Java Programming, Second Edition

1
Java Programming, Second Edition

• Chapter Three
• Using Methods, Classes, and Objects

2
In this chapter, you will

• Create methods with no arguments, a single
  argument, and multiple arguments
• Create method that return values
• Learn about class concepts
• Create a class
• Use instance methods
• Declare objects
• Organize classes
• Use constructors

3
Methods

• A method is a series of statements that carry out
  a task
• Any class can contain an unlimited number of
  methods

4
Methods

• Methods must include
• A declaration
• An opening curly brace
• A body
• A closing brace

5
Methods

• Method declarations contain
• Optional access modifiers
• The return type for the method
• The method name
• An opening parenthesis
• An optional list of method arguments
• A closing parenthesis

6
Access Modifiers

• Access modifiers for a method can be
• public- most often methods are given public
  access
• private cant be used by all classes
• protected cant be used by all classes
• static objects arent required in order for
  these methods to be used

7
Creating Methods that Require a Single Argument

• Arguments- Are communications to a method
  (parameters)
• Implementation hiding- Allows that the invoking
  program must know the name of the method and what
  type of information to send it, but the calling
  program does not need to know how the method
  works

8
Creating Methods that Require a Single Argument

• The method declaration must include
• The type of the argument
• A local name for the argument
• For example
• public void predictRaise (double moneyAmount)

9
Calling a Method

• Can use either argument when calling the method
  sending the parameter
• Constant
• Variable
• predictRaise(472.55)
• predictRaise(mySalary)

10
Creating Methods that Require Multiple Arguments

• Methods can require more than one argument
• Pass multiple arguments by
• Listing the arguments in the call to the method
• Separating them with commas
• The declaration for a method that receives two or
  more arguments must list the type for each
  argument separately
• public void predictRaise(double
  moneyAmount, float balance)

11
A Methods Return Type

• The return type is known as the methods type
• For example
• public static void nameAndAddress()
• This method is public and void means it returns
  no value

12
Return Statement

• The return statement is the last statement in a
  method
• return balance
• Usually you use the returned value, but this is
  not required
• The return statement sends back a value to the
  calling program

13
Learning about Class Concepts

• In object-oriented programming
• Everything can be considered an object
• An object is an instantiation of a class, or a
  tangible example of a class
• Every object is a member of a class
• Your desk is an object and is a member of the
  Desk class
• These statements represent is-a relationships

14
Learning about Class Concepts

• The concept of a class is useful because
• Objects inherit attributes from classes
• All objects have predictable attributes because
  they are members of certain classes
• You must
• Create the classes of objects from which objects
  will be instantiated (Car class)
• Write other classes to use the objects (a
  program/class is written to drive to the airport
  uses the car class to create a car object to
  drive)

15
Creating a Class

• You must
• Assign a name to the class
• Determine what data and methods will be part of
  the class
• To begin, create a class header with three parts
• An optional access modifier
• The keyword class
• Any legal identifier you choose for the name of
  the class

16
Access modifiers for Classes

• Access modifiers include
• public
• This is the most used modifier
• Most liberal form of access
• Can be extended or used as the basis for other
  classes
• final- used only under special circumstances
• abstract- used only under special circumstances

17
Field/Variable Modifiers

• Private
• Public
• Static
• Final

18
Field Modifiers

• Private
• No other classes can access a fields values
• Only methods of the same class are allowed to
  set, get, or otherwise use private variables
• Highest level of security
• Also called information hiding
• Provides a means to control outside access to
  your data

19
Using Instance Methods

• Methods used with object instantiations are
  called instance methods
• You can call class methods without creating an
  instance of the class (these methods have static
  as a modifier)
• Instance methods require an instantiated object.

20
Declaring Objects

• To declare an object
• Supply a type and an identifier
• Allocate computer memory for the object
• Use the new operator
• Car myCar new Car( )
• Or
• Car myCar new Car(blue, V8)
• The first example uses the defaults as the
  attribute values the second sets the color
  engine attributes

21
Organizing Classes

• Most programmers place data fields in some
  logical order at the beginning of a class
• For example, use a unique identifier for each
  employee
• empNum
• Last names and first names are organized together

22
Using Constructor Methods

• Constructor methods- Methods that establish an
  object
• Employee chauffer new Employee()
• Calls a method named Employee()
• When classes are written for the purpose of
  creating objects, they do not have a main method.
  Instead, they have a constructor method that
  sets the default attribute values.

23
Attribute Default Values

• Numeric 0
• Character Unicode \u0000
• Boolean false
• Object types null