Error caused on runtime of java class file

Question

How do I compile and run java files if you don't know the specific package used by the developer...

I got a folder of java files written by someone else, and I'm trying to compile and run the java files, I can compile the files by running javac *.java, but if I try to run the main class by calling java MainClass, I get an error like so: Error could not find or load main class MainClass, Caused by: java.lang.NoClassDefFoundError: SomePackageName/MainClass (wrong name: MainClass)

I have no idea whats the SomePackageName its defined by the developer and I have no control over that.

so the question is how do I compile and run the java files if all I know is which is the main class?

Answer 1

1. Fully qualified class names are of the form: com.foo.Bar, where com.foo is the package (e.g. the file defining the Bar class starts with the line package com.foo;), and Bar is the name (e.g. the file contains the line public class Bar {).
2. When running java and specifying classpath+classname, you pass a class name. Not a file name. So to run this file you'd write java -cp /path/to/base com.foo.Bar.
3. fully qualified names must match directory structure both at runtime (class files) and compile time (java files). So, here you should have say /Users/Foghunt/projects/myproj/src/com/foo/Bar.java and after compilation you should have e.g. /Users/Foghunt/projects/myproj/build/com/foo/Bar.class
4. The 'base dir' is the one that contains the first package directory. So, the base dir for the source file is /Users/Foghunt/projects/myproj/src and for the compiled result is /Users/Foghunt/projects/myproj/build.
5. The base dir needs to be in the classpath. Classpath entries are file paths. So, to run this thing you'd write java -cp /Users/Foghunt/projects/myproj/build com.foo.Bar.
6. javac doesn't mind if you don't go through the trouble of properly defining things (e.g. running javac *.java in /Users/Foghunt/projects/myproj/src/com/foo does work), but it gets complicated quickly if you have multiple packages.
7. In general you should be using build systems. If you've inherited this project, check for a file named build.xml or pom.xml or build.gradle in the root of the project. If it is there, use a web search engine (build.xml -> search for java ant, pom.xml -> java maven, build.gradle -> java gradle), and use that to build the project, don't run javac yourself.

There you go. Armed with that knowledge you can now analyse, build, and run any java project.