Introduction to Computer Systems and the Java Programming Language

1. Introduction to Computer Systems and the Java Programming Language

2. Computer systems • A computer is a system made up of components • Two categories of components: • hardware: electronic and mechanical parts • software: programs and data • Main hardware components of a computer system • Processor (CPU) • Main memory • Secondary memory • Input devices (keyboard, mouse) • Output devices (monitor, printer)

3. Processor • Central processing unit (CPU), processor chip • “Brain” of the computer • Contains the logic circuitry controlling the interpretation and execution of machine instructions • programmed instructions • arithmetic and logical operations on data • controls input/output functions • Instructions • electronic operations • executed one at a time • millions per second • collection of instructions  executable program

4. Computer memory • Holds data and programs • Main memory • temporary memory • stores programs and data when the processor is actively using them • random access memory (RAM) • Secondary memory • permanent • stores (saves) programs and data on a long-term basis • hard disk, floppy disks

5. Binary • In both main and secondary memory, information is stored as patterns of bits • Binary  “Two states” • 1 and 0 • true and false • on and off • Binary devices • can be in just one of two possible states at a time • A single binary value is called a bit • binary digit

6. Example • A light switch is a binary device • holds one bit of information • can be in one of two states: on or off • can not be in any other possible state • A light dimmer is not a binary device • can be on, off, or some state in-between • difficult to characterize in-between states • how to turn a light exactly 50% or “half” on? • how to tell “how much” a light is on?

7. Binary representation • Bits can be used to represent patterns • Specifically, any system or set of symbols can be translated into bit patterns • patterns of ones and zeros • 10100001101 • Example: characters from any language alphabet • Require enough bits so that all symbols have a unique bit pattern to represent them • How many bits are needed to represent the English alphabet? • Require set of symbols is finite

8. Bits and Bytes • Bit • single binary or 0/1 value • One bit of information is so little that usually computer memory is organized into groups of eight bits • Byte: group of eight bits • kilobyte (KB): 210 = 1024 bytes • megabyte (MB): 220 = 1,048,576bytes • 1MB = 210KB • gigabyte (GB): 230 = 1,073,741,824 bytes • 1GB = 210 MB

9. High-level programming languages • Most programs are created using a high level programming language • closer to human language than machine language (low-level) • Java, C, C++, Pascal, FORTRAN • easier to read, write, and maintain • Source programs are translated to executable (machine language) programs by a compiler • Different programming languages require different compilers • Language can have many compilers • computer type, software package

10. Source program  Executable program • Create source program using a text editor • written (typed) instructions in a high-level language • Save source program on disk • Compile source program • compiler translates source program to executable program • source program remains unchanged • a new executable program is created • executable program is saved on disk • Run executable program • copied into main memory and run by processor

12. HelloWorld.java import java.awt.*; public class HelloWorld extends java.applet.Applet { TextField t; public void init() { t = new TextField(50); t.setText(“Hello World!"); add(t); } }

13. Java programs • Java programs are created as text files using a text editor (like emacs!) • Save to disk with.java file extension HelloWorld.java • The file contains characters (stored as bytes) • file can be printed, displayed on monitor, or edited • file cannot be directly executed (run) by the computer system • Java must first translate the program into bytecodes before it can be run

14. Bytecodes • Java bytecode • machine instruction for the Java processor • Java compiler javac translates the source program into bytecodes • Bytecode file has same name as the source program with a with.class file extension HelloWorld.class HelloWorld.java javac HelloWorld.class Javacompiler source program Java bytecodes

15. Java Virtual Machine (JVM) • Bytecode (class) file will contain exactly the same bytecodes no matter what computer system is used • Bytecode file is executed by a Java bytecode interpreter • processor specific executable program • Each type of computer system has its own Java interpreter that can run on that system. • Any computer system can execute Java bytecode programs if it has a Java interpreter • Computers with Java interpreters are called Java Virtual Machines • a “computer” with a Java processor that can run Java bytecodes

16. Java applets • An applet is a Java bytecode program that runs on a Web browser • Most newer Web browsers have Java interpreters • Web pages on the Internet contain instructions that send Java bytecodes to your computer • Web browser runs the Java applet with its built-in interpreter

17. Data Types • Computer memory stores arbitrary bit patterns • Meaning of a bit pattern depends its use • The particular pattern used for a particular string of bits is a data type. • uses bits to represent values • values are any kind of data a computer can process • all values are represented using some data type • Example: What does the following pattern of 16 bits represent? 0000000001100111 • No way to know without more information • If data type is short (a Java type) it represents 103

18. Java data types • Primitive • types of data that are so fundamental ways to represent them are built into Java • Reference • reference to (an address of) the value or set of values represented by the variable

19. Primitive data types • Primitive data values use fixed number of bytes • There are exactly eight primitive data types: byte, short, int, long, float, double, char, boolean • A programmer can not create new primitive data types • Any data type you invent will be a type of object

20. Java primitive data types

21. Declaration and initialization • Declaration: type <variable-name>; type <variable-name> = <value>; • Variable names • any combination of letters, numbers, and the underscore character • may not start with number • may not be reserved word (int, return) • may not be same as method name • case-sensitive (num and Num are different)

22. Examples • int x, y, z; • int sum = 0; • float f; • double pi = 3.14; • char first = ‘T’, middle = ‘L’, last = ‘B’; • char first = ‘T’; char middle = ‘L’; char last = ‘B’;

23. Basic operators

24. Conditional statements • if-else if( expression ){… statements …}else{… statements …}

25. Conditional statements • while while( expression ){… statements …} • for for( initialization; test; update ){… statements …}

26. Java methods int sum_between(int x, int y) { int k, sum = 0; for(k = x; k <= y; k++) { sum = sum+k; } return sum; }