Object Oriented Systems

Lecture 01First Java Programming

Jaeki Song

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Objectives

• Learn about programming

• Be introduced to object-oriented programming concepts

• Learn about Java

• Analyze a Java application that uses console output

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What is a program?

• A computer program is a set of instructions that enable the computer to solve a problem or perform a task• Display a message on the screen: “How many

hours did you work?”

• A computer’s CPU can only process instructions that are written in machine language• A stream of binary numbers

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What is a Program made of?

• Language elements• Keywords (reserved words)

• There are words that have a special meaning in the programming language

• E.g.: public, class, static, etc

• Operators• Symbols or words that perform operation son one or more

operands• E.g.: “=“, “*”, etc

• Punctuation• Most programming languages require the use of punctuation

characters such as the beginning or ending of a statement• E.g.: semicolon in Java – similar to a period in English

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What is a Program made of?

• Programmer-defined names• Unlike key words, these are words or names

that are defined by the programmer• E.g.: names of variables

• Syntax• Rules that must be followed when writing a

program

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Programming Approaches

• Procedural programming• Divide a problem into smaller sub-problems

• Each sub-problem is analyzed and a solution for the sub-problem is obtained

• Also known as top-down design, stepwise refinement and modular programming

• Object-oriented programming• Object-oriented Design (OOD)

• Identify components called object• Specify the relevant data for each object and possible operations to

be performed on that data• In OOD, the final program is a collection of interacting objects• A programming language that implements OOD is called an OOP

language

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Introduction to Object Concepts

• Object-oriented programming differs from traditional procedural programming• Basic concepts

• Objects • Classes• Inheritance• Polymorphism

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What is Java?

• Designed in the early of 1990s by Sun Microsystems• Code name “Green”

• Used in consumer devices such as an intelligent television “set-up” boxes

• The language was designed to be simple and architecture-neutral, so that it could be executed on a variety of hardware

• Rewrite the program: architecture-neutral, real-time, reliable, and secure• Applets, now called Java provides animation and

interactivity on the World Wide Web• Web browsers have provided the opportunities to run

Java applets

• The fastest growing language

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Java Language

• Standard language used for programming, creating applets, servlets, JavaBeans, and enterprise components

• Java is simple

• Java is object-oriented language

• Java is distributed

• Java is portable

• Java is multithreaded

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Java Environment

Java source code

Java compiler

Java interpreter

Computer OS

Java virtual machine

Source code is stored on a diskIn a file with a name ending in

.java

Compiler creates byte codes that are stored on a disk a file with a

name ending in .class

JVM (named java.exe) performssecurity chekcs and translates byte codes to machine language, which executes

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Java Program Types

• Applets• Programs embedded in Web page

• Java applications• Called Java stand-alone programs• Console applications

• Support character output

• Windowed applications• Menus• Toolbars• Dialog boxes

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First Java Application• Eclipse

• Tutorial

public class First {

public static void main (String [ ] args) { System.out.println (“First Java Application”) }}

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Understanding First Class

• Literal string• Will appear in output exactly as entered• Written between double quotation marks

• Arguments• Pieces of information passed to method

• Method • Requires information to perform its task

• Define Java class using any name or identifier • Requirements for identifiers

• Must begin with:• Letter of English alphabet• Or non-English letter (such as α or π)

• Cannot begin with digit

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Understanding First Class (continued)

• Requirements for identifiers• Can only contain:

• Letters• Digits• Underscores• Dollar signs

• Cannot be Java reserved keyword (p. 10)• Cannot be true, false, or null

• Access modifier• Defines how class can be accessed

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Java Naming Conventions

• Packages• The prefix of a unique package name is always

written in all-lowercase ASCII letters and should be one of the top-level domain names

• Use dots to separate the parts• E.g.: com.sun.eng, com.objectcentral.javatools

• Classes• Class (and interface) names should be nouns

descriptive of the purpose of the class• Names are in mixed case, beginning with a capital

and with the first letter of each internal word capitalized

• Use complete words and avoid abbreviations• E.g.: Point, Shape, MovieEditor, ClientList

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Java Naming Conventions (continued)

• Methods• Methods should be verbs descriptive of the purpose of the method• Names are mixed case with the first letter lowercase and the first letter

of each internal word capitalized • There are prefix conventions for general types of methods, such as

using get and set for getters and setters• E.g.: getOrigin, findSmallest, drawGraph, saveMoney

• Variables• Except when used as constants, all variables are named using mixed

case with a lowercase first letter, and with intenral words starting with capital letters

• Use one-letter variable names only for temporary variables• E.g.: myMovie, editedMovie, backgroundColor

• Constants• Names should be all uppercase with words seperated by underscores

(“_”)• e.g.: MAX_SIZE, TERM_LIMIT

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main( ) Method

•Static• Reserved keyword • Means method accessible and usable

• Even though no objects of class exist

•void • use in main() method header• Does not indicate main() method empty• Indicates main() method does not return

value when called• Doesn’t mean main() doesn’t produce output

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Adding Comments to a Java Class

• Types of Java comments• Line comments

• Start with two forward slashes (//) • Continue to end of current line • Do not require ending symbol

• Block comments • Start with forward slash and asterisk (/*)• End with asterisk and forward slash (*/)

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Programming Errors

• Syntax error• Result from errors in cod construction

• E.g.: mistyping, omitting some necessary punctuation, using an opening brace without a corresponding closing brace

• Logical error• Occur when a program does not perform the way it

was intended to

• Run-time error• Cause a program to terminate abnormally

• E.g.• Input error: the user enters an unexpected input value that the program cannot

handle• Division by zero

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Using Java Swing Class

• Refers to the new library of GUI• A component set that makes up all the objects

of GUI• Displays output using windows or dialog boxes

• Input Dialog and Output Dialog

• Use packages• Predefined classes grouped into categories of

related classes called packages (sometimes called java class libraries or java applications programming interface (API))

• JOptionPane• Defined in a package called javax.swing

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GUI Output

•JOptionPane• Produce dialog boxes

• Dialog box • GUI object resembling window• Messages placed for display

• Package• Group of classes

•import statement• Use to access built-in Java class

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Output Dialog

• showMessageDialog ( null, “string”);• A method of class JOptionPane• Two arguments

• Syntax

JOptionPane.showMessageDialog(null, “string”);

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Common Errors to Avoid

• Mismatched braces, quotation marks, or parentheses

• Misspelling key words• Using capital letters in key words

• Java is a case-sensitive• All key words are written in lower case in Java

• Using a key word as a variable name• Using inconsistent spelling • Forgetting the semicolon at the end of a

statement• Not using the required import statement