New to Java

I tried compiling a program that instantiates an object of another class from a separate .java file. However, both .java files are in the same directory. I've never encountered this problem before, and I know some path variable must be wrong somewhere but I don't where. I also tried added a main method to the class I was trying to instantiate an object of ( from the separate .java file ), and then successfully compiled that separate file, but when I try to run it I get a runtime exception that says: "Exception in thread "main" java.lang.NoClassDefFoundError". I know the problem is not in the program itself because the entire program compiled and ran seemingly without any kind of error on a different computer, so there must be some sort of problem with my computer. Thanks in advance for any help.