java - abstract class and arraylist -

i don't understand lines on code

public abstract class subject{ 

the class abstract subject super class

    public subject() {  

and constructor?

like inside constructor passed on sub classes?

observers = new arraylist<ioserver> ();  

here have no idea.

the constructor passed on subclass constrained. observers variable initialized within constructor, don't see declared.

like:

arraylist<ioserver> observers = new arraylist<ioserver> ();