Chapter 6 A first look at classes

Chapter 6 Chapter 6 A First Look at Classes 2 Contents 1 Classes and objects 2 Instance Fields and Methods 3 Constructors 4 Overloading Methods and Constructors 5 Scope of Instance Fields 6 Packages a.

Chapter 6

A First Look at Classes

1 Classes and objects

2 Instance Fields and Methods

3 Constructors

4 Overloading Methods and Constructors

5 Scope of Instance Fields

6 Packages and import Statements

1 Classes and Objects

Procedures typically operate on data items that are separate from the procedures

Procedural programming is centered on creating procedures

Object oriented programming is centered on

1 Classes and Objects

An object's fields are called attributes

A class is the blueprint for an object It specifies the fields and methods a particular type of object has

From the class, one or more objects may be

created

1 Classes and Objects

Object oriented programming addresses the

problem of code/data separation through:

Only the object's methods may directly access

and make changes to the object's data

An object hides its data but allows outside code

1 Classes and Objects

Object reusability

An object is not a stand-alone program, but is

used by programs that its services

An everyday example of an object

Your alarm clock: An object

The current second (0-59)

The current minute (0-59)

The current hour (1-12)

1 Classes and Objects

Fields are merely data values that define the state

of the alarm clock is currently in

The user cannot directly manipulate these fields because they are private

To change a field's value, we must use one of the object's methods

Some of the alarm clock object's methods

Set time

Set alarm time

Turn alarm on

These methods can be activated

by users, who are outside the alarm clock.

They are public methods.

1 Classes and Objects

The alarm clock also has private methods, which are

parts of the object's private, internal workings

External entities (such as you, the user of the alarm clock) do not have directly access to the alarm

clock's private methods

The object is designed to execute these methods

automatically and hide the details from you

Some of the alarm clock object's private methods

Increment the current second

1 Classes and Objects

 Before an object is created, it must be designed by a

 A class is not an object, but it is a description of an objects.

 When the program is running, it can use the class to create,

in memory, as many objects as needed.

 Each object that is created from a class is called an

instance of the class.

1 Classes and Objects

Insect class

housefly object

mosquito

The Insect class

describes

the fields and methods

that a particular type of

object may have.

The Insect class

describes

the fields and methods

that a particular type of

object may have.

The housefly object is an instance of the Insect class It has the fields and methods described by the Insect class.

The mosquito object is an instance of the Insect class It

A primitive data type is called “primitive” because

a variable created with a primitive data type has

no built-in capabilities other than storing a value

Objects versus Primitive

Variables

String class allows you to create String

objects

In addition to storing strings, String object s

have numerous methods that perform operations

on the strings they hold

String name = “Peter”;

address “Peter”

A String object The name variable

holds the address of

a String object.

A Rectangle object will have the following fields

length: Hold the rectangle's length

width: Hold the rectangle's width

The Rectangle class will also have the following

methods

setLength: Store a value in an object's length field

setWidth: Store a value in an object's width field

Building a Simple Class Step by

Step

getWidth: Return the value in an object's width field

getArea: Return the area of the rectangle

When designing a class it is often helpful to draw

a UML diagram

UML stands for Unified Modeling Language

It provides a set of standard diagrams for

UML Diagram for a Class

The general layout of a UML diagram for a class:

The diagram is a box that is divided into three

UML Diagram for a Class

UML diagram for the Rectangle class

Rectangle length width

setLength() SetWidth() GetLength() GetWidth() getArea()

Writing the Code for a Class

Create a file named Rectangle.java

In the Rectangle.java file, we start by writing a

general class skeleton as follows

public class Rectangle

Writing the Code for the Class

Fields

We will write the code for the class's two fields

width and length We use variables of the

double data types

public class Rectangle

{

private double length;

private double width;

Writing the Code for the Class

Fields

Access specifier

private: When the private access specifier is

applied to a class member, the member cannot be accessed by the code outside the class The

member can be accessed only by methods that

are members of the same class

public: When the public access specifier is

applied to a class member, the member can be

accessed by the code inside the class or outside

Writing the setLength Method

public class Rectangle

{

private double length;

private double width;

/** The setLength method stores a value in

the length field.

@param len The value to store in length.

*/

public void setLength(double len)

{

Using Rectangle Class

Using Rectangle Class

Using Rectangle Class

Using Rectangle Class

Using Rectangle Class

Rectangle box = new Rectangle();

box.setLength(10.0);

box.setWidth(20.0);

address

length width

A Rectangle object The box variable

holds the address of

a Rectangle object.

0.0 0.0

address length

width

A Rectangle object The box variable

holds the address of

a Rectangle bject.

10.

0 0.0

address length

width

A Rectangle object The box variable

holds the address of

a Rectangle object.

10.

0 20.

0

Accessor and Mutator Methods

 It is a common practice to make all of a class's fields

private and to provide public methods for accessing and changing those fields.

 A method that gets a value from a class's field but does

not change it is known an accessor method (getter).

 A method that stores a value in a field or changes the

value of a field is known as a mutator method (setter).

 In the Rectangle class

Avoiding Stale Data

In the Rectangle class, the getArea method

returns the result of a calculation

The area of the rectangle is not stored in a field Why?

The area is not stored in a field because it could potentially become stale

When the value of an item is dependent on other data and that item is not updated when the other data is changed, it is said that the item has

become stale.

Avoiding Stale Data

If the area is stored in a field, the value of this

field becomes incorrect as soon as either the

length or width fields changed

When designing a class, you should take care not

to store in a field calculated data that can

potentially become stale

Instead, provide a method that returns the result

of the calculation

Data Type and Parameter Notation in UML Diagrams

The UML diagram also provides notation that you may use to indicate

Data types of fields, methods

Layout of Class Members

Typical layout of class members

public class ClassName

{

}

Field declarations

Methods definitions

2 Instance Fields and Methods

Each instance of a class has its own set of fields,

which are known as instance fields or instance

variables.

You can create several instances of a class and store different values in each instance's fields

The methods that operate on an instance of a

class are known as instance methods.

Instance methods do not have the keyword

static in their headers

2 Instance Fields and Methods

Rectangle kitchen = new Rectangle();Rectangle bedroom = new Rectangle();Rectangle den = new Rectangle();

address

length width

A Rectangle object

The box variable holds the address of

a Rectangle bject.

0.0 0.0

address length

width

The bedroom variable holds the address of

a Rectangle bject.

0.0 0.0

A Rectangle object

0

14.

0 The bedroom

variable hold the address of

a Rectangle object.

The kitchen variable hold the address of

a Rectangle object.

6.10 Assume that r1 and r2 are variables that

reference Rectangle objects, and the following statements are executed:

r1.setLength(5.0);

r2.setLength(10.0);

r1.setWidth(20.0);

r2.setWidth(15.0);

Fill in the boxes in the figure that represent each object's

length and width fields.

Checkpoint

address

length width

0.0 0.0

address length

width

0.0 0.0

A Rectangle object

A Rectangle object r1

r2

0.0 0.0

A Rectangle object

20.

0

3 Constructors

A constructor

 Has the same name as the class.

A method that is automatically called when an

instance of a class is created

Performs initialization or setup operations

Storing initial values in instance fields

It is called a constructor because it helps

construct an object

The constructor's header does not specify a

return type

3 Constructors

Write a constructor in the class Rectangle:

The constructor has the name Rectangle

The constructor has two arguments

The length of the rectangle

The width of the rectangle

Two arguments are then assigned to the lengthand width fields

3 Constructors

This constructor accepts two arguments

3 Constructors

Constructor's header does not specify a return

type – not even void

The method header for a constructor

3 Constructors

Uninitialized Local Reference

Variables

Reference variables can be declared without

being initialized

Rectangle box;

Does not create a Rectangle object

Only declares a variable named box

Does not reference to any Rectangle object

It is an uninitialized local reference variable.

box = new Rectangle(7.0, 14.0);

Uninitialized Local Reference

Variables

Rectangle box;

box = new Rectangle(7.0, 14.0);

Rectangle box = new Rectangle(7.0, 14.0);

Be careful when using uninitialized reference

variables:

Reference variables must be initialized or

assigned a value before they can be used

The Default Constructor

When an object is created, its constructor is

always called.

If we do not write a constructor

Java automatically provides one

It is known as default constructor.

The default constructor

Does not accept arguments

Sets all object's numeric fields to 0

The Default Constructor

We wrote no constructor for the Rectangle

class

Rectangle r = new Rectangle();

// Calls the default constructor

Now we wrote our own constructor for the

Rectangle class

Rectangle r = new Rectangle(); // Error!

Java does not provide the default constructor

We must call the constructor we wrote

No-Argument Constructor

A constructor that does not accept arguments is

known as a no-argument constructor.

The default constructor is a no-argument

The String Class Constructor

The String class has a constructor

Accepts a string literal as its argument

Uses the string literal to initialize the String

object

String name = new String(“Pham Dai Xuan”);

Java provides the shortcut notation for creating

and initializing String objects

String name = “Pham Dai Xuan”;

6.11 How is a constructor named?

A constructor must have the same name as the class

6.12 What is a constructor's return type?

A constructor does not have a return type

b) Write a statement that create an object and

passes the value 25 as an argument to the

constructor

Overloading Methods and

Constructors

Problem: Write a Java class that has seven

methods

1) Calculate the sum of two integer numbers

2) Calculate the sum of two double numbers

3) Calculate the sum of an integer number and a double number

4) Calculate the sum of a double number and an integer number

Overloading Methods and

Constructors

Overloading Methods and

Constructors

Overloading Methods and

Constructors

Overloading Methods

The Addition class is not easy to use

We have to choose the right method for each

We use the same + operator as long as

their operands are different.

Overloading Methods

In Java, two or more methods in a class may

have the same name as long as their parameter lists are different

Overloading methods

When a method is overloaded, it means that

multiple methods in the same class have the

same name, but use different types of parameters

Overloading Methods

Overloading Methods

Overloading Methods

Overloading Methods

Java uses a method's signature to distinguish it

from other methods of the same name

A method's signature

The method name

The data type of the method's parameters

add(int, int)add(String, String)

add(double, String)add(String, double)

Overloading Constructors

Constructors can be overloaded

A class can have more than one constructors

The rules for overloading constructors are the same for overloading methods

 Rectangle box1 = new Rectangle();

Rectangle box2 = new Rectangle(5.0, 10.0);

The BankAccount Class

A bank account is simulated by an object of the BankAccount class The BankAccount object allows us

To have a starting balance

To make deposits

To make withdrawals

To get the current balance

The BankAccount Class

UML class diagram

BalanceAccount

- balance: double + BankAccount():

+ BankAccount(startBalance: double):

+ BankAccount(str: String):

+ deposit(amount: double): void + deposit(str: String): void + withdraw(amount: double): void + withdraw(str: String): void + setBalance(b: double): void + setBalance(str: String): void + getBalance(): double

The BankAccount Class

The BankAccount Class

The BankAccount Class

The BankAccount Class

The BankAccount Class

The BankAccount Class

5 Scope of Instance Fields

A variable's scope is the part of a program where the variable may be accessed by its name

Location of a variable's declaration determines

the variable's scope

Instance fields can be accessed by any instance method in the same class as the field

If the instance field is declared with the public

However, we can have a local variable or a

parameter variable with the same name as a field

The name of the local variable or parameter

variable shadows the name of the field

For example, assume that the Rectangle

class's setLength method had been written in

the following manner:

public void setLength(double len)

{

double length; // local variable

length = len; // length : local

Checkpoint

6.14 Is it required that overloaded methods have different return values, different parameter lists, or both?

Different parameter lists

6.15 What is a method's signature?

A method's signature is used to distinguish it from other methods of the same name

A method's signature consists of

Method name

Data types of the method's parameters in order

Look at the following class:

public class Checkpoint

Checkpoint

What will the following code display?

Checkpoint cp = new Checkpoint();

cp.message(“1”);

cp.message(1);

This is the 2nd version of the method

This is the 1st version of the method

6.17 How many default constructors may a class have?

1

Packages and import Statements

The classes in the Java API are organized into

packages An import statement tells the

compiler which package a class is located in

A package is simply a group of related classes

Each package has a name

We have to import an API class in order to use it