4-1 Writing Classes (Chapter 4) We've been using predefined classes. Now we will learn to write our...

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Writing Classes (Chapter 4)• We've been using predefined classes. Now we will

learn to write our own classes to define objects

• Chapter 4 focuses on:

– class definitions– instance data– constructors– encapsulation and Java modifiers– method declaration and parameter passing

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Classes, Objects, & Methods

• Object-oriented programming uses classes, objects, and methods as basic programming components.

• These components help to– organize a large program into small modules– design and think about an intricate program– find and remove errors (bugs)

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predefined classes

• In Java, we have used predefined classes defined in the Java standard class library such as Scanner, Random, and String.

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Writing Classes

• Now we will begin to design programs that rely on classes that we write ourselves

• True object-oriented programming is based on defining classes that represent objects with well-defined characteristics and functionality

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Classes and Objects

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Classes and Objects

• We can represent a bank account in software by designing a class called Account that models this state and behavior

– The class serves as the blueprint for a Account object

• We can then instantiate as many Account objects as we need for any particular program

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Classes

• A class can contain data declarations and method declarations

double balance; Data declarations

Method declarations

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Classes

• An object has state and behavior

• The values of the data define the state of an object created from the class

• The functionality of the methods define the behaviors of the object

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Bank Account Example

• We’ll represent a bank account by a class named Account

• It’s state can include the account number, the current balance, and the name of the owner

• An account’s behaviors (or services) include deposits and withdrawals, and adding interest

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Driver Programs

• A driver program drives the use of other, more interesting parts of a program

• Driver programs are often used to test other parts of the software

• The Transactions class contains a main method that drives the use of the Account class, exercising its services

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Bank Account Example

• To compile and run

– Place Account.java and Transactions.java in the same folder

– compile Account.java

– compile Transactions.java

– run Transactions

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Constructor

• A constructor is a special method that is used to set up an object when it is initially created

• A constructor has the same name as the class

• The programmer does not have to define a constructor for a class: Each class has a default constructor that accepts no parameters

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Constructor

• Note that a constructor has no return type specified in the method header, not even void

• A common error is to put a return type on a constructor, which makes it a “regular” method that happens to have the same name as the class

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public Account(…)

Account acct1 = new Account ("Ted Murphy", 72354, 102.56);

main

Constructor

Transactions.java Account.java

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Instance Data: Bank Account Example

• The objects of a class share the method definitions, but each object has its own data space

acct1 72354acctNumber

102.56balance

name “Ted Murphy”

acct2 69713acctNumber

40.00balance

name “Jane Smith”

Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

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Instance Data

• A class declares the type of the data, but it does not reserve any memory space for it

• Every time a Account object is created, a new name variable is created as well

• Instance variables, declared at the class level, exists as long as the object exists

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UML(Unified Modeling Language) Diagrams

• UML(Unified Modeling Language) diagrams show relationships among classes and objects

• A UML class diagram consists of one or more classes, each with sections for the class name, attributes (data), and operations (methods)

• A dotted arrow shows that one class uses the other (calls its methods)

Transactions

main (args : String[]) : void

Accountname: String…

deposit(double amount) : double…

Exercise

• Write a class called Dog. The Dog class assuming a Dog object is described by three pieces of instance data: name (a String), breed (a String), age(an int). – Write a constructor that has three parameters

—a String containing the name of the dog, a String containing the breed of the dog, and a int containing the age of the dog.

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Method

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Method Declarations

• A method declaration specifies the code that will be executed when the method is invoked (called)

• When a method is invoked, the flow of control jumps to the method and executes its code

• When complete, the flow returns to the place where the method was called and continues

• The invocation may or may not return a value, depending on how the method is defined

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deposit

acct1.deposit(25.85);

main

Method Control Flow

Transactions.java Account.java

• The called method is often part of another class or object

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Method Header

• A method declaration begins with a method header

int add(int num1, int num2)

methodname

returntype

parameter list

The parameter list specifies the typeand name of each parameter

The name of a parameter in the methoddeclaration is called a formal parameter

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Method Body

• The method header is followed by the method body


{ int sum = num1 + num2;

return sum;}

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The return Statement

• The return type of a method indicates the type of value that the method sends back to the calling location

• A method that does not return a value has a void return type, for example,

public static void main(String[] args)

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The return Statement

• A return statement specifies the value that will be returned

return expression;

• Its expression must conform to the return type



return sum;}

The return expressionmust be consistent withthe return type

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Parameters

• Formal Parameters: the names of the parameters in the header of the method declaration.

• Actual Parameters: In an invocation, the values passed into a method.

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Parameters

• When a method is called, the values of actual parameters in the invocation are copied into the formal parameters in the method header

int num = obj.add (24, 192);



return sum;}

actual parameters

formal parameters


Exercise

• Dog program continued• Add a method:

– A method ageInPersonYears that takes no parameter. The method should compute and return the age of the dog in “person years” (seven times the dog’s age).

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Encapsulation

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Encapsulation

• We can take one of two views of an object:

– internal - the details of the variables and methods of the class that defines it

– external - the services that an object provides and how the object interacts with the rest of the system

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Encapsulation

• From the external view, an object is an encapsulated entity, providing a set of specific services

• These services define the interface to the object

• The client invokes the interface methods of the object, which manages the instance data

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Encapsulation

• An encapsulated object can be thought of as a black box -- its inner workings are hidden from the client

Methods

Data

Client


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Encapsulation: Why?

• Take Bank Account as an example,

• The client of an object may request its services (call its methods) such as withdraw and deposit, but it should not have to be aware of how those services are accomplished

• Any changes to the object's state (its variables) such as balance should be made by that object's methods

• We should make it difficult, if not impossible, for a client to access an object’s variables directly

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Visibility Modifiers

• In Java, we accomplish encapsulation through the appropriate use of visibility modifiers

• A modifier is a Java reserved word that specifies particular characteristics of a method or data


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• We've used the final modifier to define constants

• Java has three visibility modifiers: public, protected, and private

• The protected modifier involves inheritance, which we will discuss later.


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• Members of a class that are declared with public visibility can be referenced anywhere, for example,

public double deposit() { … }

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• Members of a class that are declared with private visibility can be referenced only within that class, for example,

private double balance;

• Members declared without a visibility modifier have default visibility and can be referenced by any class in the same package (same directory)

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public private

Variables

Methods Provide servicesto clients

Support othermethods in the

class

Enforceencapsulation

Violateencapsulation


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• Methods that provide the object's services are declared with public visibility so that they can be invoked by clients

• A method created simply to assist a service method is called a support method

• Since a support method is not intended to be called by a client, it should not be declared with public visibility

Exercise

• Dog program: Add visibility modifiers to the instance data and method.

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Data Scope

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Data Scope

• The scope of data is the area in a program in which that data can be referenced (used)

• Data declared at the class level can be referenced by all methods in that class, for example,

private String name;

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Local Data

• Data declared within a method is called local data

• the variable sum is declared inside the add method -- it is local to that method and cannot be referenced anywhere else

sum is local data

It is created each time the method is called, and is destroyed when it finishes executing

int add(int num1, int num2){ int sum = num1 + num2;

return sum;}

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Local Data

• As we’ve seen, local variables can be declared inside a method

• The formal parameters of a method create automatic local variables when the method is invoked

• When the method finishes, all local variables are destroyed.

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More on Methods: toString, getters and setters

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The toString Method

• All classes that represent objects should define a toString method

• The toString method returns a character string that represents the object in some way

• It is called automatically when an object is concatenated to a string or when it is passed to the print/println method.

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The toString Method

• For example, in Transactions.java

System.out.println (acct1);

toString method defined in Account.java

public String toString () {

NumberFormat fmt =

NumberFormat.getCurrencyInstance();

return (acctNumber + "\t" + name + "\t" +

fmt.format(balance));

}

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Accessors and Mutators

• Because instance data is private, a class usually provides services to access and modify data values

• An accessor method returns the current value of a variable

• A mutator method changes the value of a variable

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• The names of accessor and mutator methods take the form getX and setX, respectively, where X is the name of the value

• They are sometimes called “getters” and “setters”

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• Examples:

//Returns the current balance of the account public double getBalance () {

return balance; }

// Sets name of a student public void setName (String newName) {

name = newName; }

Exercise

• Dog program: Add a toString method that returns a string containing the name, breed, the age of the dog, and the age in person years in a format similar to the following:

Name: yoyo

Breed: shih tzu

Age: 5

Age in person years: 35

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Exercise

• Dog program: • Complete the driver program that creates a

dog object and then implicitly calls the toString() method.

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Readings and Assignments

• Reading: Chapter 4.1-4.5 • Lab Assignment: Java Lab 7

• Exercises and Programming Projects:– After Chapter Exercises

• EX 4.1, 4.5, 4.6, 4.7

– Programming Projects• PP 4.1, 4.4, 4.5, 4.6, 4.7, 4.8

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