Java Tutorial

Java Tutorial
Object-Oriented Programming

Rajan Gupta
Singsys Pte. Ltd.

Different Programming Paradigms
Functional/procedural programming:
program is a list of instructions to the computer
Object-oriented programming
program is composed of a collection objects that
communicate with each other

Main Concepts
 Object

Class
Inheritance
Encapsulation

Objects
identity – unique identification of an object
attributes – data/state
services – methods/operations
− supported by the object
− within objects responsibility to provide these
services to other clients

Inheritance
Class hierarchy
Generalization and Specialization
− subclass inherits attributes and services from its
superclass
− subclass may add new attributes and services
− subclass may reuse the code in the superclass
− subclasses provide specialized behaviors (overriding
and dynamic binding)
− partially define and implement common behaviors
(abstract)

Encapsulation
 Separation between internal state of the object and its external
aspects

 How ?

− control access to members of the class
− interface “type”

Why Java ?
Portable
Easy to learn
[ Designed to be used on the Internet ]

Platform Dependent

myprog.c
C source code

gcc

myprog.exe
machine code
OS/Hardware

Platform Independent

myprog.java

javac

myprog.class
bytecode

Java source code
JVM
OS/Hardware

Primitive types
•
•
•
•
•
•
•
•

int
4 bytes
short 2 bytes
Behaviors is
long
8 bytes
exactly as
byte
1 byte
in C++
float
4 bytes
double 8 bytes
char
Unicode encoding (2 bytes)
Note:
boolean {true,false}
Primitive type
always begin
with lower-case

Wrappers
Java provides Objects which
wrap primitive types and supply
methods.
Example:
Integer n = new Integer(“4”);
int m = n.intValue();

Read more about Integer in JDK Documentation

Hello World
Hello.java

class Hello {
public static void main(String[] args) {
System.out.println(“Hello World !!!”);
}
}

C:javac Hello.java
C:java Hello

( compilation creates Hello.class )
(Execution on the local JVM)

Arrays
• Array is an object
• Array size is fixed
Animal[] arr; // nothing yet …
arr = new Animal[4]; // only array of pointers
for(int i=0 ; i < arr.length ; i++) {
arr[i] = new Animal();
// now we have a complete array

Arrays - Multidimensional
 In C++ a
Animal arr[2][2]

Is:
• In Java
Animal[][] arr=
new Animal[2][2]
What is the type of
the object here ?

Static data
Member data - Same data is used for all the
instances (objects) of some Class. Assignment performed
on the first access to
the
Class.
Only one instance of
‘x’
exists in memoryb
a

Class A {
public int y = 0;
public static int x_ =
1;
};
A a = new A();
A b = new A();
System.out.println(b.x_);
a.x_ = 5;
A.x_ = 10;

Output:
1
5
10

0
y

0
y

1
A.x_

Static
• Member function
– Static member function can access only static
members
– Static member function can be called without an
instance.
Class TeaPot {
private static int numOfTP =
0;
private Color myColor_;
public TeaPot(Color c) {
myColor_ = c;
numOfTP++;
}
public static int
howManyTeaPots()
{ return numOfTP; }
// error :
public static Color getColor()
{ return myColor_; }

}

String is an Object
• Constant strings as in C, does not exist
• The function call foo(“Hello”) creates a String
object, containing “Hello”, and passes reference
to it to foo.
• There is no point in writing :
String s = new String(“Hello”);

• The String object is a constant. It can’t be
changed using a reference to it.

Flow control
Basically, it is exactly like c/c++.
do/while
if/else

If(x==4)
{
//
act1
} else {
//
act2
}

int i=5;
do {
// act1
i--;
} while(i!=0);
for

int j;
for(int i=0;i<=9;i++)
{
j+=i;
}

switch

char
c=IN.getChar();
switch(c) {
case ‘a’:
case ‘b’:
// act1
break;
default:
// act2
}

Access Control
• public member (function/data)
– Can be called/modified from outside.

• protected
– Can be called/modified from derived classes

• private
– Can be called/modified only from the current class

• default ( if no access modifier stated )
– Usually referred to as “Friendly”.
– Can be called/modified/instantiated from the same package.

Inheritance
Base

Derived

class Base {
Base(){}
Base(int i) {}
protected void foo() {…}
}
class Derived extends Base {
Derived() {}
protected void foo() {…}
Derived(int i) {
super(i);
…
super.foo();
}
}

As opposed to C++, it is possible to inherit only from ONE class.
Pros avoids many potential problems and bugs.
Cons might cause code replication

Inheritance (2)
• In Java, all methods are virtual :
class Base {
void foo() {
System.out.println(“Base”);
}
}
class Derived extends Base {
void foo() {
System.out.println(“Derived”);
}
}
public class Test {
public static void main(String[] args) {
Base b = new Derived();
b.foo(); // Derived.foo() will be activated
}
}

Abstract
• abstract member function, means that the function
does not have an implementation.
• abstract class, is class that can not be instantiated.
AbstractTest.java:6: class AbstractTest is an abstract
class.
It can't be instantiated.
new AbstractTest();
^
1 error
NOTE:
An abstract class is not required to have an abstract method in it.
But any class that has an abstract method in it or that does
not provide an implementation for any abstract methods declared
in its superclasses must be declared as an abstract class.

Interface
Interfaces are useful for the following:
• Capturing similarities among unrelated classes
without artificially forcing a class relationship.
• Declaring methods that one or more classes are
expected to implement.
• Revealing an object's programming interface
without revealing its class.

Interface
interface IChef {
void cook(Food
food);
}
interface BabyKicker {
void kickTheBaby(Baby);
}

interface SouthParkCharacter {
void curse();
}

class Chef implements IChef, SouthParkCharacter {
// overridden methods MUST be public
// can you tell why ?
public void curse() { … }
public void cook(Food f) { … }
}
* access rights (Java forbids reducing of access rights)

Collections
• Collection/container
– object that groups multiple elements
– used to store, retrieve, manipulate, communicate
aggregate data
• Iterator - object used for traversing a collection and
selectively remove elements
• Generics – implementation is parametric in the type of
elements

Java Collection Framework
• Goal: Implement reusable data-structures and functionality
• Collection interfaces - manipulate collections
independently of representation details
• Collection implementations - reusable data structures
List<String> list = new ArrayList<String>(c);

• Algorithms - reusable functionality
– computations on objects that implement collection interfaces
– e.g., searching, sorting
– polymorphic: the same method can be used on many different
implementations of the appropriate collection interface

Collection Interface
• Basic Operations
–
–
–
–
–
–

int size();
boolean isEmpty();
boolean contains(Object element);
boolean add(E element);
boolean remove(Object element);
Iterator iterator();

• Bulk Operations
–
–
–
–
–

boolean containsAll(Collection<?> c);
boolean addAll(Collection<? extends E> c);
boolean removeAll(Collection<?> c);
boolean retainAll(Collection<?> c);
void clear();

• Array Operations

– Object[] toArray(); <T> T[] toArray(T[] a); }

Final
• final member data
Constant member
• final member function
The method can’t be
overridden.
• final class
‘Base’ is final, thus it
can’t be extended

final class Base {
final int i=5;
final void foo() {
i=10;
//what will the compiler
say about this?
}
}
class Derived extends Base
{ // Error
// another foo ...
void foo() {
}

(String class is final)

}

final
Derived.java:6: Can't subclass final classes: class Base
class class Derived extends Base {
^
1 error

final class Base {
final int i=5;
final void foo() {
i=10;
}
}
class Derived extends
Base { // Error
// another foo ...
void foo() {
}
}

Exception - What is it and why do I care?
Definition: An exception is an event that occurs during the
execution of a program that disrupts the normal flow of
instructions.

• Exception is an Object
• Exception class must be descendent of
Throwable.

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