The document provides an introduction to object-oriented programming concepts in Java including classes, methods, and variables. It explains how to write a simple "Hello World" program in Java with a main method and how to declare and initialize variables of different data types. It also covers basic input/output using print statements and the Scanner class.

1. Java in two semesters by Quentin Chratan and Aaron Kans

2.  public class Hello
 Object Oriented languages require that program is to be
written in separate units called classes
 Telling the compiler that we are writing a class named
as Hello
 Public indicates that we are placing this class accessible
to every one
 A public class should always be saved in file with the
same name as of the class
 So what will be name of our program??
 Hello.java
 Our program will interact with multiple built in java
libraries classes

3. {}
 Everything in class has to be contained between two curly
brackets (Braces) to indicate the beginning and end of class
 Java is case sensitive
 Upper case letters and lower case letters are different
 public static void main(String[] args)
 Every program will have at least one class with this line
 This is method
 Main is a special method, this is where the program begins
 A program starts with the first instruction of main and then
obeys each instruction in sequence.
 The program terminates when it has finished obeying the final
instruction of main
 { } indicates the body of main and marks its starting and
ending

4.  System.out.println(“Hello World”);
 Display “Hello World”
 println is short for “print line”
;
 Java instruction has to end with a semi colon.

5.  Although you can choose any name such as x, its best to pick a
name that describe the purpose of data item
 E.g.
 A computer game might need a piece of data to record the
player’s score as secret key.
 What should be the name of variable to store such data
 Score
 What should be the data type of such variable (byte, short, int
and long)
 int
 You can choose any name for variable as long as
 The name is not already a word in java language (class, void,
public)
 Name has no spaces in it
 Name does not include operators or mathematical symbols such
as + or –
 Name starts with either a letter, an underscore (_) or a dollar sign
($)
 You can use any letter as starting letter but java convention is to
begin the name with lower case letter

6. Computer Memory Java Instruction
int score; hits;
score,
char level;

7.  Allows values to be put into variables
 Written with equality symbol (=)
 Assignment Operator
 variableName = value;
 score =0;
 Set value of score to zero OR score becomes equal to zero
 Puts the number zero into memory we called score
 Combine variable statement with a variable declaration
int score = 0; int score;
score =0;
 What will be the effect of following statement in java?
 int score = 2.5
 This statement will not compile as 2.5 is a real number

8.  double something =1000;
 Though 1000 is an integer but this statement is legal as
will not result in information loss. It will be considered
as 1000.0
 Character Assignment
 char level = ‘A’;
 Enclosed he character in single quotes
 Assignment at the time of declaration
 level =‘B’
 Assignments changed

9.  Data items in program have values that do not change
 Following are examples of such items
 Maximum score in a exam
 Number of hours in a day
 Mathematical value of  (3.14176)
 In these cases the values do not vary
 Values remain constant
 Such items should be named and declared as
constants
 Add the keyword “final” before declaration
 final int HOURS = 24
 The statement HOURS = 12 will not compile now

10. Operation Java Operator
Addition +
Subtraction -
Multiplication *
Division /
Remainder %

11. int x;
x =10 + 25;
double cost;
cost = 500*(1+17.5/100);
int x;
x=30/4;
What Will be the answer? 7 or 7.5?
The answer is 7 as division operator is overloaded
double price, tax, cost;
price = 500;
tax =17.5;
cost =price+(1+tax/100);
price =price+(1+tax/100);

12.  X = X + 1;
 X++;
 X = X – 1;
 X--;
 int X = 5;
 int Y = 0;
Postfix Prefix
Y=X--;
Y=X++; Y=--X;
Y=++X;
Y value will be 5 Y value will be 6
4
X value will be 6
4 X value will be 6
4
•Y = Y + X;
•Y + = X;

15.  System.out.println(“Hello World”);
 System.out.println(“Welcome to java world”);
Hello World
Welcome to java world
 System.out.print(“Hello World”);
 System.out.println(“Welcome to java world”);
Hello WorldWelcome to java world
 System.out.println();
Blank line in the program

16.  Strings
 Collection of characters
 Always enclosed in speech marks “ “
 Print statements print strings
 Several strings can be combined using + operator
 Concatenation Operator (+)
 System.out.println(“Hello” + “World”);
 HelloWorld
 Spaces included in speech marks are printed
 System.out.println(“Hello ” + “World”);
 Hello World
 System.out.println(10*10);
 The instruction prints 100 on screen.
 Java converts value/expression to a string before displaying it
 As these numbers are converted into string so they can be
combined
 System.out.println(“Cost = ”+(10*10));
 Cost = 100

17.  Complex data type
 Name
 Address
 Car Registration Number
 Any meaningless sequence of characters
 Declare it in the same was as declare variables
 String Name;
 Name = “Saira”;
OR
 String Name = “Saira”;

18.  Part of Java release 5.0 and later
 Class that makes it easy for us to write a program
that obtains information that is typed in at the
keyboard
 Scanner is part of Java package called util
 A package is a collection of pre-compiled classes
 To make Scanner class accessible to compiler, we
have to tell the compiler that it should look in util
package
 import java.util.*;
 * means all classes in util package are made available
 import java.util.Scanner;
 Only Scanner class is accessible

19.  Create Object
 Scanner sc = new Scanner(System.in);
 System.in represent KeyBoard
 new instantiates a class by allocating memory for a new object
and returning a reference to that memory.
 Integer Input
 int x;
 x=sc.nextInt();
 Double Input
 double x;
 x=sc.nextDouble();
 String Input
 String x;
 x=sc.next(); OR x=sc.nextLine();
 Character Input
 char x;
 x=sc.next().charAt(0);