Java introduction

Java Introduction

Prepared by: Ragab Mustafa

Agenda
• Overview of Programming Languages.
• What’s and why Java.
• What’s Eclipse .
• Writing first program in Java.
• Basics of java programming Language.
• Variables, Operators and Types.
• Methods, Conditionals and Loops.

Agenda cont.
• Classes, Arrays and objects.
• Solving a problems in Java.
• Object-oriented Programming(OOP).
• Inheritance.

Overview of Programming Languages.

• There are three types of programming
languages:
 Machine Languages that encoded in 1’s and 0’s,
that’s hard to write program in these languages.
 Low Level Languages(e.g. Assembly)it’s close to
the machine code but not the same also it’s easer
to write and read(understand).
 High Level Languages (e.g. C,C++,Java and
C#)these Languages needs a compiler to
translate the program into machine code.

What’s Java
• Java is an OOP language developed at Sun
Microsystems Labs by James Gosling at 1991.
• The first version called “OAK” but at 1995
“Netscape” announced that Java would be
incorporated into Netscape Navigator.
• Sun formally announced Java at a major
conference in May 1995.
• it is also a good not only in “object-oriented
programming “but also in “general programming
language”.

Why Java
• Java has some advantages make it differ
from the other languages as:
o That it is platform independent(mean work well in the internet). It
achieves this by using something called the ‘Java Virtual Machine’
(JVM).

o It’s a free source language.

Why Java cont.
• Also it’s
1. Simple Architecture neutral .
2. Object oriented Portable .
3. Distributed High performance .
4. Multithreaded Robust .
5. Dynamic Secure.

What’s Eclipse
• Eclipse is a portable IDE for java it’s easy and
powerful.
• There are more than one IDE for java as
NetBeans, JBuilder and JDeveloper.

Prepare your PC and yourself!
• First you must download JDK from sun or go
to here.
• Then run the IDE that you will treat with it
here we will work by Eclipse.
• To download Eclipse IDE go to here or go to
the home page of Eclipse .
• Just you download the IDE (hint: Eclipse is portable not installed)
run it that will Explained by video.

The first program in java
• Write the program:
public class FirstProgram
{
public static void main (String[] args)
{
System.out.print("Hello, 2 + 3 = ");
System.out.println(5);
System.out.println("Good Bye");
}
}

Basics of Java
• Program Structure
class CLASSNAME {
public static void main(String[] args)
{
STATEMENTS
}
}

Variables
• Named location that stores a value
• Form:
TYPE NAME;
• Example:
String fName;
class Hello {
public static void main(String[] arguments)
{
String fName = “ragab”;
System.out.println(fName);
fName = "Something else";
System.out.println(fName);
}
}

Types
• Limits a variable to kinds of values
String: plain text (“hello”)
double: Floating-point, “real” valued
number(3.14, -7.0)
int: integer (5, -18)
String fName = “hello”;
double Pi = 3.14;

Types
• Order of Operations :Precedence like math, left
to right.
Right hand side of = evaluated first.
double x= 20
double x =3 / 2+ 1; // x= 2.0

• Conversion by casting
Int a = 2; // a = 2
double a = (double)2; // a = 2.0
double a = 2/3; // a = 0.0
double a = (double)2/3; // a = 0.6666…
int a = (int)18.7; // a=18

Types
• Conversion by method:
o Int to String:
String five = Integer.toString(5);
String five = “” + 5; // five = “5”
o String to int:
Int a=Integer.parseInt(“18”);
• Mathematical Functions:
o Math.sin(x)
o Math.cos(Math.PI / 2)
o Math.log(Math.log(x + y))

Operators
• Symbols that perform simple computations
• Assignment: =
• Addition: +
• Subtraction: -
• Multiplication: *
• Division: /

Example
class Math {
public static void main(String[] arguments)
{
int score;
score = 1 + 2 * 3;
System.out.println(score);
double copy = score;
copy = copy / 2;
System.out.println(copy);
score = (int) copy;
System.out.println(score);
}
}

Methods
• Adding Methods
public static void NAME() { STATEMENTS }
• To call a method:
NAME();
• Parameters
public static void NAME(TYPE NAME) { STATEMENTS }
• To call:
NAME(EXPRESSION);

Methods
• Example:
class Square {
Public static void printSquare(int x){
System.out.println(x*x);}
public static void main(String[] arguments){
int value = 2;
printSquare(value);
printSquare(3);
printSquare(value*22);
}
}

Methods
• Multiple Parameters
*…+ NAME(TYPE NAME, TYPE NAME) {STATEMENTS }
NAME(arg1, arg2);
• Return Values
public static TYPE NAME() {
STATEMENTS
return EXPRESSION;
}
void means “no type”

Conditionals
• if statement
if (COMPARISON) {STATEMENTS }
• Example:
class If {
public static void test((int x)) {
if (x > 5){
System.out.println((x + " is > 5");
}
}
test(6);
test(5);
test(4);
}
}

Conditionals
• Comparison operators
x > y: x is greater than y
x< y: x is less than y
x >= y: x is greater than or equal to y
x <= y: x is less than or equal to y
x == y: x equals y (assignment: =)

Conditionals
• else
if (COMPARISON) {STATEMENTS
} else {
STATEMENTS
}
• else if
if (COMPARISON) {STATEMENTS
} else if (COMPARISON) { STATEMENTS
} else if (COMPARISON) { STATEMENTS
}
else {
STATEMENTS
}

Conditionals
• Example
public static void test(int x){
if (x > 5){
System.out.println(x + " is > 5");}
else if (x == 5){
System.out.println(x + " equals 5");}
else {
System.out.println(x +”is < 5"); -
}
test(6);
test(5);
test(4);
}

Loops
static void main (String[] arguments) {
System.out.println(“This is line 1”);
}

• What if you want to do it for 200 lines?
• Loop operators allow to loop through a block
of code.
• There are several loop operators in Java.

Loops
• The while operator:
while (condition) {statement}
• Example:
int i = 0;
while (i < 3) {
System.out.println(“This is line “ + i);
i = i+1;}
• Count carefully
• Make sure that your loop has a chance to finish.

Loops
• The for operator:
for (initialization;condition;update){statement}
• Example:
int i;
for (i = 0; i < 3; i=i++) {
System.out.println (“This is line “ + i);}
• Embedded loops:
for (int i = 0; i < 3; i++) {
for (int j = 2; j < 4; j++){
System.out.println (i + “ “ + j);
} }
• Scope of the variable defined in the initialization:
respective for block

Loops
• Branching Statements :
• break terminates a for or while loop
for (int i=0; i<100; i++) {if(i ==
terminationValue)
break;
else {...}
}
• continue skips the current iteration of a for or while loop
and proceeds directly to the next iteration
for (int i=0; i<100; i++) {
if(i == skipValue)
continue;
else {...}
}

Arrays
• An array is an indexed list of values.
• You can make an array of int, of double, of
String,etc.All elements of an array must have
the same type.
0 1 2 3 n-1

• This array contain n elements.

Arrays
• An array is noted using [] and is declared in the
usual way:
int values[]; // empty array
int[] values; // equivalent declaration
• To create an array of a given size, use the
operator new :
int values[] = new int[5];
• or you may use a variable to specify the size:
int size = 12;
int values[] = new int[size];

Arrays
• Array Initialization:
• Curly braces can be used to initialize an array in
the array declaration statement (and only there).
int values[] = { 12, 24, -23, 47 };

• To access the elements of an array, use the []
operator: values[index]
• Example:
int values[] = { 12, 24, -23, 47 };
values[3] = 18; // write
int x = values[1] + 3; // read

Arrays
• Each array has a length variable built-in that contains the length of the array.
int size = values.length; // 12
• Looping through an array
• Example :
for (int i=0; i<values.length; i++) {
values[i] = i;
int y = values[i] *values[i];
System.out.println(y);
}
• Another one:
double values[] = new double[25];
int j=0;
while (j<values.length) {...
j++;
}

Objects
• Objects are a collection of related data and
methods.
• Example:
• Strings A string is a collection of characters
(letters) and has a set of methods built in.
String nextTrip = “Mexico”;
int size = nextTrip.length(); // 6

Objects
• To create a new object, use the new operator.
If you do not use new, you are making a
reference to an object (i.e. a pointer to the
same object).
Point p;
p.x = 23;
p.y = -12;
public static Point middlePoint (Point p1, Point p2) {
Point q = new Point ((p1.x+p2.x)/2, (p1.y+p2.y)/2);
return q; }

Classes
• A class is a prototype to design objects.
• It is a set of variables and methods that
encapsulates the properties of the class of
objects.
• In Java, you can (will) create several classes in
project.
• A class constructor is called each time an
object of the class is instantiated (created).

Packages
• A package is a set of classes that relate to a
same purpose.
• Example: math, graphics, input/output...
• In order to use a package, you have to import
it. package

class class class

object

Object Oriented Programming
• Objects are a collection of related data and
methods.
• To create a new object, use the new operator.
If you do not use new, you are making a
reference to an object (i.e a pointer to the
same object).

Objects and references
Point p1 = new Point (12,34);
Point p2 = p1;
System.out.println(“x = “ + p2.x); // 12
p1.x = 24;
System.out.println(“x = “ + p2.x); // 24!

The null object
• null simply represents no object. It is
convenient
• as a return value to mean that a method failed
for example. It may also be used to “remove”
an element from an array.
String values[] = { “ragab”, “ahmed”, “zidan” };
values[1] = null;

Object methods v.s. class methods
class Bicycle {
static int gear, speed;
public static void speedUp (Bicycle b) {
b.speed += 10;}
public static void main (String[] arguments) {
Bicycle bike1 = new Bicycle();
speedUp (bike1); }}

• static means that the variable is going to be same
for all objects of the class.
• Class methods are declared static.
• static = the same for all objects
• nonstatic = different for each object

Abstraction
• Objects are tools for abstraction.
• Abstraction is a fundamental concept in
computer science.
• In Java, objects are instances of a class.
• A class contains variables and methods that
capture the essence of the object.
• An object is instantiated using new

Why is abstraction important?
• Modularity (allows to subdivide a big problem
into smaller sub-problems)
• Powerful representation of real-world
problems

Inheritance
• In the world, people inherit characteristics
from their parents.
• In computer science, objects inherit variables
and methods from their parents.
• When you define a class in Java, you may
define it as a subclass of another class (its
parent).

Why inheritance and it’s rules?
• Avoid duplicate code
• Improve modularity
Inheritance rules
• The subclass inherits all variables and methods
• Use the extends keyword to indicate that one
class inherits from another
• Use the super keyword in the subclass
constructor to call the parent constructor

Example
public class Vehicle {
public int nWheels; // number of wheels
public Vehicle (int n)
{nWheels = n;}
int getNWheels ()
{return nWheels;}}
• ----------------------------------------
public class Car extends Vehicle {
public Car (int n)
{super (n);}
public void openWindow() {}
}

Example
public class Bicycle extends Vehicle {
public Bicycle (int n)
{super (n);}
public void freeStyle() {}
}
• -------------------------------------
public class World {
public static void main (String[] args) {
Bicycle bike = new Bike (2);
Car car = new Car (4);
System.out.println (“Car has “ +car.getNWheels() + “ wheels”);
System.out.println (“Bike has “ +bike.getNWheels() + “ wheels”);--

OOP: Scope & Visibility
• Scope:
– A variable cannot be used outside of
the curly braces (“,...-”) in which it is declared.
– E.g., Class-level variables (fields) are usable
everywhere in the class; loop variables are only
usable in the scope of the loop

OOP: Scope & Visibility
• Visibility:
– public fields, methods, & constructors can be “seen”
or used directly by all classes & subclasses
– protected fields, methods, & constructors can only
be seen or used from within their class or subclasses
– private fields, methods, & constructors can only be
seen or used from within their exact class (in general,
most or all of a class’s fields should be private)
– Fields, methods, and constructors with unspecified
(“default” or “package”) visibility can be seen by
some classes – more on this later

OOP: Getter & Setter Methods
• Another way to access or modify fields
• Good programming practice
• Widely-used convention in Java
• Also called accessors and mutators
• Don’t come for free, but very useful when
dealing with private or protected fields

OOP: Ecapsulation
• The Principle of Encapsulation
• Allows one to access and change the internals
of a class, without having “direct access”
• Visibility, Accessors, & Mutators are vital tools
for this OOP principle

OOP: this
• A way for an instance of class to refer to itself
– E.g.: say Engineer has a field called trafficHours and a
setter method with this signature:
– public void setTrafficHours(int trafficHours);
– How to resolve scope?
– Solution: use this.trafficHours = trafficHours;
– this does not work inside static methods

• Good programming practice
• Standard Java convention
• Does come for free, has many uses

Inheritance & Abstraction

• A class can extend another (single) class
• Subclasses inherit methods and variables from
their parents
• This allows us to use objects to form an
hierarchy of representations
• Classifications of objects and relationships
between them can now be modeled!

Inheritance & Abstraction:The Object
class
• Every class implicitly extends Object
• So, in Java, every object is an Object…
• “Class Object is the root of the class hierarchy.
Every class has Object as a superclass. All
objects, including arrays, implement the
methods of this class.” – from the Java API
• Core and backbone of OOP in Java
• Methods of note are equals(Object) and
toString() – more on these later

Inheritance & Abstraction:Overriding
& Overloading

• What if we want a subclass’s inherited method
to do something different than that of its
parent? – Override it!
• What if we want multiple constructors for a
class, or methods with the same name but
different arguments? – Overload them!

& Overloading
• Overriding: re-defining an inherited method
• E.g., Say every Manager gets a weekly bonus
of amount weeklyBonus (which would be
defined somewhere in the class)
• To reflect this, we can override the
weeklyPay() method simply by explicitly
defining it again in Manager, with the desired
changes

& Overloading

• Overloading: method or constructor with
same name and type as another, but with a
different signature (i.e., takes different
number, ordering, or types of arguments)
• Cannot have methods with same name and
arguments and different return type

Method Overriding
• Method overriding occurs when a subclass
implements a method that is already implemented in a
superclass. The method name must be the same, and
the parameter and return types of the subclass's
implementation must be subtypes of the superclass's
implementation. You cannot allow less access than the
access level of the superclass's method.
eg,
class Timer {
public Date getDate(Country c) { ... } }
class USATimer {
public Date getDate(USA usa) { ... } }

Method Overloading

• Method overloading is when two methods
share the same name but have a different
number or type of parameters.
eg,
public void print(String str) { ...
}
public void print(Date date) { ...
}

References
• Java how to program, book.
• Essential Java for scientists and Engineering,
book.
• MIT Lectures.
• Tutorial from sun.

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