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I am trying to run this project called "hello user". I am new to Java, so wrote a simple program that takes your name, and displays "Hello ". while Running it, I get the following error:

run:
Error: Could not find or load main class hello.world.HelloWorld
Java Result: 1
BUILD SUCCESSFUL (total time: 0 seconds)

But when I run file HelloWorld.java, it does it fine

I am doing this on Netbeans IDE 7.2

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  • Where you do get that Error at, are you trying to run it at the command line or inside of netbeans? To run your program at the command line you have to specify the package name in front of the class name. For example to run yours you would type java hello.worl.HelloWorld to run it. Commented Oct 7, 2012 at 1:04
  • Rather than the coding error, it could be related to IDE. Since the "Run File" runs okay, but 'Run Project" does not, I believe you have something to set up in IDE itself. Right click the project, and select "Set is as Main" or something similar, now run the project. I am just giving it a guess, may not help you. But it worth a shot.Otherwise, paste your whole code. Commented Oct 7, 2012 at 1:26

6 Answers 6

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Rather than the coding error, it could be related to IDE. Since the "Run File" runs okay, but 'Run Project" does not, I believe you have something to set up in IDE itself. Right click the project, and select "Set is as Main", now run the project. I am just giving it a guess, may not help you. But it worth a shot.If it does not help, please paste your code too.

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Your class needs a public static void main(String[] args) function. And moreover I suspect that the error could be in the package.

If you want your class in <main_package>.<sub_package>, The directory structure is

- main_package
    - sub_package
        -HelloWorld.java

And be sure to write your class like this.

package main_package.sub_package;

public class HelloWorld {

    public static void main(String[] args){
        System.out.println("Hello " + args[o]);
    }

}

This is all due to the naming convention in Java

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1

You need to run the .class file containing the public static void main(String[] args) method..

Here, your HelloWorld.java file might contain a class with main() method.. So, you can run it..

This is because, execution of any Java program starts with the invocation of main().. JVM needs an entry point to your code.. Which is main().. If it doesn't find one.. It will not run..

So, make sure, whatever class file you are running, it should have main() method..

UPDATE :- And for the starting point, may be you can skip using packages.. Just go with plain Java class without packages..

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6 Comments

Hey Rohit, this will run even without Main. public class trial{ static{ System.out.print("It works"); System.exit(0); } }. Run it from command mode. I think the entry point is Main is thread concept. The first thread to be executed is Main , however static block executes even before main.
@vandey.. What?? Have you tried it?? It's not possible to run a class which does not have main() method.. Unless you are running an Applet..
I have tried it . Just try it. I know you can not run any thread , without having Main thread , but this works like a charm.
@vandey.. Yeah using just static block is an exception case.. They are executed when the class is loaded.. It won't require any main method..
It was kind of tricky, If you remove system.exit(0), it will not run because it wont find the main thread. however the point is , execution starts way before. But we exit the system before JVM calls the main thread.
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This message can also appear in Eclipse (Juno 4.2.2 in my case) and I have found two potential causes for it. In my cases: 1. a DTD was in error. I deleted the file and that solved the issue*. 2. having cleaned the project, an external Jar that I had built externally had been deleted as could be seen from Properties -> Java Build Path -> Libraries.*

*Having solved either of the above issues, it was necessary to restart Eclipse

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0

if you are using intellij idea then just rebuilding (clean and build) project might solve your problem . because intellij might be still trying to load the old classes which are not there or changed

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-1

Make sure you call looks like below:

    public class HelloWorld {

        public static void main(String[] args) {
            System.out.println("hello user");
        }
    }

To run a Java class in stand alone mode, public static void main(String[] args) is the entry method, which is must.

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2 Comments

I have a file first.java containing as you have mentioned, when I compile it, I get error message :first.java:3: error: class HelloWorld is public, should be declared in a file named HelloWorld.java This is my very first java program and I don't understand what error is about.
As it says, save the file in a file named "HelloWorld.java". public classes should be created in a file of the "<class name>.java". Since your class name is HelloWorld, the file name should be HelloWorld .java, not first.java.

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