Develop Reference

Java runs in eclipse and will compile, but wont execute on cmd, but still runs in eclipse. How can I get it to execute in cmd?

So I have a basic hello world set up in eclipse and I can compile it using cmd easily (I have set all the necessary paths), however when I then try to use the java command to execute the hello world, it always returns the same error:
Error: Could not find or load main class helloWorld
Caused by: java.lang.NoClassDefFoundError: net/codejava/helloWorld (wrong name: helloWorld)
This is the code used:
package net.codejava;
public class helloWorld {
public static void main(String[] args) {
System.out.println("Hello World");
}
}
I am cd in the right directory (I think, I cd into the src directory and then into the package file stored in src) and am using Windows 10 with java 18.0.1 and JRE build 18.0.1+10-24
Any help would be greatly appreciated, as this is highly frustrating, when the code runs effortlessly on the eclipse console. Thanks.

Your file has a 'package' of net.codejava and a name of helloWorld, meaning, the full name of this class is net.codejava.helloWorld.
The java command, at least in the way you're using it, requires that you pass the full name, thus, you must run java net.codejava.helloWorld. Just java helloWorld simply isn't going to work.
But that's not all.
Java needs to then find the class file that contains the code for class net.codejava.helloWorld. It does this by first turning that full name into a path of sorts: net/codejava/helloWorld.class, and it will then scan each entry in the classpath for that. You can put directories and jar files on the classpath.
Thus, you have a directory on your system; let's call this directory X. X contains a directory named net, which contains a directory named codejava, which contains a file named helloWorld.class. If there is no such X (i.e. your class file is not in a dir named codejava for example), you're going to have to fix that by making these directories.
Then, X (and not the codejava dir!) needs to be on the classpath. Usually (it depends on how you configured things), 'the current dir' is by default on the classpath.
Given that your code is in, say, /home/PythonSux/workspace/learningjava/net/codejava/helloWorld.class, that means the dir that needs to be on the classpath is /home/PythonSux/workspace/learningjava. After all, if you, from there, look for net/codejava/helloWorld.class, you find the right file.
Therefore, either cd to that directory, or run java -cp /home/PythonSux/workspace/learningjava net.codejava.helloWorld
Note that this isn't usually how you actually run java apps. You either run them from your IDE, or you ask your build tool to run it, or you package your java app into a jar file and run that, etcetera.

run java file in windows command prompt

I want to run a java project in windows. I first compiled the .class file in linux. Copy back to windows. Now under the path H:\deletefiles has delete.class, delete.java, a.jar, b.jar. The package for class delete is deleteFiles.
My java class path is C:\program Files\Java\jre7\bin, Where I have no access to write.
I run in command prompt C:\program Files\Java\jre7\bin>
java -cp H:\deleteFiles\deleteFiles.delete
always has the problem could not find or load main class, what's the problem? thanks

You are missing the actual class to be run. The -cp H:\deleteFiles\deleteFiles.delete only defines the classpath to be used, but not which class you want to run (and you limit the classpath to a single class as well).
What you want is:
java -cp H:\deleteFiles\deleteFiles delete
Note the blank (space) between H:\deleteFiles\deleteFiles which means you are passing two parameters to the java command:
-cp H:\deleteFiles\deleteFiles - the classpath to use
delete - the class to run
If you need the classes that are part of the jar files, you need to add them to the classpath as well:
java -cp H:\deleteFiles\deleteFiles;H:\deleteFiles\deleteFiles\a.jar;H:\deleteFiles\deleteFiles\b.jar delete

You need to set the classpath to the location which contains the package hierarchy. If your package is named deleteFiles the location needs to contain a directory named deleteFiles which contains the class file.
In your example you would run it with
java -cp H:\ deleteFiles.delete

you should call delete.class in your java command line, like this:
java -cp H:\deleteFiles\delete

To execute a Java program you have two options. Using a class file or using a jar file.
If your program only contains a single source file, executing the class file would be fine. But if you have multiple sources, you would have to copy all of them. Then a jar would be more practicable.
For class:
java -cp <class path> <class name>
For jar (if the main class is set):
java -jar <jar file>

Why do I need to delete the package line to run Java outside of NetBeans?

So I'm completely new to programming, and I've been writing some Java with the NetBeans IDE. My code runs fine within NetBeans, but I've tried to run it using the command line as well. However, if I run it from the command line, I have to delete the line:
package firstprogram;
which NetBeans automatically places at the top of each new file, or I get the error:
Error: Could not find or load main class FirstProgram
However, if I do delete the line, then the program no longer runs within NetBeans! It doesn't seem right that I have to choose whether to run a .java from within NetBeans or without.
The research I've done makes me think that this is something to do with directory structure? But everything I read on that goes straight over my head. NetBeans has a structure with "build", "dist", "nbproject", and "src", but when I use the command line I just place the .java file in an empty directory and javac from there.
Any explanation is appreciated! The books and tutorials I'm learning from either assume you're just using NetBeans or don't have the package line at all.

You can compile your class using javac command from anywhere, as long as you provide correct relative or absolute path. The problems come when you want to run your program using the java program.
You have to provide the correct path corresponding to your package declaration. For example, if I had 'MyClass' in package mypackage, first line would look like this:
package mypackage;
class source stored on disk:
c:/MyNetbeansProject/src/mypackage/MyClass.java
Compiled bytecode:
c:/MyNetbeansProject/build/classes/mypackage/MyClass.class
Now, if I would have opened a command prompt/terminal in folder c:/MyNetbeansProject/build/classes/, I could run the program using java mypackage/MyClass or java mypackage.MyClass.
However, if i would be somewhere else, I would have to say where the class files are located using the cp option: java -cp c:/MyNetbeansProject/build/classes mypackage/MyClass. The path in cp option can be relative or absolute, use "" when it contains spaces.

Package are directory architecture.
If your class is in the package com.acme.test, the class should be in the com/acme/test directory.
Instead of placing your class in an empty folder, place it in a folder named firstprogram and do javac firstprogram/youclass.java
The package (and folder) permit you to arrange your architecture with logical pattern.
More info here : http://www.tutorialspoint.com/java/java_packages.htm

So like OcterA said, you should keep organized, but with only one class this is not the issue. I believe that your problem is that you are not entering the correct command into the command line.
First cd to the correct directory and when you want to execute a file within a package in that directory you need to enter
java packageName.className
In this case
java firstprogram.FirstProgram

Error; Could not find or load main class (Java using Windows CMD)

I am trying to compile and run some java files I have made in Eclipse. The full path to the .java file is C:\Users\MYNAME\Documents\Java\Introduction\src\tests\Test.java. tests is the package I created in Eclipse and src is a folder that Eclipse made under Introduction (which is the project name).
In my environment variables, I have the following relevant variable:
JAVA_HOME C:\Program Files (x86)\Java\jdk1.7.0_40\bin
Under system variables I have the following:
CLASSPATH %JAVA_HOME%
I go to my cmd and cd into the tests directory (cd C:\Users\MYNAME\Documents\Java\Introduction\src\tests). Then I compile using javac Test.java. This seems to work as I then have a Test.class file under the same directory. Now I want to run the file, I type java Test and I get the error, "could not find or load main class". I've tried a variety of things including appending .class and .java to the end but I keep getting the error. I looked at some answers and docs and I managed to get it to work if I cd into:
cd C:\Users\MYNAME\Documents\Java\Introduction\src (i.e, get out of the package)
and then run:
java -cp . tests.Test
So that seems to temporarily set the class path to the current directory, and run Test from the package tests. However, I want to simply be able to type java Test. I know it's possible as I used to be able to do it, but now for some reason I cannot (I must have changed something along the way...).
Any help is appreciated.

However, I want to simply be able to type java Test
That will only work if Test is in the default package - it's as simple as that. You need to pass the java executable the fully-qualified name of the class you want to launch. There's no way round that.
Of course, you could create your own launcher which looks in the current directory for class files, finds out the fully-qualified name of the classes within those files, and launches java providing the full name and probably specifying an appropriate classpath... but that seems like a lot of hassle compared with just including the package name in the command.

You could be making the same mistake I made. So, try the following.
Here is my code for your reference.
class A{
public static void main(String args[]) {
System.out.println("Hello world");
}
}
Once you saved this as "C:\JavaStudy\ClassA.java", try the following.
c:\JavaStudy>javac ClassA.java
c:\JavaStudy>java A.class
Error: Could not find or load main class A.class
c:\JavaStudy>java A
Hello world
c:\JavaStudy>
Note: You don't need to use " java.exe -cp . " if you have class file in the same directory from where you are executing.

What does "Exception in thread \"main\" java.lang.NoClassDefFoundError" mean when executing java .class file?

Java and Gradle beginner's question.
I made a project directory for java and gradle test:
The directory hierarchy :
HelloWorld.java:
package foo.bar;
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, world");
}
}
build.gradle:
apply plugin:'java'
Then,gradle build this project and generated what i need.
As you see above, my problem is why doesn't this execute correctly? Even through I cd to .class path.
======================================================================
While, if I remove package foo.bar; in HelloWorld.java, and repeat gradle commands and execute at he.bak directory then the error remained the same.
But when I cd to the directory where HelloWorld.java placed. everything goes OK！Why? something related with CLASSPATH environment variables or other causes?
////////////////////////////////////////////////////////////////////
UPDATE
////////////////////////////////////////////////////////////////////
Thought you guys' warm replies, I know that I should combine the CLASSPATH and the period-separated executable .class file to figure out what's going on when executing java class file.
I experiment my thought resulting in 2 point to this question:
The -cp option path parameter A/B plus the executable file c.d.e.class finally form the A/B/c.d.e.class full path where the class is actually located.
If I specify the package in source code file with package d,I must split the full path in the form of java -cp A/B/c/d e.class. split in other ways all will result in errors.
something I am not sure here is :
When I specify my package path in my source code file, It determined the only classpath when executing corresponding executable, right?
If it is the truth, How does a project with lots of package and sources files work?
What's the root principle?

When in build/classes/main try java foo.bar.HelloWorld instead of java HelloWorld
The reason you need to specify foo.bar.HelloWorld is because you specified package foo.bar;. This tells java that the class should be in foo/bar/HelloWorld and the fully qualified name for HelloWorld is foo.bar.HelloWorld. If you want to execute the class from a different working directory however, you can specify the classpath explicitly using the -cp option, e.g., java -cp c:\myproject\build\classes\main foo.bar.HelloWorld.
By the way, the classpath default is the current working directory (i.e., .) but java -cp c:\myproject\build\classes\main foo.bar.HelloWorld will NOT have the classpath set to the current working directory if it is explicitly set using the -cp option. If you want to include the current working directory but explicitly set it, or even add more directories, you can chain them using semicolons like this: java -cp .;c:\myproject\build\classes\main foo.bar.HelloWorld. So this will include both the current working directory and the directory I specified.