http://www.javasprint.com/java_training_tutorial_blog/object_oriented_programming_oops.htm In a program, you create an object for every thing in the real world and establish relationships between them. Hence you recreate the world in Computer's memory. How the relationship is established between objects is show below using two diagrams. Lets say, you are creating a software for running a University. You will create an Object of University Class, Five Objects of Department Class if there are five departments in the University. 25 Objects of Professor Class if there are 25 professors in the University (Lets say 5 professors per department,) 20 Semester Objects if there are four semesters per department, 600 Student Objects if there are 30 students per semester. Java Training and Reference 1 Copyright Bharat Chhajer RSSGenerator.net Free RSS Feed Generator en-us http://www.javasprint.com/java_training_tutorial_blog/object_oriented_programming_oops.htm http://www.javasprint.com/java_training_tutorial_blog/object_oriented_programming_oops.htm Thu, 08 Dec 2011 00:00:00 EST After creating the Objects now we need to establish relation ships between them. How do we do that ? There are only two kinds of relations possible in Java. "Is A" and "Has A". Their meaning in Java is very literal. Like, An apple "Is A" fruit and An apple "Has A" seed (actually has lot of seeds). Also like, An Orange "Is A" fruit. The "Is A" relation is established between two classes by means of Inheritance (using extends keyword). The "Has A" relation is established between two classes by means of Association (using fields).