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Transcript of 610 Chapter 02
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Comprensin de las
definiciones de clase
Mirando dentro de las clases
5.0
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Conceptos principales a
tratarse campos
constructores
mtodos parmetros
instrucciones de asignacin
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Mquinas de entradas - una
visin externa Explorando el comportamiento de una mquinatpica de entradas.
Utilice el proyecto ingenuo-ticket-mquina.
Mquinas suministran entradas de un precio fijo. Cmo se determina ese precio?
Cmo es el "dinero" dentr en una mquina?
Cmo una mquina de hacer un seguimientodel dinero que se ingresa?
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Las mquinas del boleto
demostracin
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Mquinas de entradas - una
vista interna La interaccin con un objeto nos da
pistas sobre su comportamiento.
Mirando el interior nos permitedeterminar cmo se proporciona oimplement ese comportamiento.
Todas las clases de Java tienen unavisin interna de aspecto similar.
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Estructura de clases bsica
public class TicketMachine
{
Parte interna omitirse.
}
public class ClassName
{
Los campos
Constructores
Mtodos
}
La capa exterior deTicketMachine
El contenido
interno de unaclase
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Palabras clave
Palabras con un significado especialen el lenguaje:
pblica clase
privado
int Tambin conocido como palabras
reservadas.
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Los campos Los campos almacenan
los valores de un objeto. Tambin son conocidos
como variables deinstancia.
Los campos definen elestado de un objeto. Utilice Inspeccione para
ver el estado. Algunos valores cambian
a menudo. Algunos cambios rara vez
(o nunca).
public class TicketMachine{
private int price;
private int balance;
private int total;
Further details omitted.
}
private int price;
visibility modifier type variable name
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Constructores
Inicialice un objeto.
Tener el mismo nombre que su clase.
Estrecha asociacin con los campos.
Guarde los valores iniciales en los campos. Valores de los parmetros externos de este.
public TicketMachine(int cost){
price = cost;
balance = 0;
total = 0;
}
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Passing data via parameters
Los parmetros son otro
tipo de variable. Los
parmetros son otro
tipo de variable.
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Asignacin
Los valores se almacenan en campos(y otras variables) a travs de
sentencias de asignacin:variable = expression;
price = cost;
A variable almacena un solo valor,por lo que cualquier valor anterior sepierde.
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La eleccin de los nombres devariables
Hay mucho de la libertad sobre la eleccin delos nombres. salo con sabidura!
Elegir nombres expresivos para hacer el cdigo
ms fcil de entender:price, amount, name, age, etc.
Evite una sola letra o nombres crpticos:
w, t5, xyz123
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Conceptos principales atratarse
mtodos
incluyendo mtodos de acceso y
mutadores sentencias condicionales
concatenacin de cadenas
variables locales
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Mtodos
Mtodos implementan el comportamiento de losobjetos.
Mtodos tienen una estructura coherente
compuesta de una cabecera y un cuerpo. Mtodos de acceso proporcionan informacin acerca
de un objeto.
Mtodos de mutador alteran el estado de un objeto.
Otros tipos de mtodos de lograr una variedad detareas.
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Metodo estructura
La cabecera ofrece la firma del mtodo :public int getPrice()
La cabecera nos dice : el nombre del mtodo lo parmetros que toma si se devuelve un resultado su visibilidad a los objetos de otras clases
El cuerpo incluye sentencias del mtodo.
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Mtodos de acceso (GET)
public int getPrice()
{
return price;
}
tipo deretorno
nombre del mtodo
ista deparmetros(empty)
inicio y final de un cuerpo de mtodo(bloque)
sentencia return
modificador de visibilidad
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Mtodos de acceso
Un mtodo de acceso siempre tieneun tipo de retorno que no es nula.
Un mtodo de acceso devuelve unvalor (resultado) del tipo dado en lacabecera.
El mtodo contiene una sentencia
return para devolver el valor. NB: Volviendo no imprime!
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Pruebapublic class Maquinacocacola{
private price;
public CokeMachine(){
price = 300
}
public int getPrice
{
return Price;
}}
;
()
int
-
Qu est
mal aqu?
(hay cinco
errores!)
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mtodos mutadores
Tener una estructura mtodo similar:cabecera y el cuerpo.
Se utiliza para mutar (es decir, elcambio) el estado de un objeto. Conseguido a travs de cambiar el
valor de uno o ms campos.
Normalmente contienen instrucciones deasignacin.
A menudo reciben parmetros.
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Mutator methods
Objects First with Java - A Practical Introduction using BlueJ, David J. Barnes, Michael Klling
public void insertMoney(int amount)
{
balance = balance + amount;
}
return typemethod name parameter
visibility modifier
assignment statementfield being mutated
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setmutator methods
Fields often have dedicated setmutator methods.
These have a simple, distinctiveform: voidreturn type method name related to the field name
single parameter, with the same type asthe type of the field
a single assignment statement
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A typical setmethod
public void setDiscount(int amount)
{
discount = amount;
}
We can infer that discount
is a field of type int, i.e:
private int discount;
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Protective mutators
A set method does not have to assignthe parameter to the field.
The parameter may be checked forvalidity and rejected ifinappropriate.
Mutators thereby protect fields. Mutators support encapsulation.
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Printing from methods
Objects First with Java - A Practical Introduction using BlueJ, David J. Barnes, Michael Klling
public void printTicket()
{
// Simulate the printing of a ticket.
System.out.println("##################");
System.out.println("# The BlueJ Line");System.out.println("# Ticket");
System.out.println("# " + price + " cents.");
System.out.println("##################");
System.out.println();
// Update the total collected with the balance.total = total + balance;
// Clear the balance.
balance = 0;
}
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String concatenation
4 + 59
"wind" + "ow""window" "Result: " + 6
"Result: 6" "# " + price + " cents"
"# 500 cents"
Objects First with Java - A Practical Introduction using BlueJ, David J. Barnes, Michael Klling
overloading
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Quiz
System.out.println(5 + 6 + "hello");
System.out.println("hello" + 5 + 6);
Objects First with Java - A Practical Introduction using BlueJ, David J. Barnes, Michael Klling
11hello
hello56
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Method summary
Methods implement all object behavior.
A method has a name and a return type. The return-type may be void.
A non-voidreturn type means the method willreturn a value to its caller.
A method might take parameters.
Parameters bring values in from outside for themethod to use.
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Reflecting on the ticketmachines
Their behavior is inadequate inseveral ways: No checks on the amounts entered. No refunds.
No checks for a sensible initialization.
How can we do better? We need more sophisticated behavior.
Objects First with Java - A Practical Introduction using BlueJ, David J. Barnes, Michael Klling
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Making choices in everyday life
If I have enough money left, then Iwill go out for a meal
otherwise I will stay home and watcha movie.
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Making a choice in everyday life
if(I have enough money left) {
go out for a meal;
}
else {stay home and watch a movie;
}
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Making choices in Java
Objects First with Java - A Practical Introduction using BlueJ, David J. Barnes, Michael Klling
if(perform some test) {Do these statements if the test gave a true result
}
else {
Do these statements if the test gave a false result
}
ifkeywordboolean condition to be tested
actions if condition is true
actions if condition is falseelsekeyword
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Making a choice in theticket machine
Objects First with Java - A Practical Introduction using BlueJ, David J. Barnes, Michael Klling
public void insertMoney(int amount)
{
if(amount > 0) {
balance = balance + amount;
}
else {
System.out.println(
"Use a positive amount: " +
amount);}
}
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How do we write'refundBalance'?
Objects First with Java - A Practical Introduction using BlueJ, David J. Barnes, Michael Klling
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Variables a recap
Fields are one sort of variable. They store values through the life of an object. They are accessible throughout the class.
Parameters are another sort of variable: They receive values from outside the method. They help a method complete its task. Each call to the method receives a fresh set of
values. Parameter values are short lived.
Objects First with Java - A Practical Introduction using BlueJ, David J. Barnes, Michael Klling
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Local variables
Methods can define their own, localvariables: Short lived, like parameters.
The method sets their values unlikeparameters, they do not receive externalvalues.
Used for temporarycalculation and storage.
They exist only as long as the method is beingexecuted. They are only accessible from within the
method.
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Scope highlighting
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Scope and lifetime
Each block defines a new scope. Class, method and statement.
Scopes may be nested: statement block inside another block
inside a method body inside a classbody.
Scope is static (textual). Lifetime is dynamic (runtime).
Objects First with Java - A Practical Introduction using BlueJ, David J. Barnes, Michael Klling
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Local variables
Objects First with Java - A Practical Introduction using BlueJ, David J. Barnes, Michael Klling
public int refundBalance()
{
int amountToRefund;
amountToRefund = balance;
balance = 0;
return amountToRefund;
}
A local variable
No visibilitymodifier
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Scope and lifetime
The scope of a local variable is theblock in which it is declared.
The lifetime of a local variable is thetime of execution of the block inwhich it is declared.
The scope of a field is its whole class. The lifetime of a field is the lifetime
of its containing object.
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Review (1)
Class bodies contain fields,constructors and methods.
Fields store values that determine anobjects state. Constructors initialize objects
particularly their fields. Methods implement the behavior of
objects.
Objects First with Java - A Practical Introduction using BlueJ, David J. Barnes, Michael Klling
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Review (2)
Fields, parameters and local variablesare all variables.
Fields persist for the lifetime of anobject. Parameters are used to receive values
into a constructor or method. Local variables are used for short-lived
temporary storage.
Objects First with Java - A Practical Introduction using BlueJ, David J. Barnes, Michael Klling
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Review (3)
Methods have a return type.
void methods do not return anything.
non-voidmethods return a value. non-voidmethods have a return
statement.
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Review (4)
Correctbehavior often requiresobjects to make decisions.
Objects can make decisions viaconditional (if) statements.
A true-or-false test allows one of two
alternative courses of actions to betaken.
