---------ex Class A located in  folder \your\a_pkg---
package your.a_pkg
class A{
 void methold1(){...}
}
----------ex Class B,also located in folder \your\a_pkg----
package your.a_pkg
ClassB{
  void myMethold(
    A a=new A();
    a.methold();//it's ok.Class B and Class A in same package.
    ....
  }
}
------------Class C located in other folder \your\b_pkg----
package your.b_pkg
Class C {
   void cMothold{
    A a=new A();
    a.methold(); <--error!!can not invoke this a.methold out of package "your.a_pkg"
   }
}