Unit 2final

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Object Oriented Programming Unit 2

Sikkim Manipal University DDE Page No. 16

Unit 2 Java Basics

Structure:

2.1 Introduction

Objectives

2.2 Keywords

2.3 Working of Java

2.4 Including Comments

2.5 Data Types in Java

Primitive Data Types

Abstract / Derived Data Types

2.6 Variables in Java

2.7 Using Classes in Java

2.8 Declaring Methods in Java

2.9 Code to Display Test Value

2.10 The main() Method

2.11 Invoking a Method in Java

2.12 Saving, Compiling and Executing Java Programs

2.13 Summary

2.14 Terminal Questions

2.15 Answers

2.1 Introduction

In the last unit, we have discussed various features of Java. In this unit, we

will discuss the basic programming structure in Java. Before that we all

know that the English language has a vocabulary a set of words that have

certain meaning. It also provides us with rules for using the vocabulary

English grammar. The Java language also provides a vocabulary and a set

of rules to use the vocabulary. The vocabulary is represented through a set

of keywords and the grammar is the syntax of the language.

This unit explains how to write object-oriented program using the Java

language syntax.Objectives:

After studying this unit, you should be able to:

list the keywords significant to Java compiler

explain the working of Java


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describe different data types in Java

explain the variables and their naming in Java use classes and coding standards in Java

discuss declaring methods in Java

use code to display test value

discuss the main methods with its rules

describe invoking a method in Java

save, compile and execute a Java program

2.2 Keywords

Keywords are special words that are of significance to the Java compiler.

There are 48 reserved keywords currently defined in the Java language (seeTable 2.1). These keywords combined with the syntax of the operators and

separators, form the definition of the Java language. These keywords

cannot be used as names for a variable, class, or method.

Table 2.1: Java Reserved Keywords

abstract boolean break byte case

catch char class const continue

default do double else extends

final finally float for goto

if implements import instanceof intinterface long native new package

private protected public return short

static strictfp super switch synchronized

this throw throws transient try

void volatile while

The keywords const and goto are reserved but not used. In addition to the

keywords, Java reserves the following: true, false, and null. These are

values defined by Java. You may not use these words for the names of

variables, classes, and so on.

2.3 Working of Java

A compiler converts the Java program into an intermediate language

representation called bytecode which is platform independent. A Java file


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will have the extension .java, similar to a word file having the extension .doc,

a Pascal file having the extension .pas and a text file having theextension .txt.

Let us assume that there exists a Java file named Hello.java. When this file

is complied we get a file called as Hello.class.

This class file is run using an interpreter as and when necessary. The

Figure 2.1 shows the java program saved as Hello.java.

Figure 2.1: A Sample Java Program

The steps for compiling and running the program are shown in Figure 2.2.

The program is stored in a subdirectory called java. When you run theabove program, it prints the message Hello! How are you?

Figure 2.2: Compiling and Executing


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The concept of write once, run anywhere is possible in Java. The Java

program can be compiled on any platform having a Java compiler. Theresulting bytecode can then be run on Window NT or Solaris or Macintosh

or any other machine. The machine should have a Java platform to run Java

code. Java platform consists of Java Virtual Machine (JVM) and a package

of ready made software components. This package is known as Java

Application Programming Interface (Java API). The compiled Java

program can run on any hardware platform having Java Virtual Machine

(JVM) installed on it.

Self Assessment Questions

1. A compiler converts the Java program into an intermediate language

representation called ____________.2. The concept of _________________ is possible in Java.

2.4 Including Comments

Comments can be included in a Java program as follows:

Type 1

/* Comments go here

More comments here

*/

Type 2

// This information is ignored by the compiler till the end of the line

Type 3

/* Documentation comment */

In the first type comments can spread over multiple lines. In the second type

the information written after // is ignored by the compiler. This is used for

writing single line comments. The third type of comment is used by a tool

calledjav ad ocfor automatic generation of documentation.

Self Assessment Questions:

3. The third type of comment is used by a tool called ___________ forautomatic generation of documentation.


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2.5 Data Types in Java

There are two kinds of data types in Java: Primitive / Standard data types.

Abstract / Derived data types.

2.5.1 Primitive Data Types

Primitive data types (also know as standard data types) are the data types

that are built into the Java language. The Java compiler holds detailed

instructions on each legal operation the data type supports. There are eight

primitive data types in Java (see Table 2.2).

Table 2.2: Primitive / Standard Data Types

Data Type Size/Format (bits)

Description Range

byte 8 Byte-length integer -128 to 128 (signed)0 to 255 (unsigned)

short 16 Short integer -215 to 215 -1

int 32 Integer -2 to 2 -1

long 64 Long integer -2 to 2 -1

float 32 Single precision floatingpoint

+/- about 10

double 64 Double precision floatingpoint

+/- about 10317

char 16 A single character Any single character

boolean 1 A boolean value true orfalse

The data types byte, short, int, long, float and double are numeric data

types. The first four of these can hold only whole numbers whereas the last

two (float and double) can hold decimal values like 5.05. All these data

types can hold negative values. However, the keyword uns ignedcan be

used to restrict the range of values to positive numbers. Amongst others,

boolean can hold only the value true or false and char can hold only a

single character.

2.5.2 Abstract / Derived Data Types

Abstract data types are based on primitive data types and have more

functionality than the primitive data types. For example, String is an

abstract data type that can store alphabets, digits and other special


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characters like /, (); :$#. You cannot perform calculations on a variable of the

String data type even if the data stored in it has digits.Self Assessment Questions

4. String is a ___________ data type in Java.

2.6 Variables in Java

When you learned algebraic equations in school, you used x and y to

represent values in equations. Unlikepiwhich has a constant value of 3.14,

the values ofxand yare not constant in equations. Java provides constants

and variables to store data in programs.

Java allocates memory to each variable and constant you use in your

program. As in algebra, the values of variables may change in a program,

but the values of constants, as the name suggests, do not change. You

must assign unique names to variables and constants. Variable names are

used in a program in much the same way as they are in ordinary algebra.

Each variable used in a program must be declared. That is to say, the

program must contain a statement specifying precisely what kind of

information (data type) the variable will contain. This applies to every

variable used in the program, regardless of the type.

Naming Variables

A program refers to a variable using its name. Certain rules and conventionsgovern the naming of variables. You must adhere to rules. Conventions help

improve the readability of the program, but following them is not mandatory.

Rules for Naming Variables in Java

A variable name:

Must not be a keyword in Java.

Must not begin with a digit.

Must not contain embedded spaces.

Can contain characters from various alphabets, like Japanese, Greek,

and Cyrillic.

Syntax for Defining Variables

All the attributes of a class are defined as data members. The syntax used

to declare a class variable is:

;


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As the brackets { } are used to mark the beginning and end of a class, a

semicolon ; is used to mark the end of a statement.Self Assessment Questions:

5. Keywords can be used as a variable name. (True or False.)

2.7 Using Classes in Java

The data members and methods of a class are defined inside the class body.

In Java, braces { } mark the beginning and end of a class or method.

Any program that you write in Java can be written using a simple text editor.

We will use Windows Notepad for writing programs.

Example:

class Employee

{

}

Standard for Coding

By following coding standards and conventions, you can write code that is

easy to read and understand.

One of the conventions is indentation. Notice that the code written below

clearly indicates the data members defined in the class.

Example:class Employee

{

String employeeName;

String employeeAddress;

}

2.8 Declaring Methods in Java

The syntax for declaring a method in Java is as follows:

([argument_list])

{

}


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access_specifier

An access specifier defines where a method can be accessed. A publicspecifier allows the method to be executed from another class. A private

provides access to methods for only the current class.

return_type

The return_type of a method is the data type of the value that is returned by

the method.

Example:

public void displayEmpName(); // returns no value, therefore, the return

// type of the method is void.

public float calculateAllowance(); // returns a value of float data type// therefore, the return type of the method

// is float


6. A ____________ access specifier allows the method to be executed

from another class.

2.9 Code to Display Test Value

The Systemclass

To communicate with the computer, a program needs to use certain systemresources such as the display device and the keyboard.

Java makes use of all these resources with the help of a class called

System, which contains all the methods that are required to work with these

resources.

System is one of the most important and useful classes provided by Java. It

provides a standard interface to common system resources like the display

device and the keyboard.

The outObject

It is an object encapsulated inside theSystem class, and represents thestandard output device. This object contains theprintln () method.

The print ln ()method

Theprintln () method displays the data on the screen.


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Example:

System.out.println (Hello World);The above statement will display Hello World on the screen.

2.10 The main()Method

In a Java application, you may have many classes. Within those classes,

you may have many methods. The method that you need to execute first

should be the main()method.

Syntax for the main()method:

public stat ic void m ain(Str ing args[])

{

}

The main () method should exist in a class that is declared as public.

Rules for the main ()method:

The primary name of the file in which the code is written, and the name

of the class that has the main() method should be exactly the same.

If you try to execute a Java application that does not have a main ()

method, the following error message will be printed:

Exception in thread main Java.lang.NoSuchMethodError: main

2.11 Invoking a Method in Java

Creating an Object of a class

The newoperator is used to create a class object.

Example:

Employee emp = new Employee ();

Invoking a Method

To invoke a method, the method name must be followed by parentheses

and a semicolon. One method of a class can invoke another method of the

same class using the name of the method.

Example:

class Employee

{

String employeeName;

String employeeAddress;


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public Employee()

{employeeName=Bala;

employeeAddress = Manipal;

}

public void display()

{

System.out.println (Name: +employeeName);

System.out.println(Address: +employeeAddress);

}

public static void main(String args[])

{

Employee emp =new Employee();

emp.display ();

}

}


7. _____________ operator is used to create an object.

2.12 Saving, Compiling and Executing a Java Programs

The programs that you write in Java should be saved in a file, which has the

following name format:

.java

Compiling

A program is a set of instructions. In order to execute a program, the

operating system needs to understand the language. The only language an

operating system understands is in terms of 0s and 1s i.e. the binary

language. Programs written in language such as C and C++ are converted

to binary code during the compilation process. However, that binary code

can be understood only by the operating system for which the program iscompiled. This makes the program or application as operating system

dependent.


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In Java, the program is compiled into bytecode (.class file) that run on the

Java Virtual Machine, which can interpret and run the program on anyoperating system. This makes Java programs platform-independent.

At the command prompt, type

javac .java

to compile the Java program.

Executing

When the code is compiled and error-free, the program can be executed

using the command:

java


8. _____________ is the extension for Java source code files.

9. _____ command is used to compile the Java source code.

10. _____ command is used to execute the Java class file.

2.13 Summary

In this unit, you have learnt the following:

Creating Classes Using Java

The data members and methods of a class are defined inside the class

body. In Java, brackets {} mark the beginning and end of a class or

method. The class keyword is used to declare a class.

Coding Methods of a Class

Methods provide functionality to classes. In Java, methods are declared

in the class body.

Declaring Objects

The new operator is used to create a class object.

Displaying Data on Screen

The Systemclass is one of the most important and useful classes

provided by Java. It provides a standard interface to common system

resources like the display device and the keyboard. The print ln()

method displays the data on the screen.


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Compiling a Java Program

In Java, the program is compiled into bytecode (.class file) that runs onthe Java Virtual Machine, which can interpret and run the program on

any operating system. This makes Java programs platform-independent.

Executing a Java Program

When the code is compiled and error-free, the program can be executed

by issuing the following command:

java

2.14 Terminal Questions

1. The candidate class for the employee referral process has the following

attributes and behaviors:Candidate

candidateNamecandidateAddresscandidateCourse

displayDetails()

Create the candidate class with its attribute and methods.

2. What do you mean by the statement :

3. System.out.println( );

4. How do you compile a Java program?5. How do you execute a Java program?

2.15 Answers


1. bytecode

2. write once, run anywhere

3. javadoc

4. abstract / derived

5. false

6. public

7. new

8. java

9. Javac

10. java


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Terminal Questions

1. class Candidate{

String candidateName;

String candidateAddress;

String candidateCourse;

public Candidate()

{

candidateName=Arun;

candidateAddress=Manipal;

candidateCourse=BScIT;

}

public void displayDetails()

{

System.out.println(Name: +candidateName);

System.out.println(Address: +candidateAddress);

System.out.println(Course: +candidateCourse);

}

} (Refer section 2.11)

2. System.out.println( ); will print a blank line. (Refer section 2.9 fordetail.)

3. Java programs are compiled using the command javac.

(Refer section 2.12 for detail.)

4. Java class files are executed using the command java.

(Refer section 2.12 for detail.)

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