Class 02: Introduction to Java

Held Tuesday, January 26

Summary

• Due: Introductory survey
• Introduction to object-oriented programming
• An introduction to Java
• Handout (electronic): Using Java in the MathLAN
• Reading: Introduction to Experiments in Java
• Lab: J1: An introduction to Java

Contents

Notes

• Today is primarily a lab day. After a short discussion of Java and object-oriented programming, we'll dig right in to the lab.
• Thanks to those of you who filled out the introductory survey.
  • Those of you who didn't need to do so ASAP.
  • If you care about my answers to your questions, you can find them online.

An introduction to object-oriented programming

• As you might guess, object-oriented languages concern themselves with objects.
  • You might have a sense of what a ``real world'' object is.
  • In computerese, an object is a collection of information (sometimes called attributes or fields) and operations that the object can perform (called methods).
• For example, we might say that the textbook for our class
  • is currently title ``Java Plus Data Structures'' (attribute)
  • is under development (attribute)
  • is or will be written by Sam Rebelsky, Chip Weems, and Nell Dale (attribute)
  • permits you to search it, at least in the electronic form (method)
  • contains text that you can extract (the text is an attribute, the extracting of the text might be considered a method)
• We often categorize objects into classes. A class specifies common aspects of a set of objects. These aspects are often generic attributes and specifications of methods (E.g., ``all objects in this class have a color, a size, and can draw themselves''.)
  • Almost every book has a title
  • Almost every book has one or more authors
  • Almost every book has text that you can extract
• Most object-oriented languages support inheritance, in which classes can be based upon other classes. For example, we might say that library books are a subclass of books and inherit all the attributes of books. Library books also extend books.
  • Since books have titles, library books also have titles
  • Since books have authors, library books also have authors
  • Library books also have ``loan records'': information on who has the book.
• Some notes on subclasses:
  • A subclass inherits the attributes and methods of its parent class.
  • The parent class is often called a superclass.
• Most object-oriented languages support polymorphism, in which a member of a subclass can be used in place of a member of a superclass.
  • If we know how to read a book, we know how to read a library book.
• Many object-oriented languages are event-driven. That is, you can specify ``when this event happens, call this method of this object''.
• A key aspect of object-orientation (and good program design in any language) is information hiding. In general, objects should know what other objects do, but not how they do it.
  • For example, you might know that a Menu object can draw menus on the screen, but it's not important to you how it does it.

An introduction to Java

• Java is an object-oriented language developed by Sun Microsystems.
  • It was ostensibly developed for developing large programs.
  • Many of these programs were to run on autonomous systems, like toasters.
  • With the popularization of the World-Wide Web, it has also become popular as a language for creating web applications.
  • It is also a reasonably good introductory language.
• Java looks a lot like C and C++, two popular languages. It has many differences from the two.
  • It is object-oriented (which C is not but C++ is)
  • It has a different view of object-orientation than C++.
  • It provides automatic memory management (yay!).
  • We'll see some others as time goes on.
• Sun says that Java is ``a simple, object-oriented, distributed, interpreted, robust, secure, architecture neutral, portable, high-performance, multithreaded, and dynamic language.''
  • Simple. Compared to many modern languages, there is not too much to the core of Java. However, a great deal of functionality exists in the standard libraries, and you will eventually need to learn those libraries.
  • Object-oriented. Java focuses on the creation of classes and objects. Every Java program begins with a central class or object.
  • Distributed. Java permits you to place different portions of a program on different servers, to segment processing between servers, and so on and so forth.
  • Interpreted. Although a compiler translates Java code to another format (byte codes), an interpreter is required to execute or further translate the byte codes.
  • Robust. Java provides a number of facilities for ensuring that things don't go wrong or that, when they do, the programmer provides for problems.
  • Secure. The Java interpreter will often limit what a program can do (from accessing the file system to reading memory outside of a valid range). We often say that Java gives each program a sandbox to play in.
  • Architecture neutral. A Java program will run identically (except for speed) on every platform. Unlike most languages, which can have a different implementation on every platform, Java has specific requirements for essentially every part of the language (from the amount of storage used for various data types to the algorithms used for numeric computation).
  • Portable. See the sections above on architecture neutral and distributed.
  • Multithreaded. A program can have separate threads of computation, and it is relatively easy to manage those threads.
  • Dynamic. New objects can easily be created, and new classes can be loaded as the program runs.
• Unfortunately, Java is also a moving target. There have been at least two major changes to Java since it has been released.

History

• Created Monday, January 11, 1999
• Filled in some details on Monday, January 18, 1999. Those details were primarily based on materials I had written for a prior session of CS152.
• Added the section on object-oriented programming on Tuesday, January 26, 1999.