Chapter 2

Chapter 2 Basics in Java Programming

13-04-2021 Introduction to Java and OO Concepts 2
Outline of this chapter
 Structure of java Program
 Variable types and identifiers
 Lexical Components of Java Tokens
 Constants and Data Types
 Overview of Java statements
 if statement
 switch statement
 for loop
 while, do…. while loop
 break and continue Keywords

 Java is a pure object-oriented language, and hence
everything is written within a class block. The structure
of a java program is:

 Structure of java Program
[Documentation] --------- suggested
[package statement] ------ optional
[import statements] ------ optional
[interface statements] ------ optional
[class definitions] ------ optional
[main-method class] ------ Essential

Creating, Compiling and Running a Java Program
 To create java program, you will:
•Create a source file and write in the Java program.
•Compile the source file into a bytecode file using the compiler, javac.
•Run the program contained in the bytecode file using The Java
interpreter installed on your computer.
 The file name should be the same as the name of the class containing our
main-method.
 Eg. A main-class named MyFirstClass has to be in the file called
MyFirstClass.java
 A program can contain one or more class definitions but only one public
class definition.
 This class is called main-class because it contains the main method.
 If a file contains multiple classes, the file name must be the class name of
the class that contains the main method

 Open notepad and add the code as above.
 Save the file as: MyFirstJavaProgram.java.
 Open a command prompt window and go to the directory where you saved the
class. Assume it's C:.
Programming paradigms
public class MyFirstJavaProgram
{ public static void main(String []args)
{ System.out.println("Hello World");
}
}
C : > javac MyFirstJavaProgram.java
C : > java MyFirstJavaProgram
Hello World

About Java programs, it is very important to keep in mind the following points.
 Case Sensitivity -Java is case sensitive, which means identifier Hello and hello
would have different meaning in Java.
• Class Names - For all class names the first letter should be in Upper Case.
Example: class MyFirstJavaProgram
• Method Names - All method names should start with a Lower Case letter.
Example: public void myMethodName()
• Program File Name - Name of the program file should exactly match the class
name.
Example: Assume 'MyFirstJavaProgram' is the class name. Then the file should be
saved as 'MyFirstJavaProgram.java'
public static void main(String args[]) - Java program processing starts from the
main() method which is a mandatory part of every Java program.

Lexical Components of Java Tokens
 Java program = Comments + java statements + white space
 Java statements are also combinations of java Token(s).
 Java tokens are meaningful words and symbols of java programming
language.
 Java Tokens
 Tokens are meaningful words and symbols used by a programming
language. They are the smaller individual units inside a program the
compiler recognizes when building up the program.
 There are five types of Tokens in Java:
Reserved keywords, Identifiers, Literals, operators, separators.

1. Reserved words

 All Java components require names. Names used for classes, variables, and
methods are called identifiers.
 In Java, there are several points to remember about identifiers. They are as
follows:
 All identifiers should begin with a letter (A to Z or a to z), currency
character ($) or an underscore (_).
After the first character, identifiers can have any combination of characters.
 A key word cannot be used as an identifier.
 Most importantly, identifiers are case sensitive.
 Examples of legal identifiers: age, $salary, _value, __1_value.
 Examples of illegal identifiers: 123abc, -salary, my age.
2. Identifiers

Literals are values to be stored in variables. They are a sequence of
characters (digits, letters, & other characters).
4. Operators
 Operators are a symbol that take one or more arguments (operands)
and operates on them to produce a result.
 Eg. +, *, -, /, %...
 In general, there are 8-kinds of operators:
1. Arithmetic operators 5. Conditional operators
2. Logical operators 6. Increment and decrement
3. Relational operators. 7. Bit wise operators
4. Assignment operators. 8. Special operators
3. Literals

 Separators are symbols used to indicate where groups of codes are
divided and arranged. They define the shape and function of our code.
Some of them are:
5. separators
Name Symbol
Parenthesis ( )
braces { }
brackets [ ]
semicolon ;
comma ,
period .

 Java allows putting our comments for making clarifications to our
java codes. The compiler skips comments during compiling. These
comments can be written using three ways.
Java Comments
Start End Purpose
/* */ The enclosed text is treated as a comment.
// (none) The rest of the line is treated as a comment.
/** */ The enclosed text is treated as a comment by the
compiler but is used by JavaDoc to automatically
generate documentation.

Java Statements
 Statements are roughly equivalent to sentences in natural languages.
 A statement is terminated using a semi colon.
 It forms a complete unit of execution.
 Java statements are categorized as follows:
White space
Java white spaces include: space, tab, newline.

The basic concepts of OOP

Constants
 Constants are fixed values (literals to be stored in variables) that do not
change during the execution of a program
Constants, Variables and Data Types

 Variables are identifiers that denote a storage location to store a data
values. i.e. they are names of storage locations.
Variables

 Data type means “item type to be stored in memory”.
 They are used to identify
 amount of memory size that should be assigned to a given data/item
 kinds of operations that could be made on the item.
 Based on java programming language, data types are categorized as
primitive/built-in and derived/user-defined.
Data types

 An operator performs a function on one, two, or three
operands.
 An operator that requires one operand is called a unary
operator.
 The unary operators support either prefix or postfix notation.
 Prefix notation means that the operator appears before its
operand.
 Postfix notation means that the operator appears after its
operand
operator operand; //prefix notation operand
operand operator; //postfix notation
Operators

 The character pair ?: is a conditional which is ternary operator
of Java, which is used to construct conditional expressions of
the following form:
 Expression1 ?Expression2 : Expression3
 variable x = (expression) ? value if true : value if false
Conditional operator

public class Test {
public static void main(String args[])
{
int a , b;
a = 10;
b = (a == 1) ? 20: 30;
System.out.println( "Value of b is : " + b );
b = (a == 10) ? 20: 30;
System.out.println( "Value of b is : " + b );
}
}
Example of Conditional operator
Value of b is : 30
Value of b is : 20

 There are two types of decision making statements
in Java. They are:
 if statements
 switch statements
Java selection statements

if statements
An if statement consists of a Boolean expression
followed by one or more statements.
Syntax:
The syntax of an if statement is:
if(Boolean_expression)
{ //Statements will execute if the Boolean expression is true
}

Example of if statements
public class Test {
public static void main(String args[]){
int x = 10;
if( x < 20 ){
System.out.print("This is if statement");
} }}

The if...else Statement:
 The if single-selection statement performs an indicated action only
when the condition is true; otherwise, the action is skipped.
 The if...else double-selection statement allows the programmer to
specify an action to perform when the condition is true and a different
action when the condition is false. For example, the pseudocode
statement
 Syntax:
if(Boolean_expression){
//Executes when the Boolean expression is true}
else{
//Executes when the Boolean expression is false
}

public class Test {
public static void main(String
args[]){
Float grade;
if ( grade >= 60 )
{
System.out.println( "Passed" );
}
else
{
System.out.println( "Failed" );
}}}
Example of The if...else Statement:
public class Test {
int x = 30;
if( x < 20 ){
System.out.print("This is if
statement");
}
else{
System.out.print("This is else
statement");
} }}

The if...else Statement:
A program can test multiple cases by placing if...else statements inside
other if...else statements to create nested if...else statements.

For example, the following
pseudocode represents a nested
if...else write java program
If student’s grade is greater than or
equal to 90 Print “A”
else
equal to 80
Print “B” else
equal to 70 Print “C”
else
equal to 60
Print “D” else
Print “F”
Example of if…else if…else
public class Test {
int x = 30;
if( x == 10 ){
System.out.print("Value of X is 10");
}else if( x == 20 ){
}
else if( x == 30 ){
}
else{
System.out.print("This is else
statement");
} }}
Value of X is 30

The switch Statement:
 A switch statement allows a variable to be tested for equality
against a list of values.
 Each value is called a case, and the variable being switched
on is checked for each case.
switch(expression){
case value :
//Statements
break; //optional
case value :
//Statements
break; //optional
//You can have any number of case statements.
default : //Optional
//Statements
}

public class Test {
char grade = 'C';
switch(grade) {
case 'A' :
System.out.println("Excellent!");
break;
case 'B' :
case 'C' :
System.out.println("Well done");
break; case 'D' :
System.out.println("You passed");
case 'F' :
System.out.println("Better try again");
break; default :
System.out.println("Invalid grade");
System.out.println("Your grade is " + grade);
}}}
Well doneYour
grade is a C
Example of Switch program
Write a java switch program
that compute the 5 arithmetic
operator (+,-,*,/ and %)

Java Loops/Iteration and jump statements
Java provides three repetition statements (also called looping statements)
that enable programs to perform statements repeatedly as long as a
condition (called the loop-continuation condition) remains true.
while, do...while and for
The while Loop
 A repetition statement (also called a looping statement or a loop) allows
the programmer to specify that a program should repeat an action while
some condition remains true.
 A while loop is a control structure that allows you to repeat a task a
certain number of times.
The syntax of a while loop is:
while(Boolean_expression){
//Statements}

public class Test {
public static void main(String args[]) {
int x = 10;
while( x < 20 ) {
System.out.print("value of x : " + x );
x++;
System.out.print("n");
} }}
Example of while loop
value of x : 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x : 15
value of x : 16
value of x : 17
value of x : 18
value of x : 19
Write a program that display a number start from 0 to 10 using
while loop and revers order?

 A do...while loop is similar to a while loop, except that a do...while loop
is guaranteed to execute at least one time.
 Notice that the Boolean expression appears at the end of the loop, so the
statements in the loop execute once before the Boolean is tested.
 If the Boolean expression is true, the flow of control jumps back up to do,
and the statements in the loop execute again.
 This process repeats until the Boolean expression is false.
The syntax of a do...while loop is:
do{
//Statements
++;
}
while(Boolean_expression);
The do...while Loop:

Example of do… while loop
public class Test {
int x = 10;
do{
x++; System.out.print("n");
}while( x < 20 );
}
}
do… while loop and revers order?

The for Loop:
A for loop is a repetition control structure that allows you to efficiently write
a loop that needs to execute a specific number of times.
The syntax of a for loop is:
for(initialization; Boolean_expression;update){
//Statements
}
Here is the flow of control in a for loop:
 The initialization step is executed first, and only once. This step allows you to declare
and initialize any loop control variables. You are not required to put a statement here, as
long as a semicolon appears.
 Next, the Boolean expression is evaluated. If it is true, the body of the loop is executed.
If it is false, the body of the loop does not execute and flow of control jumps to the next
statement past the for loop.
 After the body of the for loop executes, the flow of control jumps back up to the update
statement. This statement allows you to update any loop control variables.

Example of for loop
do… while loop and revers order?
public class Test {
for(int x = 10; x < 20; x = x++) {
} }}

 The break keyword is used to stop the entire loop. The
break keyword must be used inside any loop or a switch
statement.
 The break keyword will stop the execution of the
innermost loop and start executing the next line of code
after the block.
Syntax:
The syntax of a break is a single statement inside any loop:
break;
The break Keyword:

public class Test {
int [] numbers = {10, 20, 30, 40, 50};
for(int x : numbers ) {
if( x == 30 ) {
break;
}
System.out.print( x );
} }}
This would produce the following result:
10
20

The continue keyword can be used in any of the loop control structures.
It causes the loop to immediately jump to the next iteration of the loop.
In a for loop, the continue keyword causes flow of control to immediately
jump to the update statement.
In a while loop or do/while loop, flow of control immediately jumps to
the Boolean expression.
Syntax:
The syntax of a continue is a single statement inside any loop:
continue;
The continue Keyword:

Example:
public class Test {
int [] numbers = {10, 20, 30, 40, 50};
for(int x : numbers ) {
if( x == 30 ) {
continue;
}
System.out.print( x );
}
}
}
The continue Keyword:

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