fundamento java

PJ-020

FUNDAMENTOS DE JAVA www.profesorjava.com

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Anatomy of a Simple Java Program

Built-In Data Types

Autoincrement/Decrement Operators

Java Expressions

Casting

Block Structured Languages and the Scope of a Variable

Controlling a Programs Execution Flow.

Exercises

Java Language Basics

Anatomy of a Simple Java Program.

Comments

class wrapper

main

method

Anatomy of a Simple Java Program.

Anatomy of a Simple Java Program. Examples

Built-In Data types. Example.

Built-In Data types.

Built-In Data types. Example.

++/-- Operators.

Java provides autoincrement(++) and autodecrement(--) operators;

++/-- Operators Example.

Java Expressions.

An expression is a combination of one or more operators and operands.

Expressions usually perform a calculation. The value calculated does not have to

be a number, but it often is. The operands used in the operations might be

literals, constants, variables, or other sources of data.

Many programming statements involve expressions. Expressions

are combinations of one or more operands and the operators used

to perform a calculation.

Java Expressions. Example.

Expr

Casting

Java automatically casts implicitly to larger data types.

When placing larger data types into smaller types, you must use explicit casting to state the type name to which you are converting.

Casting

The rules governing automatic casting by the Java compiler are as follows when

considering two operands within an arithmetic expression:

If either type is double, the other is cast to a double

If either type is float, the other is cast to a float

If either type is long, the other is cast to a long

Else both operands are converted to int

Casting

int num1 = 53;

int num2 = 47;

byte num3 = (byte)(num1 + num2) //ok nhpp

int valor;

long valor2 = 99L;

valor = (int)valor2; //no hay prdida de precisin

int valor;

long valor2 = 123987654321;

valor = (int)valor2; //el nmero se trunca

Casting

short s = 259; //binario 100000011

byte b = (byte)s; //casting

System.out.println(b = + b);

0 0 0 0 0 0 0 1 0 0 0 0 0 0 1 1

b = (byte)s

0 0 0 0 0 0 1 1

Casting

Casting

1 / 2 = 0

en 32 bits entero

Casting

1.0 / 2 = 0 se representa en 64 bits

Casting


Java is a block structured language. A block of code is a series of zero or more

lines of code enclosed within curly braces {}

Controlling a Programs Execution Flow.

if

while

do

for

Conditional Statement Types: if-else

An if-else statement is a conditional expression that must return a boolean value

else clause is optional

Braces are not needed for single statements but highly recommended for clarity

Controlling a Programs Execution Flow. If

Controlling a Programs Execution Flow. If-else: ?

Shortcut for if-else statement:

( ? : )

Can result in shorter code

Make sure code is still readable

Controlling a Programs Execution Flow. Switch

Switch statements test a single variable for several alternative values

Cases without break will fall through (next case will execute)

default clause handles values not explicitly handled by a case

Controlling a Programs Execution Flow. Switch

Looping Statement Types: while

Executes a statement or block as long as the condition remains true

while () executes zero or more times

do...while() executes at least once.

Looping Statement Types: while

Looping Statement Types: for

A for loop executes the statement or block { } which follows it

Evaluates "start expression" once

Continues as long as the "test expression" is true

Evaluates "increment expression" after each iteration

A variable can be declared in the for statement

Typically used to declare a "counter" variable

Typically declared in the start expression

Its scope is restricted to the loop

Looping Statement Types: for

for vs. while

These statements provide equivalent functionality

Each can be implemented in terms of the other

Used in different situations

while tends to be used for open-ended looping

for tends to be used for looping over a fixed number of iterations

for vs. while

Branching statements

break

Can be used outside of a switch statement Terminates a for, while or do-while loop Two forms:

Labeled: execution continues at next statement outside the loop Unlabeled: execution continues at next statement after labeled loop


continue

Like break, but merely finishes this round of the loop Labeled and unlabeled form

return

Exits the current method May include an expression to be returned

Type must match methods return type Return type void means no value can be returned

Abstraction and Modeling

57

Simplification Through Abstraction

Generalization Through Abstraction

Reuse of Abstractions

Inherent Challenges

Exercises

Abstraction and Modeling

58

Abstraction: a process that involves recognizing and focusing

on the important characteristics of a situation or object, and

filtering out or ignoring all of the unessential details.

Is the process of ignoring details to concentrate on essential characteristics

Is the primary means of coping with complexity

Simplifies users interaction with abstracted objects


59


60

One familiar example of an abstraction is a road map.


61

As an abstraction, a road map represents those features of a given geographic

area relevant to someone trying to navigate with the map, perhaps by a car:

major roads and places of interest, obstacles such as major bodies of water, etc.

Of necessity, a road map cannot include every building, tree, street sign,

billboard, traffic light, fast food restaurant, etc. that physically exists in the real

world. If i did, then it would be so cluttered as to be virtually unusable; none of

the important features would stand out.


62

Compare a road map with a topographical map, a climatological

map, and a population density map of the same region: each

abstracts out different features of the real world namely, those

relevant to the intender user of the map in question.


63

As another example, consider a landscape. An artist may look at the landscape

from the perspective of colors, textures, and shapes as a prospective subject for

a painting.


64

A homebuilder may look at the same landscape from the perspective of where

the best building site may be on the property, assessing how many trees will need

to be cleared to make way for a construction project.


65


66

If we eliminate enough detail from an abstraction, it becomes

generic enough to apply to a wide range of specific situations

or instances. Such generic

abstractions can often be

quite useful. For example,

a diagram of a generic cell

in the human body might

include only a few features

of the structures that

are found in an actual cell:


67

This overly simplified diagram doesnt look like a real nerve cell, or a real

muscle cell, or a real blood cell; and yet, it can still be used in a educational

setting to describe certain aspects of the structure and function of all of these

cell types namely, those features that the various cell types have in common.


68

Even though our brains are adept at abstracting concepts such as road maps and

landscapes, that still leaves us with hundreds of thousands, if not millions, of

separate abstractions to deal with over our lifetimes. To cope with this aspect of

complexity, human beings systematically arrange information into categories to

established criteria; this process is known as classification.

Organizing Abstractions Into Classification Hierarchies

69


70


71

For example, science categorizes all natural objects as belonging to either the

animal, plant, or mineral kingdom. In order for a natural object to be classified

as an animal, it must satisfy the following rules:

It must be a living being

It must be capable of spontaneous movement

It must be capable of rapid motor response to stimulation


72

The rules for what constitute a plant, on the other hand, are diferent:

It must be a living being (same as for an animal)

It must lack an obvious nervous system

It must possess cellulose cell walls


73

Given clear-cut rules such as these, placing an object into the appropriate

category, or class, is rather straightforward. We can then drill down,

specifying additional rules which differentiate various types of animal, for

example, until weve built up a hierarchy of increasing more complex

abstractions from top to bottom.


74

A simple example of an abstraction hierarchy is shown below.


Natural Objects

Plant Animal Mineral

Mammal Fish Bird Reptile Insect

Dog Cat Monkey

75

When thinking about an abstraction hierarchy such as the one shown previously,

we mentally step up and down thehierarchy, automatically zeroing in on only the

single layer or subset of the hierarchy (known as a subtree) that is important

to us at a given point in time. For example, we may only be concerned with

mammals, and so can focus on the mammalian subtree:


Mammal

Dog Cat Monkey

76

We temporarily ignore the rest of the hierarchy. By doing so, we automatically

reduce the number of concepts that we mentally need to juggle at any one

time to a manageable subset of the overall abstraction hierarchy; in the

simplistic example, we are now dealing with only four concepts rather than the

original 13. No matter how complex an abstraction hierarchy grows to be, it

neednt overwhelm us if it is properly organized.


Mammal

Dog Cat Monkey

77

Coming up with precisely which rules are necessary to properly classify an object

within an abstraction hierarchy is not always easy. Take for example, the rules

we might define for what constitutes a bird: namely, something which:

Has feathers

Has wings

Lays eggs

Is capable of flying


78

Given these rules, neither an ostrich nor a penguin could be classified as a bird,

because neither can fly.


Birds Non-Birds

79

If we attempt to make the rule set less restrictive by eliminating the flight

rule, we are left with:

Has feathers

Has wings

Lays eggs

According to this rule set, we now may properly classify both the ostrich and the

penguin as birds.


80


Birds Non-Birds

81

This rule set is still unnecessarily complicated, because as it turns out, the lays

eggs rule is redundant: whether we keep it or eliminate it, it doesnt change our

decision of what constitutes a bird versus a non-bird. Therefore, we simplify

the rule set once again:

Has feathers

Has wings


82

We try to take our simplification process one step further, by eliminating yet

another rule, defining a bird as something which:

Has wings

Weve gone too far this time: the abstraction of a bird is now so general that

wed include airplanes, insects, and all sorts of other non-birds in the mix.


83


The process of rule definition for purposes of categorization

involves dialing in just the right set of rules not too general, not to restrictive, and containing no redundancies- to define

the correct membership in a particular class.

84

When pinning down the requirements for an information systems development

project, we typically start by gathering details about the real world definition on

which the system is to be based. These details are usually a combination of:

Those that are explicitly offered to us as we interview the intended users of the system

Those that we otherwise observe.

Abstractions as the Basis for Software Development

85

We must make a judgment all as to which of these details are relevant to the

systems ultimate purpose. This is essential, as we cannot automate them all!.

To include too much details is to overly complicate the resultant system, making

it that much more difficult to design, program, test, debug, document, maintain,

and extend in the future.

As with all abstractions, all of our decisions of inclusions versus elimination when

building a software system must be made within the context of the overall

purpose and domain, or subject matter focus, of the future system.


86

Once weve determined the essential aspects of a situation we can prepare a

model of that situation. Modeling is the process by which we develop a pattern

for something to be made. A blueprint for a custom home, a schematic diagram

of a printed circuit, and a cookie cutter are all examples of such patterns.


87


A model is a simplification of the reality.

88

Modeling achieves four aims:

Helps you to visualize a system as you want it to be.

Permits you to specify the structure or behavior of a system.

Gives you a template that guides you in constructing a system.

Documents the decisions you have made.

You build models of complex systems because you cannot comprehend such a system in its entirety.

You build models to better understand the system you are developing.


89

The importance of modeling:


Paper Airplane Fighter Jet

Less Important More Important

90

Many software teams build applications approaching the problem like they were building paper airplanes

Start coding from project requirements

Work longer hours and create more code

Lacks any planned architecture

Doomed to failure

Modeling is a common thread to successful projects


91

An object model of a software system is such a pattern. Modeling and

abstraction go hand in hand, because a model is essentially a physical or

graphical portrayal of an abstraction; before we can model something effectively,

we must have determined the essential details of the subject to be modeled.


92

When learning about something new, we automatically search our mental

archive for other abstractions/models that weve previously built and mastered,

to look for similarities that we can build upon.

When learning to ride a two-wheeled

bicycle for the first time, for example,

you may have drawn upon lessons

that you learned about riding a

tricycle as a child.


93

Both have handlebars that are used to steer; both have pedals that are used to

propel the bike forward. Although the Abstractions didnt match perfectly a

two wheeled bicycle introduced the new challenge of having to balance oneself

there was enough of a similarity to allow you to draw upon the steering and

pedaling expertise you already had mastered, and to focus on learning the new

skill of how to balance on two wheels.


94


This technique of comparing features to find an abstraction

that is similar enough to be reused successfully is known as

pattern matching and reuse. A pattern reuse is an

important technique for object oriented software development

,as well, because it spares us from having to reinvent the

wheel with each new project. If we can reuse an abstraction

or model from a previous project, we can focus on those

aspects of the new project that differ from the old, gaining

a tremendous amount of productivity in the process.

95

Pattern matching


96


97


98

Despite the fact that abstraction is such a natural process for human beings,

developing an appropriate model for a software system is perhaps the most

difficult aspect of software engineering.

Inherent Challenges

99

Inherent Challenges

Objects and Classes

101

What is an object?

Methods


Inherent Challenges

Exercises

Objects and Classes

102

A class is a collection of objects with related properties and behaviours.

In real-life we group things into classes to help us reduce complexity

Example:

The set of all dogs forms the class Dog

Each individual dog is an object of the class Dog

Firulais, Terry and Rex are all instances of the class Dog

To some extent, we can interact with Firulais based on our knowledge of dogs in general, rather than Firulais himself

What Is an Object?

103

What Is an Object?

104

What is a Waiter?

A Waiter is someone who has the following properties and behaviours:

Properties of a Waiter

Full Name

Behaviours of a Waiter

Bring menus

Take orders

Bring meals

This collection of properties and behaviours defines the class of Waiters

Because these behaviours are standardized, we can deal with any Waiter just based on our general knowledge of Waiters

What Is an Object?

105

A class is a general description of the properties and behaviours of some entities.

We described the class Waiter

giving the general description

of what properties Waiters have

and what things Waiters can

do.

What Is an Object?

Waiter

fullName

bringMenu

takeOrder

bringMeal

Name of

class

Properties

Behaviours

106

An object is a specific member of a class.

An object belonging to the class of Waiters is an actual individual waiter

Pierre is an object of the class Waiter, and so is Bill and so is Jimmy they can all take orders, bring menus and bring meals

What Is an Object?

107

What Is an Object?

108

Class Object

What Is an Object?

109

What Is an Object?

Classes in Java may have methods and attributes.

Methods define actions that a class can perform.

Attributes describe the class.

110

What Is an Object?

111

What Is an Object?

The phrase "to create an

instance of an object means

to create a copy of this object

in the computer's memory

according to the definition of

its class.

112

What Is an Object?

113

What Is an Object?

114

What Is an Object?

115

The class BankAccount

A bank account has the following properties:

An account number and account name

A balance

A bank account has the following behaviours:

Money can be credited to the bank account

Money can be debited from the bank account

What Is an Object?

116

What Is an Object? BankAccount

accountName

accountNumber

credit

debit

117

Objects in Java are creating using the keyword new.

What Is an Object?

118

The arguments in the constructor are used to specify initial information about

the object. In this case they represent the account number and account name.

A constructor can have any number of arguments including zero.

What Is an Object?

Arguments

119

What Is an Object?

1. Declare a reference.

2. Create the object.

3. Assign values.

120

What Is an Object?

1. Declare a reference.

2. Create the object.

Two references to two

objects, with values

for their attributes.

121

What Is an Object?

122

What Is an Object?

Heap memory Stack memory

428802

0x99f311

0x334009 AnotherShirt

myShirt

id

\u0000

0.0

false

\u0000

0.0

false

size

size

price

price

lSleeved

lSleeved

123

What Is an Object?

Heap memory Stack memory

0x99f311

0x334009

0x99f311

AnotherShirt

myShirt

\u0000

0.0

false

\u0000

0.0

false

size

size

price

price

lSleeved

lSleeved

X X

124

Consider a class that represents a circle.

public class Circle {

int radius;

}

public class ShapeTester {

public static void main(String args[]) {

Circle x;

x = new Circle();

System.out.println(x);

}

}

What Is an Object. Examples.

125

Here is another example defining a Rectangle that stores a width and height as

doubles:

public class Rectangle {

double width = 10.128;

double height = 5.734;

}



Circle x;

Rectangle y;

x = new Circle();

y = new Rectangle();

System.out.println(x + " " + y);

}

}


126



Circle x;

Rectangle y, z;

x = new Circle();


z = new Rectangle();

System.out.println(x + " " + y + " " + z);

}

}


127



Circle x;

Rectangle y, z;

x = new Circle();


z = new Rectangle();

x.radius = 50;

z.width = 68.94;

z.height = 47.54;

System.out.println(x.radius + " " + y.width + " " + z.width);

}

}


Objects Interactions

129

Methods

Exercises

Objects Interactions

130

The interesting part of OO-Programming is getting the objects to interact

together. This is obvious when we look at real world examples:

A house not being lived in is not useful

A BankAccount in which no money is deposited or withdrawn is not useful either

A CD without a CD Player is useless too.

Behaviour represents:

the things you can do with an object (i.e., a command)

information you can ask for from an object (i.e., a question)

Methods

131

By definition an instance is created from its class definition and so it only uses

the vocabulary defined in its own class. To help us understand object behaviour,

we should try to think of objects as being living entities. When we want to

"talk to" or "manipulate" an object, we must send it a message.

A message:

is a set of one or more words (joined together as one) that is sent to an object.

is part of the "vocabulary" that an object understands.

may have additional information (parameters) which are required by the object.

You can send messages to objects, and they respond to you:

Methods

132

May have additional information (parameters) which are required by the object.

You can send messages to objects, and they respond to you:

Objects only respond if they understand what you say:

Methods

133

The message may require some parameters (i.e., pieces of data):

Methods

134

Thus, by defining behaviour, we simply add to the vocabulary of words (i.e.,

messages) that the object understands. Objects communicate by sending

messages back and forth to each other:

Methods

135

As we can see, many objects are often involved in a more difficult task. For

example, consider building a house. A person asks a house building company to

build them a house. In fact, the house building company then "sub-contracts"

out all of the work in that it then hires others to do all the work. So the house

builder actually co-ordinates the interactions with all of the contractors. The

contractors themselves contact suppliers to get their parts as well as other

helpers to help them accomplish their tasks:

Methods

136

Methods

137

To define a particular behaviour for an object, we must write a method

A method :

is the code (expressions) that defines what happens when a message is sent to an object.

may require zero or more parameters (i.e., pieces of data):

Parameters may be primitives or other objects

Primitives are passed-by-value (the actual value is copied and passed with the message)

Objects are passed-by-reference (a pointer to the object is passed with the message)

may be either a class method or an instance method.

Methods are typically used to do one or more of these things:

get information from the object it is sent to change the object in some way

compute or do something with the object

obtain some result.

Methods

138

Methods are typically used to do one or more of these things:

get information from the object it is sent to

change the object in some way

compute or do something with the object

obtain some result.

Methods

139

Methods

140

Sending a message to an object is also known as calling a method. So the

method is actually the code that executes when you send a message to an

object. Some methods return answers, others may do something useful but do

not return any answer.

Methods

141

A method is calling by specifying

The target object, following by a dot

The method name

The method arguments (is there are any)

cheque.getBalance();

The target object is the one called cheque

The getBalance method has been called

There are no arguments for this method

The result will be returned to whoever called the method

Methods

142

Methods

143

Methods

144

Methods

145

Methods

146

Methods

Calling its method

147

In general, methods calls may

Send information to the target, or not

Receive information from the object, or not

The method signature tell us whether information is to be sent,

received or both.

Methods

148

Methods

149

Methods

150

Methods

Collection of Objects

152

Data Structures.

A data structure can be thought of as container that is used to

group multiple elements into a single representation, and is

used to store, retrieve, and manipulate the contained data.

153

Basic Data Structure Mechanisms.

Before the development of the Java2 platform, only a small set

of classes and interfaces were available in the supplied

Standard Class. Library for data store manipulation.

Arrays

Vector

Stack

Hashtable

Properties

BitSet

Enumeration

154

The Vector Class.

Contains a collection of object references.

Can vary in size.

Can hold objects of different types.

The Vector class is more flexible than an Array:

155

The Vector Class.

156

The Vector Class.

157

The Vector Class.

158

The Vector Class.

159

The Vector Class.

160

The Vector Class.

161

The Vector Class.

162

The Vector Class.

163

The Vector Class.

164

The Vector Class.

165

HashTable.

Maps keys to values

Keys and values can be any non-null object

166

Enumeration Interface.

The Enumeration interface allows the developer to traverse

collections at a high level, with little concern for the underlying

collection.

Used specifically when traversal order is not important.

Vector's elements() method and Hashtable's keys() and

elements() methods return Enumeration objects.

The Enumeration interface contains two methods:

hasMoreElements() and nextElement()

167

Enumeration Interface.

168

The Collection API.

169

The Collection API.

170

Set Interface.

The Set interface adds no methods to the collection interface.

Set collections add the restriction of no duplicates.

boolean add(Object element) fails to update the collection and returns false if the element already exists.

Adds a stronger contract on the behavior of the equals and hashCode operations, allowing Set objects with different implementation types to be

compared.

171

HashSet Class.

172

TreeSet Class.

173

Iterator Interface.

The Iterator interface is used to traverse through each element

of a collection. This interface offers the same functionality as

the Enumeration interface, with an additional method that

enables us to remove an object. The presence of this

additional method makes it preferable over the Enumeration

interface.

Object next()

boolean hasNext()

void remove()

174

Iterator Interface.

175

List Interface.

A List is a collection of elements in a particular order. Also

referred to as a sequence, a List can contain duplicate

elements.

The List interface extends from the Collection interface an has

an index of elements. The index, which is an integer, denotes

the position of elements in the list. The index also helps us

include a new element into a list in the specific position

required.

176

List Interface.

177

ListIterator Interface.

178

LinkedList Class.

179

LinkedList Class.

180

Concrete Collections.

Objects 1

1. Objects

183

Overview of Object Orientation.

Technique for system modeling

Models the system as a number of related objects that interact

Similar to the way people view their environment

Object technology is a set of principles guiding software

construction together with languages, databases, and

other tools that support those principles. (Object

Technology: A Managers Guide, Taylor, 1997)

184


185

Identifying Objects.

Object can be a sentence, bank account, number, or car

Objects are:

Things

Real or imaginary

Simple or complex

An object is an entity with a well-defined boundary and

identity that encapsulates state and behavior.

186


An object is an entity with a well-defined boundary and

identity that encapsulates state and behavior.

187


188


Physical entity

Conceptual entity

(Chemical process)

Software entity

(Linked list)

189


Objects have many forms:

Tangible things (Airplane, Computer, Car)

Roles (Doctor, Teacher)

Incidents (Meeting)

Interactions (Interview, Agreement)

190

Object definition. Case Study

Throughout this course, a case study of a clothing catalog, DirectClothing, Inc., will be used to illustrate concepts.

191


Most projects start by defining the problem domain by gathering customer requirements and by writing a statement of scope that briefly states what

you, the developer, want to achieve.

For example, a scope statement for the DirectClothing project might be: Create a system allowing order entry people to enter and accept payment for an order.

After you have determined the scope of the project, you can begin to identify the objects that will interact to solve the problem.

192


Object names are often nouns, such as account or shirt. Object attributes are often nouns too, such as color or size. Object operations are usually verbs or noun-verb combinations, such asdisplay or submit order.

Your ability to recognize objects in the world around you will help you to better define objects when approaching a problem using object-oriented

analysis.

Solution

193


The problem domain of the DirectClothing, Inc. case study has the following nouns. Each could be an object in the catalogs order entry system.

catalog

clothing

subscribers

closeout items

monthly items

normal items

order

194


customer

CSR ( customer service representative)

order entry clerk

Supplier

Payment

warehouse

credit car

order entry

mail order

fax order

online order

195


inventory

back-ordered items

system

Internet

business

year

month

order form

check

196

Identifying Object Attributes and Operations

Example:

Cloud attributes: size, water content, shape

Cloud operations: rain, thunder, snow

Attributes: an objects characteristics Operations: what an object can do

197

Identifying Object Attributes and Operations

198

Identifying Object Attributes and Operations. Case Study

When you are attempting to assign operations to an object, operations performed on an object are assigned to the object itself. For example, in a

bank an account can be opened and closed, balanced and updated, receive

additional signers, and generate a statement. All of these would be the

Account objects operations.

199

Identifying Object Attributes and Operations. Case Study

For the Order object, the following attributes and operations could be defined:

Attributes: orderNumber, customerNumber, dateOrdered, amountOwed

Operations: whatCustomer, calcAmountOwed, printOrder, payOrder

What would be the attributes and operations for the Customer object?

200

Testing an Identified Object:

Use the following criteria to test object validity:

Relevance to the problem domain

Need to exist independently

Having attributes and operations

201

Relevance to the Problem Domain.

Does it exist within the boundaries of the problem statement?

Is it required in order for the system to fulfill its responsibility?

Is it required as part of interaction between a user and the system?

Can objects sometimes be a characteristic of other objects?

202

Testing an Identified Object. Case Study

The Order object exists within the boundaries of the problem statement, it is required for the system to fulfill its responsibilities, and as part of an

interaction between a user and the system. The Order object passes the test.

Test the other candidate objects in the case study. What are the results?

203


The following objects can probably be removed from the list:

Internet, system, business Not necessary within the boundaries of the problem statement

month, year May be attributes of the date of an order being placed, but not necessary as an object itself

204


The following objects can probably be removed from the list:

online order, fax order, mail order Can probably be captured as special cases of the order object, or you could have a type attribute on the order object to

indicate how the order was made. You may not want to eliminate here, but to

note these nouns are special cases.

back-ordered items, closeout items, monthly sales item Can probably be captured as special cases of a normal item object. You may not want to

eliminate these nouns, but to note these are special cases.

205

Independent Existence.

To be an object and not a characteristic of another object, the object must need to exist independently

206

Independent Existence. Case Study

Can an Order object exist without any of the other objects? It can, but in use, it must have an associated Customer object.

Address could be an attribute of Customer, but in this case study it is advantageous for Address to be a separate object.

207

Attributes and Operations.

An object must have attributes and operations

If it does not, it is probably and attribute or operation of another object

208

Attributes and Operations. Case Study

An object must have attributes and operations. If you cannot define attributes and operations for an object, then it probably is not an object but an

attribute or operation of another object.

The Order object has many attributes and operations defined, as do most of the candidate objects.

209

Encapsulation.

Encapsulation separates the external aspects of an object from the internal implementation details

Internal changes need not affect external interface

Hide

implementation

from clients.

Clients depend

on interface

210

Encapsulation.

211

Implementing Encapsulation.

An objects attributes and operations are its members

The members of an object can be public or private

In pure OO systems, all attributes are private and can be changed or accessed only through public operations

Objects 2

213


Technique for system modeling

Models the system as a number of related objects that interact

Similar to the way people view their environment

Object technology is a set of principles guiding software

construction together with languages, databases, and

other tools that support those principles. (Object

Technology: A Managers Guide, Taylor, 1997)

214

Class Overview.

A class is a description of a set of objects that share the same attributes, operations, relationships, and semantics.

An object is an instance of a class.

A class is an abstraction in that it Emphasizes relevant characteristics. Suppresses other characteristics.

215

Class Overview.

An object is an instance of a class.

216

Class Overview.

A class is represented using a rectangle with compartments.

217

Class Overview.

A class is an abstract definition of an object. It defines the structure and behavior of each object in the class. It serves as a template for creating

objects.

Classes are not collections of objects.

218

Class Overview.

An attribute is a named property of a class that describes a range of values that instances of the property may hold.

A class may have any number of attributes or no attributes at all.

219

Class Overview.

An operation is the implementation of a service that can be requested from any object of the class to affect behavior.

A class may have any number of operations or none at all.

220

Generalization

Generalization identifies and defines the common attributes

and operations in a collection of objects.

Example: Transport is a generalization of several classes that

provide transportation.

221

Generalization

A relationship among classes where one class shares the

structure and/or behavior of one or more classes.

222

Inheritance

Is a mechanism for defining a new class in terms of an existing class.

Allows you to group related classes so that they can be managed collectively.

Promotes reuse.

Allows you to hide or override inherited members.

Relevant terms: generalization, specialization, override.

223

Inheritance

224

Inheritance

225

Inheritance

226

Inheritance

227

Specialization

Specialization is inheritance with the addition and modification

of methods to solve a specific problem.

228

Polymorphism

Allows you to implement an inherited operation in a subclass

Works only when the common operation gives the same semantic result

Implementation of a polymorphic function depends on the object it is applied to

Can be used only with inheritance

229

Polymorphism

Polymorphism is the ability to hide many different

implementations behind a single interface.

230

Polymorphism

Interfaces formalize polymorphism.

Objects 3

232

Object Messaging.

One object sends a message to another (the receiving object)

The receiving object may send other messages, change its attribute, or read in any other appropriate way.

Messaging in handled by operations in the public interface of the receiving object.

233

Association and Composition.

Objects interact through one of two relationships: association or composition.

Association: Two independent objects collaborate to achieve some goal, like a person using a computer (uses a relationship)

Composition: One object contains another, like a pencil that has a lead (has a relationship)

234


a

235


a

236


a

237


a

238


a

239


a

240


a

241


a

242


a

243


a

244


a

245


a

246


a

Objects 4

248

Object-Oriented Analysis and Design.

Unified Modeling Language (UML) is used to notate the design.

UML diagrams:

Use case diagram

Sequence diagram

Class diagrams

Activity diagrams

249

Use Case Diagrams.

A use case diagram contains use cases, actors, and relationship links

A use case is an interaction of a user with the application in order to achieve a desired result

An actor is a role that a user plays when interfacing with the application

Relationship links between use cases are uses and extends.

250

Use Case Diagrams.

There are two types of relationship links that can be made in the diagram. These are the extends and uses relationships between the use cases.

The extends relationship links two use cases that are similar but one does a little more than the other. It is implied that the actor who performs the first

use case will also perform the extension use case. This relationship is

indicated by on the links line.

251

Use Case Diagrams.

The second type of link is the uses relationship, which occurs when there is a behavior that is used by many use cases. To avoid repetition, make that

behavior a use case itself, and have other use cases use it. It is implied that an actor does not perform the used use case, but that the base use case does the performing.

This relationship is indicated by on the links line.

252

Use Case Diagrams.

An actor represents anything that interacts with the system.

A use case is a sequence of actions a system performs that yields an observable result of value to a particular actor.

253

Use Case Diagrams.

254

Use Case Diagrams.

255

Use Case Diagrams.

Follow these steps to create a use case diagram:

1. Identify each use case in your application. (It might help to identify events

you need to react to.)

2. Draw and label each of the actors of the application.

3. Draw and label the use cases of the application.

4. Draw the links from the actor to the use cases they perform.

5. Write a short description of each use case. The diagram and the description

together will give a representation of the functionality that must be

implemented in the system.

256

Example: Use Case Diagrams.

A use case specifies a set of scenarios

for accomplishing something

useful for an actor. In this

example, one use case is

"Buy soda."

257


Restocking a soda machine is an important use case.

258


Collecting the money

from a soda machine

is another

important use case.

259


260

Use Case Diagrams.

Use case diagrams describe what a system does from the

standpoint of an external observer. The emphasis is on what

a system does rather than how.

Use case diagrams are closely connected to scenarios. A

scenario is an example of what happens when someone

interacts with the system.

261

Use Case Diagrams.

Here is a scenario for a medical clinic:

"A patient calls the clinic to make an appointment for a

yearly checkup. The receptionist finds the nearest empty time

slot in the appointment book and schedules the appointment

for that time slot. "

262

Use Case Diagrams.

A use case is a summary of scenarios for a single task or

goal. An actor is who or what initiates the events involved in

that task. Actors are simply roles that people or objects play.

The picture below is a Make Appointment use case for the

medical clinic. The actor is a Patient. The connection between

actor and use case is a communication association (or

communication for short).

263

Use Case Diagrams.

A use case diagram is a collection of actors, use cases, and

their communications. We've put Make Appointment as part of

a diagram with four actors and four use cases. Notice that a

single use case can have multiple actors.

264

Use Case Diagrams.

A use case describes a single task or goal and is indicated by

an oval. The task or goal is written inside the oval and usually

it contains a verb.

265

Use Case Diagrams.

TIP: Start by listing a sequence of steps a user might take in

order to complete an action. For example a user placing an

order with a sales company might follow these steps.

1. Browse catalog and select items.

2. Call sales representative.

3. Supply shipping information.

4. Supply payment information.

5. Receive conformation number from salesperson.

266

Use Case Diagrams.

267

Exercises: Use Case Diagrams.

Disear diagramas de casos de uso para las siguientes

situaciones:

Comprar una paleta en la cafetera de la escuela.

Cancelar una cita con el(la) novio(a) una salida con los amigos.

Enviar un mensaje de correo electrnico.

Enviar un mensaje de texto de un telfono celular a otro.

Copiar un archivo a la memoria USB.

Imprimir un documento de Word en el centro de cmputo.

268

Use Case Relations.

(extensin) : Los casos de uso pueden

extenderse a otros casos de uso. Se recomienda utilizar

cuando un caso de uso es similar a otro (caractersticas).

269

Use Case Relations.

(inclusin) : Los casos de uso pueden incluir a

otros casos de uso. Se recomienda utilizar cuando se tiene

un conjunto de caractersticas que son similares en ms de

un caso de uso y no se desea mantener copiada la

descripcin de la caracterstica.

270

Use Case Relations.

Cuando un nmero de casos de uso comparten un

comportamiento comn puede ser descrito por un caso de

uso que es utilizado por otros casos de uso.

271

Use Case Relations.

Es una relacin de dependencia donde un caso de uso

extiende otro caso de uso aadiendo acciones a un caso de

uso extendido.

272


Mquina Recicladora: Sistema que controla una mquina

de reciclamiento de botellas, tarros y jabas. El sistema debe

controlar y/o aceptar:

Registrar el nmero de tems ingresados.

Imprimir un recibo cuando el usuario lo solicita:

Describe lo depositado

El valor de cada item

Total

273


Existe un operador que desea saber lo siguiente:

Cuantos tems han sido retornados en el da.

Al final de cada da el operador solicita un resumen de todo lo depositado en el da.

El operador debe adems poder cambiar:

Informacin asociada a tems.

Dar una alarma en el caso de que:

Item se atora.

No hay ms papel.

274


Actores que interactuan con el sistema:

275


Un Cliente puede depositar Items y un Operador puede

cambiar la informacin de un Item o bien puede Imprimir un

Informe.

276


Un item puede ser una Botella, un Tarro o una Jaba.

277


la impresin de comprobantes, que puede ser realizada

despus de depositar algn item por un cliente o bien puede

ser realizada a peticin de un operador.

278


279


Sistema de ventas. Un sistema de ventas debe interactuar

con clientes, los cuales efectan pedidos. Adems los clientes

pueden hacer un seguimiento de sus propios pedidos. El

sistema enva los pedidos y las facturas a los clientes. En

algunos casos, segn la urgencia de los clientes, se puede

adelantar parte del pedido (pedidos parciales).

280


281


Encontrar los casos de uso para la biblioteca sencilla:

De cada libro tengo uno o varios ejemplares.

Cada usuario puede mantener un mximo de tres ejemplares en prstamo de forma simultnea.

Los usuarios pueden solicitar al bibliotecario un libro en prstamo (dando el autor o el ttulo, etc.) y el sistema debe determinar si hay al menos un

ejemplar en las estanteras. Si es as, el bibliotecario entrega un ejemplar

y registra el prstamo (usuario, fecha y ejemplar concreto).

282


El prstamo es semanal y si se produce un retraso en la devolucin, se impone una multa en forma de das sin derecho a nuevos prstamos (3 das

por cada da de retraso).

Antes de cualquier prstamo, el bibliotecario debe comprobar esta situacin.

283


Encontrar los casos de uso para las tareas uno y dos.

284

Use Case Diagrams.

285

Sequence Diagrams.

Capture the operations of a single use case and show how groups of objects collaborate on those operations.

Exist for each use case.

Contains objects, objects lifelines, messages between objects, conditions, iteration markers, activations, and object deletions.

286

Sequence Diagrams.

A Sequence Diagram is an interaction diagram that emphasizes the time ordering of messages.

The diagram show:

The objects participating in the interaction

The sequence of messages exchanged

287

Sequence Diagrams.

288

Sequence Diagrams.

:Sistema

crearNuevaVenta()

*[ms items]

descripcin, total

:cajero

ingresarItem(codItem, cant)

finalizarVenta()

total con imptos.

realizarPago()

monto cambio, recibo

Bucle

Un diagrama de secuencia del sistema muestra, para un escenario particular de un caso de uso, los eventos externos que los actores generan, su orden y los eventos inter-sistemas.

289

Sequence Diagrams.

:JuegodeDados dado1:Dados dado2:Dados

jugar() lanzar()

val1:=getValorMostrado()

lanzar()

val2:=getValorMostrado()

290

Sequence Diagrams.

:Computer :PrintServer :Printer

print(arch) print(arch) [no queue]

print(arch)

291

Sequence Diagrams.

:Computer :PrintServer :Printer

print(arch) print(arch) [no queue]

print(arch)

Mensaje

Lnea de vida

Activacin

Mensaje Sincrnico

Retorno

Condicin

Objetos participantes en la interaccin

Puede omitirse

292

Sequence Diagrams.

:ItemWindow

:Item

NuevoItem(data)

crearItem(data)

Flecha hacia un objeto

ndica creacin del objeto.

:ItemWindow :Item EliminarItem()

BorrarItem() X

X indica destruccin del objeto

293

Sequence Diagrams.

Mensaje Simple / Sincrnico No se dan detalles de la comunicacin cuando no

son conocidos o no son relevantes.

Mensaje Asincrnico

Sintaxis del mensaje: Nmero de secuencia [condicin] * [expresin iteracin]

valor de retorno := nombre del mensaje (parmetros)

Respuesta / Resultado

294

Sequence Diagrams.

a1:ClaseA b1:ClaseB

u Una ramificacin es mostrada por mltiples mensaje que

abandonan un mismo punto, cada una etiquetada con una

condicin

u Si las condiciones son mutuamente excluyentes representan

condiciones; de otra manera representan concurrencia.

:ClaseC [x>0] Op1()

X

[x

295

Sequence Diagrams.

a1:Order b1:OrderLine

Sintaxis: * [expresin-iteacin ] mensaje

*[for each] subtotal()

OrderTotal()

296

Sequence Diagrams.

297

Sequence Diagrams.

Activation boxes represent the

time an object needs to

complete a task

298

Sequence Diagrams.

Messages are arrows that represent

communication between objects.

Use half-arrowed lines to represent

asynchronous messages.

Asynchronous messages are sent

from an object that will not wait for a

response from the receiver before

continuing its tasks.

299

Sequence Diagrams.

Lifelines are vertical dashed

lines that indicate the object's

presence over time.

300

Sequence Diagrams.

Objects can be terminated

early using an arrow labeled

"< < destroy > >" that points to

an X.

301

Sequence Diagrams.

A repetition or loop within a

sequence diagram is depicted

as a rectangle. Place the

condition for exiting the loop at

the bottom left corner in

square brackets [ ].

302

Example:

303

Example:

304

Example:

305

Example:

306

Example:

307

Example:

308

Example:

309

Example:

310

Sequence Diagrams.

Follow these steps to create a sequence diagram. (These are

General guidelines; to write a sequence diagram you must

make sure you check for all interactions among all objects.)

1. Select a use case.

2. Add the first object in the use case to the diagram.

3. Add its method, the message it sends to the next object, and the next

object.

4. Check whether the second object replies to the first or sends on another

message and add the appropriate elements.

311

Sequence Diagrams.

5. Repeat steps 3 and 4 as necessary.

6. Add any necessary elements mentioned in this section such

as conditions, iteration markers, or object deletions.

312

Sequence Diagrams.

GENERAR EL DIAGRAMA DE SECUENCIA PARA LA

MAQUINA RECICLADORA.

313

Collaboration Diagrams.

314

Collaboration Diagrams.

GENERAR EL DIAGRAMA DE StECUENCIA PARA LA

MAQUINA RECICLADOR

Alternative to Sequence diagrams

Objects are connected with numbered arrows showing the flow of the information

Arrows are drawn from the source of the interaction

The object towards with the arrow points is known as the target

Arrows are numbered to show the order in which they are used within the scenario

Also marked with a description of the task required of the target object

315

Sequence Diagrams.

316

Sequence Diagrams.

317

Sequence Diagrams.

Arrays

319

Group data objects of the same type.

Declare arrays of primitive or class types:

char s[];

Point p[];

char[] s;

Point[] p;

Create space for a reference.

An array is an object; it is

created with new.

Declaring arrays.

320

Use the new keyword to create an array object.

For example, a primitive (char) array:

public char[] createArray() {

char[] s;

s = new char[26];

for ( int i=0; i

321

Initialize an array element

Create an array with initial values:

String names[];

names = new String[3];

names[0] = "Georgianna";

names[1] = "Jen";

names[2] = "Simon";

String names[] = { "Georgianna","Jen","Simon"};

MyDate dates[];

dates = new MyDate[3];

dates[0] = new MyDate(22, 7, 1964);


dates[2] = new MyDate(22, 12, 1964);

MyDate dates[] = { new MyDate(22, 7, 1964),new MyDate(1, 1, 2000), new MyDate(22, 12,

1964) };

Initializing Arrays.

322

Arrays of arrays:

int twoDim [][] = new int [4][];

twoDim[0] = new int[5];


int twoDim [][] = new int [][4]; illegal

Multidimensional Arrays.

323


324


325

Non-rectangular arrays of arrays:





Array of four arrays of five integers each:

int twoDim[][] = new int[4][5];


326

All array subscripts begin at 0:

int list[] = new int [10];

for (int i = 0; i < list.length; i++) {

System.out.println(list[i]);

}

Array Bounds.

327

Cannot resize an array

Can use the same reference variable to refer to an entirely new array:

int myArray[] = new int[6];

myArray = new int[10];

Array Resizing.

328

The System.arraycopy() method:

Copying arrays

329

The System.arraycopy() method:

Copying arrays

Class Design

331

Subclassing

332

Subclassing

333

Single Inheritance

When a class inherits from only one class, it is called single inheritance.

Interfaces provide the benefits of multiple inheritance without drawbacks.

Syntax of a Java class:

class [extends ] {

*

}

334

Single Inheritance

335

Access Control

336

Overriding Methods

A subclass can modify behavior inherited from a parent class.

A subclass can create a method with different functionality than the parents method but with the same:

Name

Return type

Argument list

337

Overriding Methods

338

The Super Keyword

super is used in a class to refer to its superclass.

super is used to refer to the members of superclass,both data attributes and methods.

Behavior invoked does not have

to be in the superclass; it can be

further up in the hierarchy.

339

Polymorphism

Polymorphism is the ability to have many different forms; for example, the Manager class has access to methods from Employee class.

An object has only one form.

A reference variable can refer to objects of different forms.

340

Virtual Method Invocation

Virtual method invocation:

Employee e = new Manager();

e.getDetails();

Compile-time type and runtime type

341

Rules About Overriding Methods

Must have a return type that is identical to the method it overrides

Cannot be less accessible than the method it overrides

342

Heterogeneous Collections

Collections of objects with the same class type are called homogenous collections.

MyDate[] dates = new MyDate[2];

dates[0] = new MyDate(22, 12, 1964);


Collections of objects with different class types are called heterogeneous collections.

Employee [] staff = new Employee[1024];

staff[0] = new Manager();

staff[1] = new Employee();

staff[2] = new Engineer();

343

The InstanceOf Operator

344

Casting Objects

Use instanceof to test the type of an object

Restore full functionality of an object by casting

Check for proper casting using the following guidelines:

Casts up hierarchy are done implicitly.

Downward casts must be to a subclass and checked by the compiler.

The object type is checked at runtime when runtime errors can occur.

345

Overloading method names

Use as follows:

public void println(int i)

public void println(float f)

public void println(String s)

Argument lists must differ.

Return types can be different.

346

Overloading Constructors

As with methods, constructors can be overloaded.

Example:

public Employee(String name, double salary, Date DoB)

public Employee(String name, double salary)

public Employee(String name, Date DoB)

Argument lists must differ.

You can use the this reference at the first line of a constructor to call another constructor.

347

Overloading Constructors

348

The Object Class

The Object class is the root of all classes in Java

A class declaration with no extends clause, implicitly uses extends the Object

public class Employee {

...

}

is equivalent to:

public class Employee extends Object {

...

}

349

The == Operator Compared with the equals Method

The == operator determines if two references are identical to each other (that is, refer to the same object).

The equals method determines if objects are equal but not necessarily identical.

The Object implementation of the equals method uses the == operator.

User classes can override the equals method to implement a domain-specific test for equality.

Note: You should override the hashCodemethod if you override the equals method.

350

The toString Method

Converts an object to a String.

Used during string concatenation.

Override this method to provide information about a user-defined object in readable format.

Primitive types are converted to a String using the wrapper classs toString static method.

351

Wrapper Classes

Advanced Class Features

353

The Static Keyword

The static keyword is used as a modifier on variables, methods, and nested classes.

The static keyword declares the attribute or method is associated with the class as a whole rather than any particular instance of that class.

Thus static members are often called class members, such as class attributes or class methods.

354

Class Attributes

Are shared among all instances of a class

Can be accessed from outside the class without an instance of the class (if marked as public)

355

Class Attributes

You can invoke static method without any instance of the class to which it belongs.

356

Static Initializers

A class can contain code in a static block that does not exist within a method body.

Static block code executes only once, when the class is loaded.

A static block is usually used to initialize static (class) attributes.

357

Abstract Classes

An abstract class models a class of objects where the full implementation is not known but is supplied by the concrete subclasses.

358

Interfaces

A public interface is a contract between client code and the class that implements that interface.

AJava interface is a formal declaration of such a contract in which all methods contain no implementation.

Many unrelated classes can implement the same interface.

A class can implement many unrelated interfaces.

Syntax of a Java class:

::=

class [extends ]

[implements [,]* ] {

*

}

359

Interfaces

360

Java Language Basics

361

Unit Objectives

After completing this unit, you should be able to:

Apply the concept of inheritance

Define a subclass and a superclass

Explain overriding methods

Describe the principle of dynamic binding

Explain polymorphism

Define abstract classes and interfaces

362

Review.

Classes in Java may have methods and attributes.

Methods define actions that a class can perform.

Attributes describe the class.

363

Review.

364

Review.

The phrase "to create an

instance of an object means

to create a copy of this object

in the computer's memory

according to the definition of

its class.

365

Review.

366

Review.

367

Review.

Inheritance - a Fish is Also a Pet

368

Review.

369

Review.

370

Inheritance.

Is a mechanism for defining a new class in terms of an existing class.

Allows you to group related classes so that they can be managed collectively.

Promotes reuse.

Allows you hide or override inherited methods.

Relevant terms: generalization, specialization, override.

371

Inheritance.

372

Inheritance.

Inheritance is often represented as a tree. Moving down the

tree, classes become more specialized, more honed toward

An application. Moving up the tree, classes are more

general;

they contain members suitable for many classes but are

often

not complete.

373

Inheritance Hierarchy.

374


375


376


377


Animal

Cat Dog Horse

378


379

The Constructor Process.

380


381


382

Overriding.

383

Polymorphism.

384

Dynamic Binding.

Dynamic Binding is when an operation and operands don't

find

each other until execution time.

Dynamic binding works with polymorphism and inheritance to

make systems more malleable.

Dynamic binding happens when the JVM resolves which

method to call at run time and is a form of polymorphism.

Dynamic binding is based on the type of the object, not the

type of the object reference.

385

Dynamic Binding and Polymorphism.

386


387


388


389

Upcast/Downcast.

390

Abstract Classes.

391

Abstract Classes.

392

Interfaces.

Interfaces encapsulate a coherent set of services and attributes, for example, a role.

Objects in order to participate in various relationships, need to state that they have the capability to fulfill a

particular role.

All interfaces must have either public or default access.

All methods (if any) in an interface are public, and abstract (either explicitly or implicitly).

All fields (if any) in an interface are public, static, and final (either explicitly or implicitly).

393

Interfaces.

394

Interfaces.

395

Interfaces.

Exceptions and Exceptions Handling

397

An exception is an event or condition that disrupts the normal flow of execution in a program

Exceptions are errors in a Java program

The condition causes the system to throw an exception

The flow of control is interrupted and a handler will catch the exception

Exception handling is object-oriented

It encapsulates unexpected conditions in an object

It provides an elegant way to make programs robust

It isolates abnormal from regular flow of control

Exceptions.

398

A JVM can detect unrecoverable conditions

Examples:

Class cannot be loaded

Null object reference used

Both core classes and code that you write can throw exceptions

Examples:

IO error

Divide by zero

Data validation

Business logic exception

Exceptions terminate

execution unless they

are handled by the

program

Exceptions.

399

Throwable is the base class, and provides a common interface and implementation for most exceptions

Error indicates serious problems that a reasonable application should not try to catch, such as:

VirtualMachineError

CoderMalfunctionError

Exception heads the class of conditions that should usually be either caught or specified as thrown

A RuntimeException can be thrown during the normal operation of the JVM

Methods may choose to catch these but need not specify them as thrown

Examples:

ArithmeticException

BufferOverflowException

The Exceptions Hierarchy

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The Exceptions Hierarchy

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Checked exceptions must be either handled in the method where they are generated, or delegated to the calling method

Handling Exceptions

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throws

A clause in a method declaration that lists exceptions that may be delegated up the call stack

Example: public int doIt() throws SomeException,

try

Precedes a block of code with attached exception handlers

Exceptions in the try block are handled by the exception handlers

catch

A block of code to handle a specific exception

finally

An optional block which follows catch clauses

Always executed regardless of whether an exception occurs

throw

Launches the exception mechanism explicitly

Example: throw (SomeException)

Keywords

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To program exception handling, you must use try/catch blocks

Code that might produce a given error is enclosed in a try block

The catch clause must immediately follow the try block

try/catch blocks

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The clause always has one argument that declares the type of exception to be caught

The argument must be an object reference for the class Throwable or one of its subclasses

Several catch clauses may follow one try block

The catch clause

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Example

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Example

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Optional clause that allows cleanup and other operations to occur whether an exception occurs or not

May have try/finally with no catch clauses

Executed after any of the following:

try block completes normally

catch clause executes

Even if catch clause includes return

Unhandled exception is thrown, but before execution returns to calling method

The finally clause

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Example

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It may be necessary to handle exceptions inside a catch or finally clause

For example, you may want to log errors to a file, but all I/O operations require IOException to be caught.

Do this by nesting a try/catch (and optional finally) sequence inside your handler

Nested Exception Handling

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Not to be confused with keyword throws

Can be used in a try block when you want to deliberately throw an exception

You can throw a predefined Throwable object or your own Exception subtype

Create a new instance of the exception class to encapsulate the condition

The flow of the execution stops immediately after the throw statement, and the next statement is not reached

A finally clause will still be executed if present

The throw keyword

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What happens when something goes wrong in the JVM?

It throws an error derived from Error depending on the type of problem

What happens if RuntimeException is thrown?

Methods are not forced to declare RuntimeException in their throws clauses; the exception is passed to the JVM

The JVM does the necessary cleaning and terminates the application or applet

Handling runtime exceptions

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An assertion is a Java statement that allows you to test your assumptions about your program

In a traffic simulator, you might want to assert that a speed is positive, yet less than a certain maximum

An assertion contains a boolean expression that you believe will be true when the assertion executes if not true, the system throws an error

By verifying that the boolean expression is true, the assertion confirms your assumptions about the behavior of your program, increasing your

confidence that the program is free of errors

Benefits:

Writing assertions while programming is a quick and effective way to detect and correct bugs

Assertions document the inner workings of your program, enhancing maintainability

Assertions

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Two forms:

assert ;

assert : ;

If the boolean expression is false:

Form 1 throws an AssertionError with no message

Form 2 throws an AssertionError with a message defined by evaluating the second expression

Assertion checking is disabled by default.

Must be enabled at Java command line using the enableassertions switch

Assertions can be enabled or disabled on a package-bypackage or class-by-class basis

assert statements are ignored if not enabled

Using assertions

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Do not use assertions:

For argument checking in public methods

To do any work that your application requires for correct operation

Use assertions to test:

Internal invariants (values that should not change)

For example, place default: assert false at the end of switch statements with no default

Control-flow invariants

For example, place assert false at locations that should never be reached

Preconditions, postconditions, and class invariants

For example, argument checking in private methods

When to use assertions

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Do not use assertions:

For argument checking in public methods

To do any work that your application requires for correct operation

Use assertions to test:

Internal invariants (values that should not change)

For example, place default: assert false at the end of switch statements with no default

Control-flow invariants

For example, place assert false at locations that should never be reached

Preconditions, postconditions, and class invariants

For example, argument checking in private methods

When to use assertions

Building Java Guis

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Abstract Window Toolkit (AWT).

Provides graphical user interface (GUI) components that are used in all Java applets and applications.

Contains classes that can be composed or extended. Classes can also be abstract.

Ensured that every GUI component that is displayed on the screen is a subclass of the abstract class Component or MenuComponent.

Has Container, which is an abstract subclass of Component and includes two subclases:

Panel

Window

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Abstract Window Toolkit (AWT).

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Containers.

Add components with the add methods.

The two main types of containers are Window and Panel.

A Windows is a free floating window on the display.

A Panel is a container of GUI components that must exists in the context of some other container, such as a window or applet.

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Containers.

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The Component class.

The Component class defines the attributes and behavior common to all visual components used in the user interface. It is an abstract class. It also defines

the way in which applications and applets can interact with users by capturing

events.

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Frames.

Are a subclass of Window

Have title and resizing corners

Are initially invisible, use setVisible(true) to expose the frame

Have BorderLayout as the default layout manager

Use the setLayout method to change the default layout manager.

Frame inherits its characteristics from the Container class so you can add

Components to a frame using the add method.

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Frames.

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Frames.

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Frames.

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Frames.

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Frames.

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Frames.

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The Panel Class.

The Panel class is a container on which components can be

placed.

After you create a Panel, you must add it to a Window or

Frame. This is done using the add method of the Container

class. Then set the frame to be visible so that the frame and

panel are displayed.

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The Panel Class.

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The Panel Class.

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Containers Layouts.

FlowLayout