• The is-a relationship allows an object to be extended or to be based on another object (inheritance).
• The has-a relationship allows an object to be built up from simpler objects (composition).

Towards the end of the semester we will work more with inheritance and composition, but the goal for the first part of the course is to understand simple objects and how to design them and use them to solve problems. We'll have a much clearer picture of is-a and has-a later in the course. In this class period, we are going to try to get an idea of what an object is and how it might be implemented on a computer.

• What does a button know about? (Ans: pushed or cleared).
• How is a button interact with the outside world?
  • Someone (or thing) pushes the button.
  • Someone (or thing) clears the button.
  • Button indicates whether pushed or cleared.

We will use Java, an object-oriented language. Next class period we'll get into why Java. Button.java is a Java implementation for part of the button. ButtonMain.java is a program that uses Button. (See http://vip.cs.utsa.edu/classes/cs1713f99/robbins/lectures/JavaExamples/Button.html .)

• class
• object
• member
• method
• constructor

• A method in Button to clear the button. Add a statement in ButtonMain to test it.
• Make the button keep track of how many times it was pushed.