You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let's say a class Employee belongs to a package com.xyz.hr; and is located in the file c:\dev\com\xyz\hr\Employee.java. In this case, you'd need to add c:\dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:

c:\>java com.xyz.hr.Employee

It's possible if these variables are final.

A single ampersand here would lead to a NullPointerException.

Use the following syntax: super.myMethod(); To call a constructor of the superclass, just call super() in the first line of the subclass's constructor, passing any required parameters.

 Yes, but abstract classes are more limited because a class can only extend one class but can implement multiple interfaces.

Garbage collection; a young generation is a recent collection of newly created objects, which is reclaimed more aggressively than older ones.