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Java Tutorials
Woxa Technologies pvt ltd
8471003400
woxatechnologies@gmail.com
Java Tutorial
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
Class
• “type”
• object is an instance of class
• class groups similar objects
▫ same (structure of) attributes
▫ same services
• object holds values of its class’s attributes
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”
What does it buy us ?
• Modularity
▫ source code for an object can be written and
maintained independently of the source code for other
objects
▫ easier maintainance and reuse
• Information hiding
▫ other objects can ignore implementation details
▫ security (object has control over its internal state)
• but
▫ shared data need special design patterns (e.g., DB)
▫ performance overhead
mainly for c++ programmer
Why Java ?
• Portable
• Easy to learn
• [ Designed to be used on the Internet ]
JVM
• JVM stands for
Java Virtual Machine
• Unlike other languages, Java “executables”
are executed on a CPU that does not exist.
OS/Hardware
machine code
C source code
myprog.c
gcc
myprog.exe
Platform Dependent
JVM
bytecode
Java source code
myprog.java
javac
myprog.class
OS/Hardware
Platform Independent
Primitive types
• int 4 bytes
• short 2 bytes
• long 8 bytes
• byte 1 byte
• float 4 bytes
• double 8 bytes
• char Unicode encoding (2 bytes)
• boolean {true,false}
Behaviors is
exactly as in
C++
Note:
Primitive type
always begin
with lower-case
• Constants
37 integer
37.2 float
42F float
0754 integer (octal)
0xfe integer (hexadecimal)
Primitive types - cont.
Wrappers
Java provides Objects which wrap
primitive types and supply methods.
Example:
Integer n = new Integer(“4”);
int m = n.intValue();
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++
Animal arr[2][2]
Is:
• In Java
What is the type of
the object here ?
Animal[][] arr=
new Animal[2][2]
Static - [1/4]
• Member data - Same data is used for all the
instances (objects) of some Class.
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;
System.out.println(b.x_);
A.x_ = 10;
System.out.println(b.x_);
Assignment performed
on the first access to the
Class.
Only one instance of ‘x’
exists in memory
Output:
1
5
10
a b
y y
A.x_
0 0
1
Static - [2/4]
• 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_; }
}
Static - [2/4] cont.
Usage:
TeaPot tp1 = new TeaPot(Color.RED);
TeaPot tp2 = new TeaPot(Color.GREEN);
System.out.println(“We have “ +
TeaPot.howManyTeaPots()+ “Tea Pots”);
Static - [3/4]
• Block
▫ Code that is executed in the first reference to the
class.
▫ Several static blocks can exist in the same class
( Execution order is by the appearance order in the
class definition ).
▫ Only static members can be accessed.
class RandomGenerator {
private static int seed_;
static {
int t = System.getTime() % 100;
seed_ = System.getTime();
while(t-- > 0)
seed_ = getNextNumber(seed_);
}
}
}
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 :
• The String object is a constant. It can’t be changed using
a reference to it.
String s = new String(“Hello”);
Packages
• Java code has hierarchical structure.
• The environment variable CLASSPATH
contains the directory names of the roots.
• Every Object belongs to a package ( ‘package’
keyword)
• Object full name contains the name full name
of the package containing it.
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
Polymorphism
• Inheritance creates an “is a” relation:
For example, if B inherits from A, than we say
that “B is also an A”.
Implications are:
▫ access rights (Java forbids reducing access rights) -
derived class can receive all the messages that the
base class can.
▫ behavior
▫ precondition and postcondition
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
}
}
Inheritance (3) - Optional
class classC extends classB {
classC(int arg1, int arg2){
this(arg1);
System.out.println("In classC(int arg1, int arg2)");
}
classC(int arg1){
super(arg1);
System.out.println("In classC(int arg1)");
}
}
class classB extends classA {
classB(int arg1){
super(arg1);
System.out.println("In classB(int arg1)");
}
classB(){
System.out.println("In classB()");
}
}
Inheritance (3) - Optional
class classA {
classA(int arg1){
System.out.println("In classA(int arg1)");
}
classA(){
System.out.println("In classA()");
}
}
class classB extends classA {
classB(int arg1, int arg2){
this(arg1);
System.out.println("In classB(int arg1, int arg2)");
}
classB(int arg1){
super(arg1);
System.out.println("In classB(int arg1)");
}
class B() {
System.out.println("In classB()");
}
}
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
• abstract “class”
• Helps defining a “usage contract” between
classes
• All methods are public
• Java’s compensation for removing the
multiple inheritance. You can “inherit” as
many interfaces as you want.
Example*
- The correct term is “to implement”
an interface
Interface
interface SouthParkCharacter {
void curse();
}
interface IChef {
void cook(Food food);
}
interface BabyKicker {
void kickTheBaby(Baby);
}
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)
When to use an interface ?
Perfect tool for encapsulating the
classes inner structure. Only the
interface will be exposed
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 Interfaces
Collection
Set List Queue
SortedSet
Map
Sorted Map
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); }
General Purpose Implementations
Collection
Set List Queue
SortedSet
Map
Sorted Map
HashSet HashMap
List<String> list1 = new ArrayList<String>(c);
ArrayListTreeSet TreeMapLinkedList
List<String> list2 = new LinkedList<String>(c);
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() {
}
}
IO - Introduction
• Definition
▫ Stream is a flow of data
 characters read from a file
 bytes written to the network
 …
• Philosophy
▫ All streams in the world are basically the same.
▫ Streams can be divided (as the name “IO” suggests) to Input
and Output streams.
• Implementation
▫ Incoming flow of data (characters) implements “Reader” (InputStream
for bytes)
▫ Outgoing flow of data (characters) implements “Writer”
(OutputStream for bytes –eg. Images, sounds etc.)
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.
Exception - What is it and why do I care?(2)
By using exceptions to manage errors, Java
programs have the following advantages over
traditional error management techniques:
1: Separating Error Handling Code from "Regular"
Code
2: Propagating Errors Up the Call Stack
3: Grouping Error Types and Error Differentiation
1: Separating Error Handling Code from "Regular" Code (1)
readFile {
open the file;
determine its size;
allocate that much memory;
read the file into memory;
close the file;
}
1: Separating Error Handling Code from "Regular" Code (2)
errorCodeType readFile {
initialize errorCode = 0;
open the file;
if (theFileIsOpen) {
determine the length of the file;
if (gotTheFileLength) {
allocate that much memory;
if (gotEnoughMemory) {
read the file into memory;
if (readFailed) {
errorCode = -1;
}
} else {
errorCode = -2;
}
} else {
errorCode = -3;
}
close the file;
if (theFileDidntClose && errorCode == 0) {
errorCode = -4;
} else {
errorCode = errorCode and -4;
}
} else {
errorCode = -5;
}
return errorCode;
}
1: Separating Error Handling Code from "Regular" Code (3)
readFile {
try {
open the file;
determine its size;
allocate that much memory;
read the file into memory;
close the file;
} catch (fileOpenFailed) {
doSomething;
} catch (sizeDeterminationFailed) {
doSomething;
} catch (memoryAllocationFailed) {
doSomething;
} catch (readFailed) {
doSomething;
} catch (fileCloseFailed) {
doSomething;
}
}
2: Propagating Errors Up the Call Stack
method1 {
try {
call method2;
} catch (exception) {
doErrorProcessing;
}
}
method2 throws exception {
call method3;
}
method3 throws exception {
call readFile;
}

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Java Tutorials

  • 1. Java Tutorials Woxa Technologies pvt ltd 8471003400 [email protected] Java Tutorial
  • 2. 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
  • 3. Main Concepts • Object • Class • Inheritance • Encapsulation
  • 4. 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
  • 5. Class • “type” • object is an instance of class • class groups similar objects ▫ same (structure of) attributes ▫ same services • object holds values of its class’s attributes
  • 6. 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)
  • 7. Encapsulation • Separation between internal state of the object and its external aspects • How ? ▫ control access to members of the class ▫ interface “type”
  • 8. What does it buy us ? • Modularity ▫ source code for an object can be written and maintained independently of the source code for other objects ▫ easier maintainance and reuse • Information hiding ▫ other objects can ignore implementation details ▫ security (object has control over its internal state) • but ▫ shared data need special design patterns (e.g., DB) ▫ performance overhead
  • 9. mainly for c++ programmer
  • 10. Why Java ? • Portable • Easy to learn • [ Designed to be used on the Internet ]
  • 11. JVM • JVM stands for Java Virtual Machine • Unlike other languages, Java “executables” are executed on a CPU that does not exist.
  • 12. OS/Hardware machine code C source code myprog.c gcc myprog.exe Platform Dependent JVM bytecode Java source code myprog.java javac myprog.class OS/Hardware Platform Independent
  • 13. Primitive types • int 4 bytes • short 2 bytes • long 8 bytes • byte 1 byte • float 4 bytes • double 8 bytes • char Unicode encoding (2 bytes) • boolean {true,false} Behaviors is exactly as in C++ Note: Primitive type always begin with lower-case
  • 14. • Constants 37 integer 37.2 float 42F float 0754 integer (octal) 0xfe integer (hexadecimal) Primitive types - cont.
  • 15. Wrappers Java provides Objects which wrap primitive types and supply methods. Example: Integer n = new Integer(“4”); int m = n.intValue();
  • 16. 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
  • 17. Arrays - Multidimensional • In C++ Animal arr[2][2] Is: • In Java What is the type of the object here ? Animal[][] arr= new Animal[2][2]
  • 18. Static - [1/4] • Member data - Same data is used for all the instances (objects) of some Class. 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; System.out.println(b.x_); A.x_ = 10; System.out.println(b.x_); Assignment performed on the first access to the Class. Only one instance of ‘x’ exists in memory Output: 1 5 10 a b y y A.x_ 0 0 1
  • 19. Static - [2/4] • 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_; } }
  • 20. Static - [2/4] cont. Usage: TeaPot tp1 = new TeaPot(Color.RED); TeaPot tp2 = new TeaPot(Color.GREEN); System.out.println(“We have “ + TeaPot.howManyTeaPots()+ “Tea Pots”);
  • 21. Static - [3/4] • Block ▫ Code that is executed in the first reference to the class. ▫ Several static blocks can exist in the same class ( Execution order is by the appearance order in the class definition ). ▫ Only static members can be accessed. class RandomGenerator { private static int seed_; static { int t = System.getTime() % 100; seed_ = System.getTime(); while(t-- > 0) seed_ = getNextNumber(seed_); } } }
  • 22. 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 : • The String object is a constant. It can’t be changed using a reference to it. String s = new String(“Hello”);
  • 23. Packages • Java code has hierarchical structure. • The environment variable CLASSPATH contains the directory names of the roots. • Every Object belongs to a package ( ‘package’ keyword) • Object full name contains the name full name of the package containing it.
  • 24. 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.
  • 25. 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
  • 26. Polymorphism • Inheritance creates an “is a” relation: For example, if B inherits from A, than we say that “B is also an A”. Implications are: ▫ access rights (Java forbids reducing access rights) - derived class can receive all the messages that the base class can. ▫ behavior ▫ precondition and postcondition
  • 27. 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 } }
  • 28. Inheritance (3) - Optional class classC extends classB { classC(int arg1, int arg2){ this(arg1); System.out.println("In classC(int arg1, int arg2)"); } classC(int arg1){ super(arg1); System.out.println("In classC(int arg1)"); } } class classB extends classA { classB(int arg1){ super(arg1); System.out.println("In classB(int arg1)"); } classB(){ System.out.println("In classB()"); } }
  • 29. Inheritance (3) - Optional class classA { classA(int arg1){ System.out.println("In classA(int arg1)"); } classA(){ System.out.println("In classA()"); } } class classB extends classA { classB(int arg1, int arg2){ this(arg1); System.out.println("In classB(int arg1, int arg2)"); } classB(int arg1){ super(arg1); System.out.println("In classB(int arg1)"); } class B() { System.out.println("In classB()"); } }
  • 30. 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.
  • 31. Interface • abstract “class” • Helps defining a “usage contract” between classes • All methods are public • Java’s compensation for removing the multiple inheritance. You can “inherit” as many interfaces as you want. Example* - The correct term is “to implement” an interface
  • 32. Interface interface SouthParkCharacter { void curse(); } interface IChef { void cook(Food food); } interface BabyKicker { void kickTheBaby(Baby); } 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)
  • 33. When to use an interface ? Perfect tool for encapsulating the classes inner structure. Only the interface will be exposed
  • 34. 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
  • 35. 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
  • 36. Collection Interfaces Collection Set List Queue SortedSet Map Sorted Map
  • 37. 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); }
  • 38. General Purpose Implementations Collection Set List Queue SortedSet Map Sorted Map HashSet HashMap List<String> list1 = new ArrayList<String>(c); ArrayListTreeSet TreeMapLinkedList List<String> list2 = new LinkedList<String>(c);
  • 39. 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)
  • 40. 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() { } }
  • 41. IO - Introduction • Definition ▫ Stream is a flow of data  characters read from a file  bytes written to the network  … • Philosophy ▫ All streams in the world are basically the same. ▫ Streams can be divided (as the name “IO” suggests) to Input and Output streams. • Implementation ▫ Incoming flow of data (characters) implements “Reader” (InputStream for bytes) ▫ Outgoing flow of data (characters) implements “Writer” (OutputStream for bytes –eg. Images, sounds etc.)
  • 42. 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.
  • 43. Exception - What is it and why do I care?(2) By using exceptions to manage errors, Java programs have the following advantages over traditional error management techniques: 1: Separating Error Handling Code from "Regular" Code 2: Propagating Errors Up the Call Stack 3: Grouping Error Types and Error Differentiation
  • 44. 1: Separating Error Handling Code from "Regular" Code (1) readFile { open the file; determine its size; allocate that much memory; read the file into memory; close the file; }
  • 45. 1: Separating Error Handling Code from "Regular" Code (2) errorCodeType readFile { initialize errorCode = 0; open the file; if (theFileIsOpen) { determine the length of the file; if (gotTheFileLength) { allocate that much memory; if (gotEnoughMemory) { read the file into memory; if (readFailed) { errorCode = -1; } } else { errorCode = -2; } } else { errorCode = -3; } close the file; if (theFileDidntClose && errorCode == 0) { errorCode = -4; } else { errorCode = errorCode and -4; } } else { errorCode = -5; } return errorCode; }
  • 46. 1: Separating Error Handling Code from "Regular" Code (3) readFile { try { open the file; determine its size; allocate that much memory; read the file into memory; close the file; } catch (fileOpenFailed) { doSomething; } catch (sizeDeterminationFailed) { doSomething; } catch (memoryAllocationFailed) { doSomething; } catch (readFailed) { doSomething; } catch (fileCloseFailed) { doSomething; } }
  • 47. 2: Propagating Errors Up the Call Stack method1 { try { call method2; } catch (exception) { doErrorProcessing; } } method2 throws exception { call method3; } method3 throws exception { call readFile; }

Editor's Notes

  • #3: flexibility, easing changes to programs easier to learn simpler to develop, maintain and analysize