An Overview of Objects and the Java API

An Overview of Objects and the Java API Joel Adams and Jeremy Frens Department of Computer Science Calvin College (1/10) 2003 Joel C. Adams. All Rights Reserved. Dept of Computer Science Calvin College

Where did Java come from? Object-oriented programming: · Late 1970 s: Xerox PARC (GUIs, Ethernet, Laserprinters, . . . ) - Smalltalk (Adele Goldberg) ¡ ¡ ¡ Classes of objects Objects communicate via messages Everything allocated dynamically ¡ ¡ ¡ Byte code compiler Virtual machine (interpreter) Garbage collector · Mid-1980 s: Bell Labs - C++ (Bjarne Stroustrup): “C with Classes”, a hybrid language · Early 1990 s: Sun Microsystems - Java (James Gosling): “Just another virtual architecture” - Syntax like C++, features like Smalltalk (2/10) 2003 Joel C. Adams. All Rights Reserved. Dept of Computer Science Calvin College

What is Java? When people say “Java” they may mean: · An object-oriented programming language · A developers kit (compiler, debugger, …) · A run-time environment small enough to be built-in to a web browser, cell-phone, cable-box, … · A platform-independent system that strives for “write once, run anywhere” portability anywhere on the Internet MS’s. Net gives much of this, except platform-independence. Much of what we will cover also applies to VB. Net. (3/10) 2003 Joel C. Adams. All Rights Reserved. Dept of Computer Science Calvin College

Classes and Objects What is a class? Think “typhoon-class submarine” or “galaxy-class starship”: · A mechanism for creating new types · A means of declaring a container to store both - variables to store an object’s attributes (internal state); and - messages to which an object will respond (behavior) · A factory for constructing related objects What is an object? · An object is an instance of a class. (4/10) 2003 Joel C. Adams. All Rights Reserved. Dept of Computer Science Calvin College

A Simple Java Class Suppose we needed to represent fractions (1/2, 3/4) exactly… class Fraction { private int my. Numerator; private int my. Denominator; public int get. Numerator() { return my. Numerator; } public int get. Denominator() return my. Denominator; }. . . } (5/10) To store a fraction’s internal state, we need variables to hold its numerator and denominator We then define a method for { each message to which we want a Fraction to respond A fair amount of design time goes into deciding what methods a class should provide for its users 2003 Joel C. Adams. All Rights Reserved. Dept of Computer Science Calvin College

Creating and Using Objects Once we have an operational Fraction class… class Fraction. Tester { public static void main(String [] args) { Fraction frac 1 = new Fraction(); we can use it to create Fraction frac 2 = new Fraction(3, 4); Fraction instances if ( frac 1. get. Denominator() != 0) { // do something useful with frac 1 } } } (6/10) and compute using those instances by sending messages The real power of OOP comes from organizing objects into hierarchies, which we’ll see in a future session. 2003 Joel C. Adams. All Rights Reserved. Dept of Computer Science Calvin College

Object-Oriented Design Rumbaugh’s Object Modeling Technique (OMT): · Write a natural-language description of your system. Our application should display a window containing a textbox in which a user enters their name. When they have entered it, the application should display a personalized greeting for that user. · The nouns (except external ones) are your objects. - If any object cannot be directly represented using predefined types, build a class to create a type for it. · The verbs (except user actions) are your operations. - If any operation cannot be directly performed using predefined operations, build a method (message) to perform it. (7/10) 2003 Joel C. Adams. All Rights Reserved. Dept of Computer Science Calvin College

OOD: Storyboarding Many designers find it useful to sketch storyboards that show the appearances of their GUIs: Hello. GUI Please enter your name: Jane Enter Jane Welcome to Java, Jane! If one storyboard can be reached from another, they are connected by an arrow labeled with the event that triggers the transition… (8/10) 2003 Joel C. Adams. All Rights Reserved. Dept of Computer Science Calvin College

The Java API The Java Application Programmers Interface (API) provides the definitive word on what classes Java predefines, and what messages instances of those classes accept: http: //java. sun. com/api/index. html Using this, we can build a working application using whatever IDE or tools we have available… I’ll use a text editor and the command-line here; you will use Eclipse that Jeremy will demo for you soon… (9/10) 2003 Joel C. Adams. All Rights Reserved. Dept of Computer Science Calvin College

Summary Java is an OO language descended from Smalltalk and C++. Java was designed to allow programs to be downloaded and executed anywhere on the Internet; portability is paramount. In object-oriented languages, much of the programming effort goes into building classes. Objects are instances of classes. Objects communicate with one another by sending messages. (10/10) 2003 Joel C. Adams. All Rights Reserved. Dept of Computer Science Calvin College