Packages in Java is a quite simple and straightforward concept of the language. It’s there from the beginning and it’s commonly used by every Java programmer. In a few words, these are the rules you have to follow to create a class inside a package (spoiler: which are not completely true, as we’ll see later):

  • Package statement must be the first one specified in a java class file
  • A package namespace must match the physical path of the file, i.e a class defined with a package a.b.c must be placed in a path a/b/c/

Easy right? Let’s check…

Turns out that the second rule is not mandatory but a recommendation (a good one, BTW), but Eclipse IDE enforces it to the point that if you don’t follow it, it throws a compilation error. Let’s open the following class in Eclipse:

package a.b.c;

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

And you’ll get the following error:

Eclipse error with package statement

But as I said, it’s not a restriction of the language but from the IDE itself:

In fact, turns out that manually compiling this class works perfectly fine (also tried maven and worked flawesly):

$ javac src/ -d bin

$ java -cp bin a.b.c.HelloWorld
Hello world!

$ tree bin/
└── a
    └── b
        └── c
            └── HelloWorld.class

Corollary: if you’re a programmer, don’t trust even the most basic and accepted statement you could think of. Even if you’re absolutely right at this very moment, it will eventually be modified and surprise you.