2. Introduction to Object Oriented Programming (OOP)

2. Introduction to Object Oriented Programming (OOP)
• Introduction to Object
• Elements of an object : attribute, behavior, state
• Characteristics of OOP : abstraction, encapsulation, inheritance, polymorphism
• Messages passing

1.0 HISTORY OF JAVA

• Java was developed by a team led by James Gosling at Sun Microsystems, a company best known for its Sun workstations.
• 1991 – designed by Sun Microsystems as an internal corporate research project originally called Green then Oak. It was designed for use in embedded consumer electronic applications.
• Mid 1990‟s - Java was used to write applets.
• 1993 - World Wide Web exploded in popularity – Sun saw the potential of using Java to create Web pages – with interactive and dynamic content.
• 1998 - Java 2 was released and become popular for writing application program.
• The power of Java is not limited to Web applications, for it is a general-purpose programming language.
• It has full programming features and can be used to develop standalone applications.

1.1 CHARACTERISTICS OF JAVA

1.1.1 Java Is Simple

• No language is simple, but Java is a bit easier that the popular object-oriented programming language C++, which was the dominant software-development language before Java.
• Java is partially modeled on C++, but greatly simplified and improved.
• For instance, Java uses automatic memory allocation and garbage collection, whereas C++ requires the programmer to allocate memory and collect garbage.

1.1.2 Java Is Object-Oriented

• Computers do not understand human languages, so you need to use computer languages to communicate with them.
• There are more than 100 programming languages. Some of the most popular language are:
• COBOL (COmmon Business Oriented Language)
• FORTRAN (FORmula TRANslation)
• BASIC(Beginner All-purpose Symbolic Instructional Code)
• C(So named because its developer designed B first)
• C++ (an object-oriented language, based on C)
• All of these languages except C++ are procedural programming languages – based on paradigm of procedures.
• Object-oriented programming models the real world in terms of objects.
• Everything in the world can be modeled as object – student, car, book, account, apple, etc.
• A Java program is object-oriented because programming in Java is centered on creating objects, manipulating objects, and making objects work together.

1.1.3 Java Is Distributed

• Distributed computing involves several computers working together on a network.
• Java is designed to make distributed computing easy.
• As an example, Figure 1.1 shows three programs running on three different system; the three programs communicate with one another to perform a joint task.

Figure 1.1 Java programs can run on different systems that work together.

1.1.4 Java Is Interpreted

• Java programs are compiled into the Java Virtual Machine code called bytecode.
• The bytecode is machine-independent and can run on any machine that has a Java interpreter.

1.1.5 Java Is Robust

• Robust means reliable.
• No programming language can ensure complete reliability.
• Java puts a lot of emphasis on early checking for possible errors, because Java compilers can detect many problems that would first show up at execution time in other languages.
• Java has a runtime exception-handling feature to provide programming support for robustness.

1.1.6 Java Is Secure

• As an Internet programming language, Java is used in a networked and distributed environment.
• If you download a Java applet and run it on your computer, it will not damage your system because Java implements several security mechanisms to protect your system against harm caused by stray program.
• The security is based on the premise that nothing should be trusted.

1.1.7 Java Is Architecture-Neutral

• Java is interpreted. This feature enables Java to be architecture-neutral/platform-independent.
• With a Java Virtual Machine (JVM), you can write one program that will run on any platform.

1.1.8 Java Is Portable

• Java is architecture neutral.
• This feature enables Java programs to be portable because they can be run on any platform without being recompiled.
• Moreover, there are no platform-specific features in the Java language.
• The Java environment is portable to new hardware and operating systems. In fact, the Java compiler itself is written in Java.

1.1.9 Java’s Performance

• Java‟s performance is sometimes criticized.
• The execution of the bytecode is never as fast as it would be with a compiled language, such as C++.
• Because Java is interpreted, the bytecode is not directly executed by the system, but is run through the interpreter.
• However the speed is more than adequate for most interactive applications, where the CPU is often idle, waiting for input or for data from other sources.
• Sun recently developed the Java HotSpot Performance Engine, which includes a compiler for optimizing the frequently used code. It can be plugged into a JVM to dramatically boost its performance.

1.1.10 Java Is Multithreaded

• Multithreading is a program‟s capability to perform several tasks simultaneously.
• For example, downloading a video file while playing the video would be considered multithreading.
• Multithread programming is smoothly integrated in Java, whereas in other languages you have to call procedures specific to the operating system to enable multithreading.

1.1.11 Java is Dynamic

• Java was designed to adapt to an evolving environment.
• New code can be loaded on the fly without recompilation.
• There is no need for developers to create, and for users to install, major new software versions.
• New features can be incorporated transparently as needed.

1.2 C++ vs. JAVA

• The following table shows the differences between C++ and Java:

1.3 INTRODUCTION TO JAVA APPLICATION

• Java programs:
• Java applications – standalone programs; applications can be executed from any computer with a Java interpreter.
• Applets – special kinds of Java programs that can run directly from a Java-compatible Web browser; suitable for Web projects.
• Writing simple Java application:

//---------------------------------------------------------------------------------------
/* Program name: FirstProgram.java
Purpose: This application programs prints
Welcome to Java Programming
*/
public class FirstProgram
{
public static void main(String args[])
{
System.out.println(“Welcome to Java Programming”);
}//end of method main
}//end of class
//---------------------------------------------------------------------------------------

1.4 IDENTIFIERS

• Identifier is a name given to a variable, class or method.
• Identifiers start with a letter, underscore ( _ ), or dollar sign ($).
• Subsequent characters can be digits.
• Identifiers are case-sensitive and have no maximum length.
• Examples:
• student
• mySchool
• student_name
• subject1
• $money
• _a
• An identifier cannot be a keyword, but it can contain a keyword as part of its name. For example, thisOne is a valid identifier, but this is not because this is a Java technology keyword.

1.5 JAVA KEYWORDS

• Keywords have special meaning to the Java technology compiler.
• They identify a data type name or program construct name.
• The following table list the keywords that are used in the Java programming language:

Table


1.6 DATA TYPES

• There are two categories of data types – class/object types and primitive types.
• Primitive types are simple values, are not objects.
• Class types are used for more complex types, including all the types you declare yourself.
• Class types are used to create objects.

• The Java programming language defines eight primitive data types, which can be considered in four categories:
• Logical – boolean
• Textual – char and String
• Integral – byte, short, int, and long
• Floating point – double and float

1.7 Logical – Boolean

• The boolean data type has two literals, true and false.
• For example, the statement:
• boolean result = true;
• declares the variable result as boolean type and assigns it a value of true.

1.8 Textual – char & String

• char – represents a 16-bit Unicode character
• Must have its literal enclosed in single quotes („ ‟)
• For example:
• char letter = „a‟;
• String – is not a primitive data type; it is a class
• Has its literal enclosed in double quotes (“ ”)
• Can be used as follows:
• String greeting = “Assalamualaikum”;
• String errorMessage = “Record not found!!”;

1.9 Integral – byte, short, int, and long

• An integer literal can be assigned to an integer variable as along as it can fit into the variable.
• Integral data types have the following ranges:

Table

• For example:
• int number = 12345;

1.10 Floating point – double and float

• Floating point literals are written with a decimal point.
• By default, a floating point literal is treated as a double type value.
• For example:
• double price = 27.70;
• double mark = 85.5;

1.11 Constants

• The value of a variable may change during the execution of the program, but a constant represents permanent data that never change.
• The word final is a Java keyword which means that the constant cannot be changed.
• For example:
• final double PI = 3.14159;

1.12 Type Casting

• Casting means assigning a value of one type to a variable of another type.
• If the two types are compatible, the Java software performs the conversion automatically.
• For example, an int value can always be assigned to a long variable.
• The desired target type is placed in parenthesis and used as a prefix to the expression that must be notified.
• For example:
• double number1 = 2.7, number2 = 7.5;
• int result1 = (int)(number1 + number2); // = ?
• int result2 = (int)number1 + (int)number2; // = ?

1.13 ARRAY OF PRIMITIVES

• An array is a collection of data values.
• If your program needs to deal with 100 integers, 500 Account objects, 365 real numbers, etc., you will use an array.
• In Java, an array is an indexed collection of data values of the same type.
• Array declaration:

• The index of the first position in an array is 0.
• Sample array processing:

//-----------------------------------------------------------------------------------------
double[] rainfall = new double[12];
double annualAverage,sum = 0.0;
for (int i = 0; i < rainfall.length; i++) {
rainfall[i] = Double.parseDouble(
JOptionPane.showinputDialog("Rainfall for month " + (i+1) ) );
sum += rainfall[i];
}
annualAverage = sum / rainfall.length;
//-----------------------------------------------------------------------------------------

Array Initialization

• Like other data types, it is possible to declare and initialize an array at the same time.
• Example:

• Variable-size Declaration
• In Java, we are not limited to fixed-size array declaration.
• The following code prompts the user for the size of an array and declares an array of designated size:
• Example:

Figure

1.14 CONTROL STRUCTURE

1.14.1 Branching Statements

• The Java programming language supports the if and switch statements for two-way and multiple-way branching, respectively.
• The if, else statement:

if (boolean expression){
statement or block;
}
if (boolean expression){
statement or block;
}
else{
statement or block;
}

• The switch statement:

switch (expr1){
case constant2: statements;
break;
case constant3: statements;
break;
default: statements;
break;
}

1.14.2 Looping Statements

• Looping statements allow you to execute blocks of statements repeatedly.
• The Java programming language supports three types of loop constructs: for, while and do loops.
• The for loops:

for(init_expr; boolean testexpr; alter_expr){
statements or block;
}

• The while loops:

while (boolean){
statements or block;
}

• The do/while loops:

do{
statements or block;
} while(boolean test);

1.15 PACKAGES

• A Java package is a group of related classes and interfaces.
• It facilitates software reuse by allowing programs to import classes from other packages libraries to be shared by many users.
• For example: to use the JOptionPane class in the javax.swing package, include the import statement before class declaration:
• import javax.swing.JOptionPane;

1.16 INPUT & OUTPUT STATEMENT

1.16.1 Using Dialog Box

• Input – the showInputDialog method in javax.swing.JoptionPane package is use.
• Has one String parameter which is the message to be displayed to the user.
• Returns the String keyed in by the user and returns the null String if user cancels. Example:

String input;
input = JOptionPane.showInputDialog(“Enter a
number”);

• To do mathematical operations on the given data, the String returned by showInputDialog must be converted to the required numerical data type.
• The parse functions in various wrapper classes are used:

int num = Integer.parseInt(input);
float nom = Float.parseFloat(input);
double x = Double.parseDouble(input);

• Output – the showMessageDialog method is used.
• Has one String parameter which is the message to be displayed to the user.

String output;
output = “The answer is “ + num;
JOptionPane.showMessageDialog(null, output);

1.16.2 Using Console I/O

• Input using the Scanner class (available in Java 5).
• Create a Scanner object by passing an input stream to the constructor

Scanner scanner = new Scanner(System.in);

• Use the nextInt(), nextDouble(), nextLine(), next() methods to read data typed on the keyboard.

System.out.println(“Enter a number: “);
int num = scanner.nextInt();

• Output is done through the System.out.print or System.out.println methods:

System.out.print(“The result is ”+ total);
System.out.println(“x = ”+ x + “y = ” + y);

Reference:

• Liang, Y. Daniel, Introduction to Java Programming, 8th Edition, Pearson, 2011.
• Wu, C. Thomas, An Introduction to Object Oriented Programming with Java, 4th Edition, Mc Graw Hill, 2006.