Java Methods - An Introduction to Software Development


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  • This chapter gives a glimpse of how software development evolved from artisanship into a professional engineering discipline. At the same time, students will get familiar with Java development tools and run their first programs.
  • Students with a penchant for history can find a lot of interesting historical data on programming languages and software methodologies on the Internet.
  • A Google search for SOFTWARE results in over 6.4 billion hits. Computers run everything from power grids, water utilities, TV satellites, and telephone networks to cars and coffee makers. Try to envision our lives if all programs were suddenly wiped out. Also try to appreciate the total effort that was needed to develop this invisible universe.
  • The idea of developing intelligent computers (AI, or artificial intelligence) came about as soon as (or perhaps before) the first computer. Over the years the expectations turned out to be too optimistic. Little progress to report so far.
  • The main premise of the current software development culture is that hardware keeps getting faster and cheaper, so efficiency is no longer an issue. As a result, the performance of some programs even on fast computers may be rather sluggish, and we are forced to upgrade our computers every couple of years. This arrangement is good for both hardware and software companies: new computers have room for new software, bigger and slower software calls for new computers. This disregard for efficiency and economy in commercial software may be partly intentional.
  • Thousands of programming languages and dialects have been described; many evolved over the years, others have disappeared, some existed for years but recently gained new popularity (e.g., Perl, Python). Java was initially meant for embedded systems (like home appliances) and for “interactive TV” but has survived due to the Internet. Machine code is called “first generation”; assembly languages are second generation; “high-level languages” (Fortran, Basic, Pascal, Smalltalk, etc.) are third generation. Fourth generation — visual prototyping systems with automatic code generation — was a buzzword in the 1980s (in the field called CASE, Computer-Aided Software Engineering), but the promise never really materialized.
  • A programming language has a strict syntax. A compiler parses the source code and checks the syntax. There are other programming tools. For example, version control software allows a team of programmers to keep track of changes to files, so that two programmers are not working with the same file at the same time.
  • This is a neat legend, but actually the term “bug” was used earlier.
  • If you read your code carefully and reason about it before compiling, the number of cycles through Edit-Compile-Link-Run will be reduced, perhaps even to one!
  • When you use an interpreted language, the program that is being executed is the interpreter; the text of your program serves as data for the interpreter.
  • A Java interpreter is built into a Java-enabled browser for running applets. Due to a legal dispute between Sun and Microsoft, Internet Explorer does not have the latest version of Java built in, but Sun provides a Java “plug-in” for it.
  • Java Virtual Machine is an imaginary computer with a CPU instruction set that is “the least common denominator” for typical real CPUs.
  • There is also JIT (Just-In-Time) compiler technology where the program is interpreted and compiled at the same time. On subsequent runs the compiled code is used; there is no need to reinterpret it.
  • Java’s culture is pretty open, and many applets are available on the Internet with their source code. Still, software vendors may want to protect their source code. For obvious reasons, the interpreter wouldn’t allow an applet to read or write files on your computer. Hackers have peculiar ethics (or rather, a gap in ethics): they won’t go around checking locks on the doors of houses or cars or spread real disease germs (even if non-lethal ones), but they will break into your computer system (which may be much more harmful and expensive) and spread computer viruses.
  • JDK lacks an editor, so we use non-JDK software to create the source code. Our main tools are javac , java , and appletviewer . IDE usually has a more convenient debugger than JDK’s debugger. We recommend not using a debugger at all while learning how to program.
  • It may be better to download Java docs together with the JDK and install them on the school server.
  • Most Java IDEs are written in Java. JCreator is an exception: it is written in C++ and is a little more compact and faster. It requires that JDK be installed. Install the JDK first, before the IDE.
  • Console applications emulate a teletype device; they are sometimes called terminal applications (comes from old text-only terminals).
  • Console applications may be not much fun, but they do the job when you need to test a simple calculation or algorithm. The path=... command tells the system where to look for java.exe and javac.exe .
  • Command-line arguments are not specific to Java: C and C++ have them, as well as other languages under Unix , MS-DOS , etc. In Windows , use the Command Prompt application. The Java source file name is a command-line argument for javac . Likewise, the HTML file name is a command-line argument for appletviewer .
  • An IDE usually provides a way to set command-line args (on a Mac, too) or prompts for them when you run your program.
  • The Scanner class has been added to the java.util package in Java 5.0. It simplifies reading numbers, words, and lines of text from the keyboard and files.
  • This screen is from the Marine Biology Simulation case study, developed by Alyce Brady for the College Board and used on the AP CS exams for several years.
  • JFrame is a Swing library class for a generic program window. In a GUI program the program itself defines the initial dimensions of the program window. In an applet, the dimensions are defined in the <applet> tag in the HTML file.
  • Like JFrame , JApplet is also a Swing library class — but for an applet. We redefine its init method.
  • “Methods” in Java are like functions in C or C++. Calling an object’s method is often phrased as “sending a message” to the object.
  • For example, a program may have a class Robot and an object of that class. A Robot object keeps in its memory its initial and current position. A robot has methods to step forward and to turn. Another class may be Position ; the initial and current positions may be objects of that class. Robot ’s method “forward” may call Position ’s method “move.” The number of classes in a project may be quite large, but they are usually short. In Java each class is stored in a separate file. (An exception are the so-called inner classes that are embedded inside another class. They are not discussed in this book.) The name of the file must be exactly the same as the name of the class (including upper and lower case) with the extension .java .
  • Event-driven applications are necessitated by GUI: the program cannot predict which button or menu the user will click next; it reacts to different events as they come. The robot program may have a “Step” button, for instance. Inheritance allows you to reuse most of the code in a class and redefine or add a couple of things. We could extend Robot into a JumpingRobot , adding a method jump . These concepts will become clearer in the next chapter and in the subsequent chapters.
  • We say that a Child IS-A (kind of) Person ; a Toddler IS-A (kind of) Child . Each subclass inherits all the methods of its superclass, and may redefine some of them and/or add new methods.
  • At least these are the claims. OOP is definitely good for GUI development. But is it universally applicable in any type of project? OOP originated in academia; in a rather unique situation, businesses have spent millions of dollars converting to OOP and retraining their programmers without serious formal cost-benefit analysis. Prior to OOP the big thing was structured programming and top-down development .
  • What are some of the current software development concerns? Team development, reusability, easier maintenance, portability, user-friendliness. What are editor, compiler, debugger used for? Creating source code (program’s text); compiling source into object code (CPU instructions); running a program in a controlled manner and correcting bugs, respectively. How is a compiler different from an interpreter? A compiler creates object modules that can be linked into an executable. The compiler is not needed to run the executable. An interpreter interprets the source and executes appropriate instructions. It must be installed and active to execute program statements. Name some benefits of Java’s compiler + interpreter approach. Bytecodes are device-independent, load faster, allow for security checks, and don’t reveal source code. Define IDE. Integrated Development Environment — combines software development tools under one GUI.
  • What is a console application? An application in the style of the old teletype or terminal (console): uses a dialog with text prompts and user input. What are command-line arguments? An array of strings passed to the program from the operating system command line that runs the program (or from a special option box in an IDE). What is a GUI application? An application with a graphical user interface, often used with a mouse or another pointing device. What is the difference between a GUI application and an applet? An application runs on the computer where it is installed. An applet is embedded in a web page and is usually downloaded from the Internet (or a local network) together with the HTML document. What is OOP? Object-Oriented Programming — a program simulates a world of active objects. Define inheritance. Inheritance is a way to derive a class (a subclass) from another class (the superclass), adding new features and/or redefining some of the features.
  • Java Methods - An Introduction to Software Development

    1. 1. An Introduction to Software Development Java MethodsJava Methods A & ABA & AB Object-Oriented Programming and Data Structures Maria Litvin ● Gary Litvin Copyright © 2006 by Maria Litvin, Gary Litvin, and Skylight Publishing. All rights reserved. Chapter 2
    2. 2. 2-2 Objectives: • Understand the software development process, tools, and priorities • Understand compilers and interpreters • Learn about Java Virtual Machine, bytecodes • Learn to set up and run simple console applications, GUI applications, and applets in Java • Learn basic facts about OOP
    3. 3. 2-3 Software Today: 6,460,000,000
    4. 4. 2-4 Software Applications • Large business systems • Databases • Internet, e-mail, etc. • Military • Embedded systems • Scientific research • AI • Word processing and other small business and personal productivity tools • Graphics / arts / digital photography • Games
    5. 5. 2-5 Software Development • Emphasis on efficiency  fast algorithms  small program size  limited memory use • Often cryptic code • Not user-friendly • Emphasis on  programmer’s productivity  team development  reusability of code  easier maintenance  portability • Better documented • User-friendly 1950-1960's: Now:
    6. 6. 2-6 Programming Languages 1940 1950 1960 1970 1980 1990 2000 Machine code Assembly languages Fortran Basic Pascal Scheme C C++ Java LISP Smalltalk Smalltalk-80 C# Logo Python
    7. 7. 2-7 Software Development Tools • Editor  programmer writes source code • Compiler  translates the source into object code (instructions specific to a particular CPU) • Linker  converts one or several object modules into an executable program • Debugger  steps through the program “in slow motion” and helps find logical mistakes (“bugs”)
    8. 8. 2-8 The First “Bug” “(moth) in relay” Mark II Aiken Relay Calculator (Harvard University, 1945)
    9. 9. 2-9 Compiled Languages: Edit-Compile-Link-Run Editor Source code Compiler Object code Linker Executable program Editor Source code Compiler Object code Editor Source code Compiler Object code 
    10. 10. 2-10 Interpreted Languages: Edit-Run Editor Source code Interpreter 
    11. 11. 2-11 Compiler vs. Interpreter • Compiler:  checks syntax  generates machine-code instructions  not needed to run the executable program  the executable runs faster • Interpreter:  checks syntax  executes appropriate instructions while interpreting the program statements  must remain installed while the program is interpreted  the interpreted program is slower
    12. 12. 2-12 Java’s Hybrid Approach: Compiler + Interpreter • A Java compiler converts Java source code into instructions for the Java Virtual Machine. • These instructions, called bytecodes, are the same for any computer / operating system. • A CPU-specific Java interpreter interprets bytecodes on a particular computer.
    13. 13. 2-13 Java’s Compiler + Interpreter Editor     Compiler  Hello.class   Interpreter Hello, World!  Interpreter
    14. 14. 2-14 Why Bytecodes? • Platform-independent • Load from the Internet faster than source code • Interpreter is faster and smaller than it would be for Java source • Source code is not revealed to end users • Interpreter performs additional security checks, screens out malicious code
    15. 15. 2-15 JDK — Java Development Kit • javac  Java compiler • java  Java interpreter • appletviewer  tests applets without a browser • javadoc  generates HTML documentation (“docs”) from source • jar  packs classes into jar files (packages) All these are command-line tools, no GUI
    16. 16. 2-16 JDK (cont’d) • Available free from Sun Microsystems • All documentation is online: • Many additional Java resources on the Internet
    17. 17. 2-17 Java IDE • GUI front end for JDK • Integrates editor, javac, java, appletviewer, debugger, other tools:  specialized Java editor with syntax highlighting, autoindent, tab setting, etc.  clicking on a compiler error message takes you to the offending source code line • Usually JDK is installed separately and an IDE is installed on top of it.
    18. 18. 2-18 Types of Programs • Console applications • GUI applications • Applets
    19. 19. 2-19 Console Applications C:javamethodsCh02> path=%PATH%;C:Program FilesJavajdk 1.5.0_07bin C:javamethodsCh02> javac C:javamethodsCh02> java Greetings2 Enter your first name: Josephine Enter your last name: Jaworski Hello, Josephine Jaworski Press any key to continue... • Simple text dialog: prompt → input, prompt → input ... → result
    20. 20. 2-20 Command-Line Arguments C:javamethodsCh02> javac C:javamethodsCh02> java Greetings Josephine Jaworski Hello, Josephine Jaworski public class Greetings { public static void main(String[ ] args) { String firstName = args[ 0 ]; String lastName = args[ 1 ]; System.out.println("Hello, " + firstName + " " + lastName); } } Command-line arguments are passed to main as an array of Strings.
    21. 21. 2-21 Command-Line Args (cont’d) • Can be used in GUI applications, too • IDEs provide ways to set them (or prompt for them) Josephine Jaworski
    22. 22. 2-22 import java.util.Scanner; public class Greetings2 { public static void main(String[ ] args) { Scanner kboard = new Scanner(; System.out.print("Enter your first name: "); String firstName = kboard.nextLine( ); System.out.print("Enter your last name: "); String lastName = kboard.nextLine( ); System.out.println("Hello, " + firstName + " " + lastName); System.out.println("Welcome to Java!"); } } Prompts
    23. 23. 2-23 GUI Applications Menus Buttons Clickable panel Slider
    24. 24. 2-24 import java.awt.*; import javax.swing.*; public class HelloGui extends JFrame { < ... other code > public static void main(String[ ] args) { HelloGui window = new HelloGui( ); // Set this window's location and size: // upper-left corner at 300, 300; width 200, height 100 window.setBounds(300, 300, 200, 100); window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); window.setVisible(true); } } GUI libraries
    25. 25. 2-25 import java.awt.*; import javax.swing.*; public class HelloApplet extends JApplet { public void init( ) { ... } < ... other code > } No main in applets: the init method is called by JDK’s appletviewer or the browser
    26. 26. 2-26 OOP — Object-Oriented Programming • An OOP program models a world of active objects. • An object may have its own “memory,” which may contain other objects. • An object has a set of methods that can process messages of certain types.
    27. 27. 2-27 OOP (cont’d) • A method can change the object’s state, send messages to other objects, and create new objects. • An object belongs to a particular class, and the functionality of each object is determined by its class. • A programmer creates an OOP application by defining classes.
    28. 28. 2-28 The Main OOP Concepts: • Inheritance: a subclass extends a superclass; the objects of a subclass inherit features of the superclass and can redefine them or add new features. • Event-driven programs: the program simulates asynchronous handling of events; methods are called automatically in response to events.
    29. 29. 2-29 Inheritance • A programmer can define hierarchies of classes • More general classes are closer to the top Person Child Adult Baby Toddler Teen
    30. 30. 2-30 OOP Benefits • Facilitates team development • Easier to reuse software components and write reusable software • Easier GUI (Graphical User Interface) and multimedia programming
    31. 31. 2-31 Review: • What are some of the current software development concerns? • What are editor, compiler, debugger used for? • How is a compiler different from an interpreter? • Name some of the benefits of Java’s compiler+interpreter approach. • Define IDE.
    32. 32. 2-32 Review (cont’d): • What is a console application? • What are command-line arguments? • What is a GUI application? • What is the difference between a GUI application and an applet? • What is OOP? • Define inheritance.