Java Programming and J2ME: The Basics

Mobile Programming with J2ME
By Oluwatosin Adesanya

Lesson 01:
Basic Java Programming

Java is ...
• A Pure OOP Language
• A Hybrid Programming Language
• First Compiled and Interpreted
• Runs on a JVM (Java Virtual Machine)

Byte Code

Source Object
Code Code

Getting Started

• JDK (Java Development Kit)
• Editor (Notepad) OR ...
• Integrated Development Environment (Netbeans, Eclipse,
JCreator etc.)

Setting Up Java

• Install the JDK
• The JDK Sits in C:Program FilesJavajdk1.6.0
• Set the PATH Environmental Variable (How?)

Setting the PATH Environmental Variable

Open your Systems Property: choose Advanced System Settings, Click
the Environmental Variable Button on the Dialog that shows Up


Scroll and Select Path , Click Edit Button


Append the current path with the path to the Java bin folder

Running Java

• Save your Java source code as a .java file
• Use the javac Command to compile your source codes (.java files)
e.g javac Result.java
• A .class file is generated called a bytecode
• Use java Command to run the .class file e.g java Result
• Do not add .class when compiling

Source Object
javac Byte Code java
Code Code

The Structure of a Java Program

• Java Programs are made up of classes
• Classes are made up of methods
• Java existing (ready-made) classes are found in the Java Class
Libraries (Also Called Java APIs)
• Execution of Java programs begin at the main method

A Simple Java Program

public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World!");
}
}

Classes and Objects

• A Class is like a factory from which Objects are churned Out.

Example
Class Car could produce objects Volkswagen bettles, Toyota Camri,
Peugeout 306

An Object is a model of the Real World and has
States (Or Properties) and
Behaviours (Or Actions)

Example
Object Car States are Color, Speed, Size, Plate Licence No, Cost
Behaviours are Start, Stop, Accelerate, Fuel, Open Door, etc.

Creating a Class

Use the class Keyword

Example
public class HelloWorld {
.
.
.
}

Do NOT Forget!
Name your java file ClassName.java

Adding States (Properties)

• States in Java are simply variables (Otherwise called fields)

Example

public class PaySlip {
int numdone;
String name;
}

Observe?!
Every Line of Java code ends with a semicolon (;)

int,float, double,
boolean, char … are Java Primitive Types

Adding Behaviours (Methods)

• Methods are functions that control states or fields or variables. They
change the state of an object.

Example

int numdone;
String name;

public double getPay() {
return 40.00 * numdone;
}
Method getPay()
}

Guess What? We already have a working Java Class!

Using Our Class PaySlip

We implement another tester class which
contains the main method.

public class TestPaySlip {
public static void main(String[] args){
PaySlip opay=new PaySlip();
System.out.println("n"+opay.getPay());
}
}

Constructors

• Initializes the Object after memory allocation
• Takes the same name as the Class
• May accept parameters OR... may not
• Every Class has a default constructor that accepts no parameter
• Has No Return Type
• Can be Overloaded (there can be multiple Constructors)

Constructors
int numdone;
String name;

public PaySlip(int numdone, String name) {
this.numdone=numdone; this.name=name;
}

public PaySlip(){
name=“Nobody”;
numdone=0;
}

public double getPay() {
return 40.00 * numdone;
}
}

Sorry...

• Confused?
• Questions?

Lesson 02:
Java 2 Micro Edition(J2ME)

J2ME is ...

• Java for small devices
• Divided into Configurations, Profiles and Optional APIs

Configurations, Profiles and Optional APIs combined together make up
a stack

Configurations: Specifies a JVM. We have CDC and CLDC

CLDC (Connected Limited Device Configuration) is designed for
devices with limited memory, displays, battery power and inputs.

Profiles: Layered on top of CLDC and adds APIs for specific devices.
We have MIDP, PDAP and Personal Profiles

MIDP (Mobile Information Device Profile) has characteristics that makes
most low end phones and PDAs fit in. J2ME is covered by MIDPs

MIDP Apps are ...

• Called MIDlets
• Portable
• Secured
• represented by instances of
javax.microedition.midlet.MIDlet class
• Distributed as JAR (Java Archive) files along with a MANIFEST file
and an Application Descriptor File (.jad file)
• Reduced to small sizes before distribution by an Obfuscator

Popular J2ME Apps: Opera Mini, 2go, Go Bible

Shall We Create a MIDlet?

package com.thinkit2ru;

import javax.microedition.midlet.*;

public class Metric extends MIDlet {

public void startApp() {
}

public void pauseApp() {
}

public void destroyApp(boolean unconditional) {
}
}

Remember the Constructor?
package com.thinkit2ru;

import javax.microedition.midlet.*;

public class Metric extends MIDlet {

public Metric() { The Contructor:
} Build Components Here
public void startApp() {
}

public void pauseApp() {
}

public void destroyApp(boolean unconditional) {
}
}

You need to Know...

• Primitive data types
• String Manipulation
• Control Statements
• Arrays and other data structures
• GUI Design
• Database Connectivity
• Multithreading
• Ethics and Conventions

System.out.println(“Thank You”);

Java Programming and J2ME: The Basics

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