Post on 26-Mar-2015
What is a Computer Program?
For a computer to be able to do anything (multiply, play a song, run a word processor), it must be given the instructions to do so.
A program is a set of instructions written by humans for computers to perform tasks.
The instructions are written in programming languages such as C, C++, Java, etc.
Programming Languages
Machine Language Assembly Language High-Level Language
Machine language is a set of primitive instructions built into every computer. The instructions are in the form of binary code, so you have to enter binary codes for various instructions. Program with native machine language is a tedious process. Moreover the programs are highly difficult to read and modify. For example, to add two numbers, you might write an instruction in binary like this:
1101101010011010
Programming Languages
Machine Language Assembly Language High-Level Language
Assembly languages were developed to make programming easy. Since the computer cannot understand assembly language, however, a program called assembler is used to convert assembly language programs into machine code. For example, to add two numbers, you might write an instruction in assembly code like this:
ADDF3 R1, R2, R3
… ADDF3 R1, R2, R3 …
Assembly Source File
Assembler
… 1101101010011010 …
Machine Code File
Programming Languages
Machine Language Assembly Language High-Level Language
The high-level languages are English-like and easy to learn and program. For example, the following is a high-level language statement that computes the area of a circle with radius 5:
area = 5 * 5 * 3.1415;
Popular High-Level Languages
COBOL (COmmon Business Oriented Language)
FORTRAN (FORmula TRANslation)
BASIC (Beginner All-purpose Symbolic Instructional Code)
Pascal (named for Blaise Pascal)
Ada (named for Ada Lovelace)
C (whose developer designed B first)
Visual Basic (Basic-like visual language developed by Microsoft)
Delphi (Pascal-like visual language developed by Borland)
C++ (an object-oriented language, based on C)
Java (We use it in this class)
Compiling Programs
Computers do not understand the languages (C++, Java, etc) that programs are written in.
Programs must first be compiled (converted) into machine code that the computer can run.
A compiler is a program that translates a programming language into machine code.
Running Programs
All programs follow a simple format:
Input Execution Output
Inputs can be from users, files, or other computer programs
Outputs can take on many forms: numbers, text, graphics, sound, or commands to other programs
Multiple Compilers
Because different operating systems (Windows, Macs, Unix) require different machine code, you must compile most programming languages separately for each platform.
program
compiler
compiler
compiler
WinMAC
Unix
Java is a little different.
Java compiler produces bytecode not machine code.
Bytecode can be run on any computer with the Java interpreter installed.
Java Program
compiler
Java Bytecode
Win
MAC
Unix
Interpreter
Interpreter
Interpreter
Java Interpreter
Advantages and Disadvantages of Java
Advantages:
Java is platform independent. Once it's compiled, you can run the bytecode on any machine with a Java interpreter. You do not have to recompile for each platform.
Java is safe. Certain common programming bugs and dangerous operations are prevented by the language and compiler.
Java standardizes many useful operations like managing network connections and providing graphical user interfaces.
Disadvantages:
Running bytecode through the interpreter is not as fast as running machine code, which is specific to that platform.
Because it is platform independent, it is difficult to use platform specific features (e.g., Windows taskbar, quick launch) in Java.
Java interpreter must be installed on the computer in order to run Java programs.
Java’s Compiler + Interpreter
Editor
Hello.java
Compiler
Hello.class
Interpreter
Hello,World!
Interpreter
Java Virtual Machine
Must Run on Any Architecture
Programin Java
JavaCompiler
JavaBytecode
Java Virtual Machine
“WRITE ONCE, RUN ANYWHERE!”pretty
portable
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
Types of Programs
Console applications GUI applications
Applets
Java on the Web: Java Applets
Clients download applets via Web browser
Browser runs applet in a Java Virtual Machine (JVM)
Interactive web, security, and client consistency
Slow to download, inconsistent VMs (besides, flash won this war)
Applet
ClientServer
Java on the Web: J2EE
Thin clients (minimize download)
Java all “server side”
ClientServer
JSPs
Servlets
EJB
JDBC
Console Applications
C:\javamethods\Ch02> path=%PATH%;C:\Program Files\Java\jdk 1.5.0_07\binC:\javamethods\Ch02> javac Greetings2.javaC:\javamethods\Ch02> java Greetings2Enter your first name: JosephineEnter your last name: JaworskiHello, Josephine JaworskiPress any key to continue...
Simple text dialog:
prompt input, prompt input ... result
GUI Applications
Menus
Buttons
Clickable panel
Slider
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.
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.
Inheritance
A programmer can define hierarchies of classes
More general classes are closer to the top
Person
Child Adult
Baby Toddler Teen
OOP Benefits
Facilitates team development
Easier to reuse software components and write reusable software
Easier GUI (Graphical User Interface) and multimedia programming