Question

I am having two problems regarding compiling and running an Eclipse java project from command line. This works fine when I am just running from the eclipse IDE. I tried googling but couldn’t really get the thing working. Any help is much appreciated.

Problem 1: When I try to compile from a location different from the directory where the .java file is, it throws the error “cannot read: myfile.java“. But if I migrate to this directory then it compiles.
The command that I was giving is (when in some other directory):
javac -cp C:\ABC\src\XYZ myfile.java
The command that I was giving when in XYZ directory:
javac myfile.java
This generated two .class files myfile.class and Testing_Thread.class(I guess this because I have a thread in my code)

Problem 2: After I have compiled by going to its directory, when I try to run the program, I get the error “Exception in thread “main” java.lang.NoClassDefFoundError: myfile (wrong name: XYZ/myfile.java)” even when I am trying to run from the XYZ directory. I get the same error when I try to run from some other place also.
The command that I was giving when in XYZ directory:
java myfile
The command that I was giving when in some other place:
java -cp C:\ABC\src\XYZ myfile

I am also attaching a hierarchy of my directory structure if it is of any help:

Answer 1

These examples assume the following source structure:

C:\temp\compile-test\src\a\b\c\D.java

Where D.java is:

package a.b.c;

public class D { }

The first problem, cannot read: myfile.java, is because it is not correct to use the cp command line option to point to your source code.

C:\temp\compile-test\src>javac -cp c:\temp\compile-test\src\a\b\c D.java
javac: file not found: D.java
Usage: javac <options> <source files>
use -help for a list of possible options

This should instead be the following, where javac is run from your source folder, and we can use relative paths to the source files (NOTE – javac is run from the source folder here):

C:\temp\compile-test\src>javac a\b\c\D.java

Or this, where we specify full paths to the source files, and javac can be run from anywhere (NOTE – javac is run from C:\ here):

C:\>javac temp\compile-test\src\a\b\c\D.java

Both of the above options will result in your class files being created in the same folder as the source. I.e.:

C:\temp\compile-test\src\a\b\c\D.class

For the second problem, if you try and run a class that has a package name from ‘inside’ the package, this will result in the name being wrong (NOTE – java being run from ‘inside’ the package here):

C:\temp\compile-test\src\a\b\c>java D
Exception in thread "main" java.lang.NoClassDefFoundError: D (wrong name: a/b/c/D)
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
        at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
Could not find the main class: D.  Program will exit.

To run the D class, you should be at the package ‘root’, and supply the Fully Qualified Class Name. I.e.:

C:\temp\compile-test\src>java a.b.c.D
Exception in thread "main" java.lang.NoSuchMethodError: main

Note I get an exception as the D class doesn’t have a main method, and so cannot be run. To fix, we add a main method:

package a.b.c;

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

and re-run:

C:\temp\compile-test\src>java a.b.c.D
main