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Java Tutorial

Object-Oriented Programming

Partly adapted with permission from Eran Toch, Technion

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

Adapted with permission from Avivit Bercovici Boden, Technion

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.

Platform Dependent
myprog.c

gcc

C source code

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
byte
1 byte
exactly as in
float
4 bytes
C++
double 8 bytes
char
Unicode encoding (2 bytes)
boolean {true,false}

Note:
Primitive type
always begin
with lower-case

Primitive types - cont.
• Constants
37
integer
37.2
float
42F
float
0754
integer (octal)
0xfe
integer (hexadecimal)

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)

More sophisticated
class Kyle {
private boolean kennyIsAlive_;
Default
public Kyle() { kennyIsAlive_ = true; }
C’tor
public Kyle(Kyle aKyle) {
kennyIsAlive_ = aKyle.kennyIsAlive_;
}
public String theyKilledKenny() {
if (kennyIsAlive_) {
Copy
kennyIsAlive_ = false;
C’tor
return “You bastards !!!”;
} else {
return “?”;
}
}
Kyle k = new Kyle();
String s = k.theyKilledKenny();
System.out.println(“Kyle: “ + s);
}
}

Results
javac Kyle.java

( to compile )

java Kyle
Kyle: You bastards !!!

( to execute )

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
Animal[][] arr=
new Animal[2][2]

What is the type of
the object here ?

Static - [1/4]
• 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 memory

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;

a
Output:
1
5
10

0
y

b
0
y
1
A.x_

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”);

• Block

Static - [3/4]

– 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 :
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
}

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”);
}
}
void foo() {
System.out.println(“Derived”);
}
}
public class Test {
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()");
}
}

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.

Example

Abstract - Example
package java.lang;
public abstract class Shape {
public abstract void draw();
public void move(int x, int y) {
setColor(BackGroundColor);
draw();
setCenter(x,y);
setColor(ForeGroundColor);
draw();
}
}
package java.lang;
public class Circle extends Shape {
public void draw() {
// draw the circle ...
}
}

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.
- The correct term is “to implement”
an interface
*

Example

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

interface BabyKicker {
void kickTheBaby(Baby);
}

interface SouthParkCharacter {
void curse();
}

class Chef implements IChef,
// overridden methods
// can you tell why ?
public void curse() {
public void cook(Food
}

SouthParkCharacter {
MUST be public
… }
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
SortedSet

List

Map
Queue

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

Map
Queue

Sorted Map

SortedSet
HashSet

TreeSet

ArrayList LinkedList

TreeMap HashMap

List<String> list1 = new ArrayList<String>(c);
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
(String class is final)

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

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;
}
}
// 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;

}

errorCodeType readFile {
initialize errorCode = 0;
open the file;
if (theFileIsOpen) {
determine the length of the file;
if (gotTheFileLength) {
if (gotEnoughMemory) {
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;
}

readFile {
try {
open the file;
determine its size;
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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