1. 1 Computer Systems -- Introduction  Chapter 1 focuses on:  the structure of a Java application  basic program elements  preparing and executing a program  basic object-oriented programming concepts  helpful support for writing software  Java applets

2. 2 Focus of the Course  Object-Oriented Software Development  problem solving  program design and implementation  object-oriented concepts objects classes interfaces inheritance polymorphism  graphics and Graphical User Interfaces  the Java programming language

3. 3 Programming Languages  A programming language specifies the words and symbols that we can use to write a program  There are four basic programming language levels: machine language assembly language high-level language fourth-generation language

4. 4 Machine Languages  Each CPU has its own specific machine language  machine-dependent code  Pentium processor has its own machine language  SPARC process has its own machine language  hard to read, write, understand 1101 0000 0000 0111 1011 1111 1110 1000 1101 0010 0000 0111

5. 5 Assembly language  Symbolic code for machine language  Machine-dependent  Easier to read, write, and understand than machine language ld [%fp-20], %o0 ld [%fp-24], %o1 add %o0, %o1, %o0  Still, assembly programming is a difficult job  Assembler assembly code machine code assembler

6. 6 High-level Languages  Machine-independent language  Fortran, COBOL, C, C++, Java, …  easy to program a + b  Program execution  compilation(translation)  interpretation  mixture of the two

7. 7 Compiler  A program must be translated into machine language  a software tool which translate source code into target code source code target code compiler Fortran C, C++ Assembly code Machine code

8. 8 Interpreter  A software tool which read a program and interpret(excute) it  Basic, Prolog, ML, …. input output interpreter program

9. 9 The Java Programming Language  Java was created by Sun Microsystems, Inc.  It was introduced in 1995 and has become quite popular  It is an object-oriented language  It is an Internet programming language

10. 10 Problem Solving  The purpose of writing a program is to solve a problem  The general steps in problem solving are:  Understand the problem  Dissect the problem into manageable pieces  Design a solution  Consider alternatives to the solution and refine it  Implement the solution  Test the solution and fix any problems that exist

11. 11 Java Program Structure  See Lincoln.java  A program is made up of one or more classes  A class contains one or more methods  A method contains program statements  A Java application always executes the main method

12. 12 Java Program Structure public class MyProgram {}{} // comments about the class class header class body Comments can be added almost anywhere

13. 13 Java Program Structure public class MyProgram {}{} public static void main (String[] args) {}{} // comments about the class // comments about the method method header method body

14. 14 Lincoln.java class Lincoln { public static void main (String[] args) { System.out.println ("Whatever you are, be a good one."); } // method main } // class Lincoln

15. 15 White Space  Spaces, blank lines, and tabs  to separate words and symbols in a program  Extra white space is ignored  A valid Java program can be formatted in many different ways  See Lincoln2.java and Lincoln3.java  Programs should be formatted  to enhance readability, using consistent indentation

16. 16 Lincoln2.java class Lincoln2 { public static void main (String[] args) { System.out.println ("Whatever you are, be a good one."); } }

17. 17 Lincoln3.java class Lincoln3 { public static void main ( String [] args ) { System.out.println ( "Whatever you are, be a good one." ) ; }

18. 18 Comments  Comments in a program are also called inline documentation  They should explain  the purpose of the program and  describe processing steps  Java comments can take two forms: // comment runs to the end of the line /* comment runs to terminating symbol, even across line breaks */

19. 19 Identifiers  The words a programmer uses in a program  Most identifiers have no predefined meaning  An identifier can be made up of  letters, digits, _, and $  they cannot begin with a digit  Java is case sensitive,  Total and total are different identifiers

20. 20 Reserved Words  Some identifiers, called reserved words, have predefined meanings. abstract boolean break byte byvalue case cast catch char class const continue default do double else extends false final finally float for future generic goto if implements import inner instanceof int interface long native new null operator outer package private protected public rest return short static super switch synchronized this throw throws transient true try var void volatile while

21. 21 Literals  A literal is an explicit data value used in a program  Integer literals: 25 69 -4288  Floating point literals: 3.14159 42.075 -0.5  String literals: "The result is: " "To thine own self be true."

22. 22 Java Translation and Execution  The Java compiler translates Java source code into a special representation called bytecode  Java bytecode  a virtual machine code  is not the machine language for any traditional CPU  Interpreter  interpret Java bytecode  Java compiler  is not tied to any particular machine  Java is considered to be architecture-neutral

23. 23 Java Translation and Execution Java source code Machine code Java bytecode Java interpreter Bytecode compiler Java compiler

24. 24 Java Translation and Execution  Compiling a Java program > javac Lincoln.java  This creates a file called Lincoln.class  Interpret the class file > java Lincoln  The.java extension is used at compile time, but the.class extension is not used with the interpreter

25. 25 Three types of program errors  Compile-time errors  The compiler will find problems with syntax and semantics  If compile-time errors exist, an executable version of the program is not created  Run-time errors  A problem can occur during program execution,  divide by zero  Logical errors  A program may run, but produce incorrect results

26. 26 Command Line Arguments  The main method accepts extra information on the command line when a program is executed > java Name_Tag John  Each extra value is called command line argument  In Java, command line arguments are always read as a list of character strings

27. 27 Java Applets  A Java applet  a Java program that is intended to be sent across a network and executed using a Web browser  A Java application is a stand alone program  Applications have a main method, but applets do not  Applets are derived from the java.applet.Applet class

28. 28 Java Applets  See No_Parking.java  Links to applets can be embedded in HTML documents  Appletviewer in JDK  appletviewer xxx.html  Web browser  Netscape navigator  Internet explorer

29. 29 No_Parking.java import java.applet.Applet; import java.awt.*; public class No_Parking extends Applet { public void paint (Graphics page) { page.drawString ("Parking", 50, 50); page.drawOval (45, 24, 43, 43); page.drawLine (82, 30, 51, 61); } // method paint } // class No_Parking

30. 30 Execution of Java Applets Java source code Java source code Java bytecode Java bytecode Java compiler Java compiler Java interpreter Java interpreter Web browser local computer remote computer remote computer