Your First Java Application

Chapter 2

Complete Java – Software Development Guide

Apex T. G. India Pvt. Ltd

2

Program Concepts

Modern object-oriented programs

help us build models to manage the complexity found in a problem

domain.

The problem domain describes real-world objects and concepts that a

computer program is trying to solve.

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Models

A model is a simplification of a complex system. A good model:

Helps to identify the most important aspects of a problem.

Helps a programmer to focus on the problem he or she is trying to solve

instead of on the complexity of the problem’s data.

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What is a metaphor?

A metaphor is a word or phrase used in place of another word or

phrase to denote a likeness.

Computer programs use “metaphors” to represent real objects.

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Objects

Objects represent real-world things:

Ship

Rudder

Wheel

Instruments

Objects have:

properties(characteristics)

methods(behaviors)

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Classes and Objects

A class is a definition of atype:

Like a template, a class defines the characteristics and behaviors of the

type.

An object is aninstanceof a class:

Can be instantiated and manipulated

An object’s characteristics are defined by the class that was used to

create the object.

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Fields

Fields define the properties of a class.

Can be intrinsic types (int,boolean…)

Can be user-defined objects

Stateis the current value of a field in an object.

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Methods

Methods describe the capabilities of the class.

Every method must be called from another method. The only exception

ismain(),which is called by the OS.

Methods can accept parameters.

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Object Relationships

Association

A method of one object calls the method of another object.

Composition

Some objects are composed of other objects.

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Specialization

Hierarchies of classes move from a

generalized class to a more specific

class.

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Creating Programs

Syntax and Semantics

A language’s exact keywords, punctuation, and order of terms are called

its syntax.

Semantics refers to the meaning of one’s instructions; what the program

is trying to accomplish.

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Writing a Java Program

Writing a Java program requires five steps:

Analyze

Design

Write

Compile

Test

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Compiling the Code

Programs must be compiled in order for the CPU to understand and

execute the instructions.

Java byte code is run through the Java Virtual Machine (JVM), which

translates the code for the CPU.

.java files are compiled by the javac (java compiler) program into .class

files.

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Types of Java Programs

Applet

Requires an .HTML file with the<applet>tag to reference the .class file

Can also be run using the appletviewer.exe program file

Application

Run by the java.exe program file

Run without a browser interface, often using the console window

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HelloWorld.java Analysis

The keywordimportallows existing class libraries to be reused.

The keywordpublicsignifies that a class is visible to other classes.

Braces{}indicate the beginning and end of a block of code.

Braces must always be “balanced”

Align opening and closing braces

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More HelloWorld.javaAnalysis

The keywordvoidsignifies that a method does not return a value.

Identifiers are the names given to classes, fields, and methods.

Identifiers are case sensitive.

Identifiers often use camel case notation.

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