How to run a .class file that is part of a package

Question

I keep getting errors when I make my .class part of a package and try to run it from cmd.

Here's the code that works after using javac and then java:

class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

and then the code that does not work:

package com;

class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

giving me this error after trying to run the program via the command: java HelloWorld:

Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld (wrong nam
e: com/HelloWorld)
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(Unknown Source)
    at java.security.SecureClassLoader.defineClass(Unknown Source)
    at java.net.URLClassLoader.defineClass(Unknown Source)
    at java.net.URLClassLoader.access$100(Unknown Source)
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at sun.launcher.LauncherHelper.checkAndLoadMain(Unknown Source)

Here's what I've tried doing so far:

java -cp . HelloWorld
java -cp . com.HelloWorld
java -cp . com/HelloWorld
java HelloWorld
java com.HelloWorld
java com/HelloWorld

Keep in mind that javac returns with no errors and that simply removing package com; solves the problem. Sometimes in other scenarios I get an error that says the main class file cannot be found or something along those lines.

What am I doing wrong?

Answer 1

When you compile the java code, use -d, in your case, it would be

javac -d . com.HelloWorld.java

After the above command, java compiler generate a folder named "com", under the "com" folder, you will see your HelloWorld.class

Then under the same folder as you run javac, run the following command

java com.HelloWorld

Answer 2

Packages are directly related to the expected directory location of the file.

That is, if you have a source file with the package directive of com, it is expected that the file will live in the com directory.

In your HelloWorld example, it would be expected that the HelloWorld.java file would be stored in the com directory, like com\HelloWorld.java

When you compile the file, it will create a class file called HelloWorld.class in the com directory, like com\HelloWorld.class

This way, when Java goes looking for the com.HelloWorld class, it would actually be searching it's class path and appending com\HelloWorld.class to it until it finds your class file or runs out of class path

Example

So, I copied your HelloWorld.java (with package) example to C:\java\com\HelloWord.java

From the command line, I changed to the C:\java directory...

C:\java>dir com
 Volume in drive C is OS
 Volume Serial Number is ####-####

 Directory of C:\java\com

09/08/2013  01:55 PM    <DIR>          .
09/08/2013  01:55 PM    <DIR>          ..
09/08/2013  01:55 PM               135 HelloWorld.java

Then I compiled the HelloWorld.java

C:\java>javac com\HelloWorld.java

Then I ran it...

C:\java>java com.HelloWorld
Hello World!

---

You might like to have a read through Packages tutorial

Answer 3

You should compile it first by typing this command in CMD, for exemple your file is in c:\ directory :

C:\com>javac HelloWorld.java

After that you can run the result by typing:

c:\com>java HelloWorld

Answer 4

The syntax is:

java -classpath /path/to/package-folder <packageName>.<MainClassName>

So you may try: java com.HelloWorld which would expect com/HelloWorld.class file to be present as classpath by default points to the current directory (if not specified).

In case you're in different folder, try specifying classpath:

$ CLASSPATH=/path/to/package-folder/ java com.HelloWorld
Hello World!
$ java -cp /path/to/package-folder/ com.HelloWorld
Hello World!
$ cd /path/to/package-folder/ && java com.HelloWorld
Hello World!

For further explanation, check: How do I run Java .class files?

Answer 5

Suppose the file locates at C:/projects/com/HelloWorld and you can try the following ways.

1.java -cp c:/projects com.HelloWorld

2.cd c:/projects
 java com.HelloWorld
(The example 2 is not working in many situation,such as java process.)

if there is no package declaration and there will be a little change.

1.java -cp c:/projects/com HelloWorld

2.cd c:/projects/com
 java HelloWorld
(It's not good with the same reason.)

alternatively,relative path will be ok but has some risk. Last remember put the class file in end of cmd.