to create desktop applications that could display things to the screen, open multiple windows, open files, create files, modify remote databases, display data in different ways......I could also write web applications that did all the same things.....but every time I did anything new I had to spend tons of time researching how to do it and much of my time was spent learning via trial and error. So after two years.....had I "learned" VB.NET or was I still just beginning?

perhaps the solution to my problem is keep studying and practicing at the same time and researching on all topics and the problems other programmers encounter

Go to the bookstore and get a "beginning Java" book and before you buy it, make sure the first chapter makes perfect sense to you - find something that is correct for where you are in your journey. That would be my advice. Looking at problems faced by programmers is not a good place to start because quite frankly it will all look like Greek to you and you could get discouraged.

Once you have some knowledge so you can understand what they are talking about then it will be different. But you have to gain a little knowledge just to start out.