Defining Classes and Methods

1
Defining Classes and Methods

• Chapter 4

2
Object-Oriented Programming

• Our world consists of objects (people, trees,
  cars, cities, airline reservations, etc.).
• Objects can perform actions which effect
  themselves and other objects in the world.
• Object-oriented programming (OOP) treats a
  program as a collection of objects that interact
  by means of actions.

3
OOP Design Principles

• OOP adheres to three primary design principles
• encapsulation
• polymorphism
• inheritance.

4
Introduction to Encapsulation

• The data and methods associated with any
  particular class are encapsulated (put together
  in a capsule), but only part of the contents is
  made accessible.
• Encapsulation provides a means of using the
  class, but it omits the details of how the class
  works.
• Encapsulation often is called information hiding.

5
Accessibility Example

• An automobile consists of several parts and
  pieces and is capable of doing many useful
  things.
• Awareness of the accelerator pedal, the brake
  pedal, and the steering wheel is important to the
  driver.
• Awareness of the fuel injectors, the automatic
  braking control system, and the power steering
  pump is not important to the driver.

6
Introduction to Inheritance

• Classes can be organized using inheritance.

7
Introduction to Inheritance, cont.

• A class at lower levels inherits all the
  characteristics of classes above it in the
  hierarchy.
• At each level, classifications become more
  specialized by adding other characteristics.
• Higher classes are more inclusive lower classes
  are less inclusive.

8
Introduction to Polymorphism

• from the Greek meaning many forms
• The same program instruction adapts to mean
  different things in different contexts.
• A method name, used as an instruction, produces
  results that depend on the class of the object
  that used the method.
• everyday analogy take time to recreate causes
  different people to do different activities
• more about polymorphism in Chapter 7

9
Outline

• Class and Method Definitions
• Information Hiding and Encapsulation
• Objects and Reference

10
Class and Method Definitions Outline

• Class Files and Separate Compilation
• Instance Variables
• Using Methods
• void Method Definitions
• Methods that Return a Value
• Accessing Instance Variables
• Local Variables
• Blocks
• Parameters of a Primitive Type
• Class and Method Definition Syntax

11
Basic Terminology

• Objects can represent almost anything.
• A class defines a kind of object.
• It specifies the kinds of data an object of the
  class can have.
• It provides methods specifying the actions an
  object of the class can take.
• An object satisfying the class definition
  instantiates the class and is an instance of the
  class.

12
Basic Terminology, cont.

• The data items and the methods are referred to as
  members of the class.
• We will call the data items associated with an
  object the instance variables of that object
  (i.e. that instance of the class).

13
A Class as an Outline
14
Class and Method Definition Syntax

• public class Class_Name
• Instance_Variable_Declaration_1
• Instance_Variable_Declaration_2
• Method_Definition_1
• Method_Definition_2
• ...

15
Class and Method Definition Syntax, cont.

• public Type_Name_Or_void Method_Name(Parameter_Lis
  t)
• where Parameter_List consists of
• a list of one or more formal parameter names,
  each preceded by a type and separated by commas
  or
• no formal parameters at all

16
Example

• The modifier public associated with the instance
  variables should be replaced with the modifier
  private, as we will do later in the chapter.

17
Class Files and Separate Compilation

• Each Java class definition should be in a file by
  itself.
• The name of the file should be the same as the
  name of the class.
• The file name should end in .java
• A Java class can be compiled before it is used in
  a program
• The compiled byte code is stored in a file with
  the same name, but ending in .class

18
Class Files and Separate Compilation, cont.

• If all the classes used in a program are in the
  same directory as the program file, you do not
  need to import them.

19
Using Methods

• two kinds of methods
• methods that return a single value (e.g. nextInt)
• methods that perform some action other than
  returning a single value (e.g println), called
  void methods

20
Methods That Return a Value

• example
• int next keyboard.nextInt()
• keyboard is the calling object.
• You can use the method invocation any place that
  it is valid to use of value of the type returned
  by the method.

21
Methods That Do Not Return a Value

• example
• System.out.println(Enter data)
• System.out is the calling object.
• The method invocation is a Java statement that
  produces the action(s) specified in the method
  definition.
• It is as if the method invocation were replaced
  by the statements and declarations in the method
  definition.

22
void Method Definitions

• example
• public void writeOuput()
• System.out.println(Name name)
• System.out.println(Age age)
• Such methods are called void methods.

23
Method Definitions

• All method definitions belong to some class.
• All method definitions are given inside the
  definition of the class to which they belong.
• If the definition of the method begins with
  public void, it does not return a value.
• public indicates that use is unrestricted.
• void indicates that the method does not return a
  value.

24
Method Definitions, cont.

• The parentheses following the method name contain
  any information the method needs.
• The first part of the method definition is called
  the heading.
• The remainder of the method is called the body,
  and is enclosed in braces .
• Statements or declarations are placed in the body.

25
The Method main

• A program is a class that has a method named
  main.
• The programs we have seen so far have no instance
  variables and no methods other than method main.
• Programs can have instance variables and other
  methods.

26
Defining Methods That Return a Value

• example
• public int fiveFactorial()
• int factorial 54321
• return factorial
• As before, the method definition consists of the
  method heading and the method body.
• The return type replaces void.

27
Defining Methods That Return a Value, cont.

• The body of the method definition must contain
• return Expression
• This is called a return statement.
• The Expression must produce a value of the type
  specified in the heading.
• The body can contain multiple return statements,
  but a single return statement makes for better
  code.

28
Using return in a void Method

• form
• return
• use
• to end the invocation of the method, usually
  prematurely, to deal with some problem
• caution
• Almost always, there are better ways to deal with
  a potential problem.

29
public Method Definitions

• syntax for a void method
• public void Method_Name(Parameters)
• ltstatement(s)gt
• syntax for methods that return a value
• public Return_Type Method_Name(Parameters)
• ltstatement(s), including a
• return statementgt

30
Accessing Instance Variables

• Outside the class definition, a public instance
  variable is accessed with
• the name of an object of the class
• a dot (.)
• the name of the instance variable.
• Example myBestFriend.name Lara
• Inside the definition of the same class only the
  name of the instance variable is used.
• Example name keyboard.nextLine()

31
Accessing Instance Variables, cont.

• equivalent assignment statements
• name keyboard.nextLine()
• and
• this.name keyboard.nextLine()
• The keyword this stands for the calling object -
  the object that invokes the method.

32
Local Variables

• A variable declared within a method is called a
  local variable.
• Its meaning is local to (confined to) the
  method definition.
• Variables with the same name declared within
  different methods are different variables.
• A local variable exists only as long as the
  method is active.

33
Local Variables, cont.

• class BankAccount
• class LocalVariablesDemoProgram

34
Blocks

• The terms block and compound statement both refer
  to a set of Java statements enclosed in braces
  .
• A variable declared within a block is local to
  the block.
• When the block ends, the variable disappears.
• If you intend to use the variable both inside and
  outside the block, declare it outside the block.

35
Variables in for Statements

• The loop control variable can be declared outside
  the for statement
• int n
• for (n 1 n lt10, n)
• in which case the variable n still exists when
  the for statement ends
• or

36
Variables in for Statements, cont.

• The loop control variable can be declared inside
  the for statement
• for (int n 1 n lt10, n)
• in which case the variable n ceases to exist when
  the for statement ends

37
Parameters of a Primitive Type

• Often it is convenient to pass one or more values
  into a method and to have the method perform its
  actions using those values.
• The values are passed in as arguments (or actual
  parameters) associated with the method invocation.

38
Parameters of a Primitive Type, cont.

• The method receives the values and stores them in
  its formal parameters (or simply parameters).
• A method invocation assigns the values of the
  arguments (actual parameters) to the
  corresponding formal parameters (parameters).
• This is known as the call-by-value mechanism.
• The formal parameters exist as long as the method
  is active.

39
Parameters of a Primitive Type, cont.

• Generally, the type of each argument must be the
  same as the type of the corresponding formal
  parameter.
• Java will perform automatic type conversion for
  an argument that appears to the left of a formal
  parameter it needs to match
• byte --gt short --gt int --gt long --gt float --gt
  double

40
Parameters of a Primitive Type, cont.

• An argument in a method invocation can be
• a literal such as 2 or A
• a variable
• any expression that yields a value of the
  appropriate type.
• A method invocation can include any number of
  arguments the method definition contains a
  corresponding number of formal parameters, each
  preceded by its type.

41
Parameters of a Primitive Type, cont.

• anObject.doStuff(42, 100, 9.99, Z)
• public void doStuff(int n1, int n2, double d1,
  char c1)
• arguments and formal parameters are matched by
  position
• Everything said about arguments and formal
  parameters applies to methods that return a value
  as well as to void methods.

42
Method with a Parameter
43
Information Hiding and Encapsulation Outline

• Information Hiding
• Precondition and Postcondition Comments
• The public and private Modifiers
• Encapsulation
• Automatic Documentation with javadoc
• UML Class Diagrams

44
Information Hiding

• Information overload is avoided by suppressing
  or hiding certain kinds of information, making
  the programmers job simpler and the code easier
  to understand.
• A programmer can use a method defined by someone
  else without knowing the details of how it works
  (e.g. the println method)
• He needs to know what the method does, but not
  how it does it.

45
Information Hiding, cont.

• What the method contains is not secret, and maybe
  not even interesting.
• Viewing the code does not help you use the method
  and may distract you from the task at hand.
• Designing a method so that it can be used how it
  performs its task is called information hiding or
  abstraction.

46
The public and private Modifiers

• The instance variables of a class should not be
  declared public.
• Typically, instance variables are declared
  private.
• An instance variable declared public can be
  accessed and changed directly, with potentially
  serious integrity consequences.
• Declaring an instance variable private protects
  its integrity.

47
The public and private Modifiers, cont.

• Analogy An ATM permits deposits and withdrawals,
  both of which affect the account balance, but it
  does not permit an account balance to be accessed
  and changed directly. If an account balance
  could be accessed and changed directly, a bank
  would be at the mercy of ignorant and
  unscrupulous users.

48
The private Modifier

• The private modifier makes an instance variable
  inaccessible outside the class definition.
• But within the class definition, the instance
  variable remains accessible and changeable.
• This means that the instance variable can be
  accessed and changed only via the methods
  accompanying the class.

49
Accessor and Mutator Methods

• Appropriate access to an instance variable
  declared private is provided by an accessor
  method which is declared public.
• Typically, accessor methods begin with the word
  get, as in getName.
• Mutator methods should be written to guard
  against inappropriate changes.

50
Encapsulation

• Encapsulation is the process of hiding details of
  a class definition that are not needed to use
  objects of the class.
• Encapsulation is a form of information hiding.

51
Encapsulation, cont.

• When done correctly, encapsulation neatly divides
  a class definition into two parts
• the user interface which communicates everything
  needed to use the class
• the implementation consisting of all the members
  of the class.
• A class defined this way is said to be well
  encapsulated.

52
The User Interface

• The user interface consists of
• the headings for the public methods
• the defined public constants
• comments telling the programmer how to use the
  public methods and the defined public constants.
• The user interface contains everything needed to
  use the class.

53
The Implementation

• The implementation consists of
• the private instance variables
• the private defined constants
• the definitions of public and private methods.
• The Java code contains both the user interface
  and the implementation.
• Imagine a wall between the user interface and the
  implementation.

54
Encapsulation
55
Encapsulation Guidelines

• Precede the class definition with a comment that
  shapes the programmers view of the class.
• Declare all the instance variables in the class
  private.
• Provide appropriate public accessor and mutator
  methods.

56
Encapsulation Guidelines, cont.

• Provide public methods to permit the programmer
  to use the class appropriately.
• Precede each public method with a comment
  specifying how to use the method.
• Declare methods invoked only by other methods in
  the class private.
• Use /.../ or /.../ for user interface
  comments and // for implementation comments.

57
Encapsulation Characteristics

• Encapsulation should permit implementation
  changes (improvements, modifications,
  simplifications, etc.) without requiring changes
  in any program or class that uses the class.
• Encapsulation combines the data and the methods
  into a single entity, hiding the details of the
  implementation.

58
Objects and Reference Outline

• Variables of a Class Type and Objects
• Boolean-Valued Methods
• Class Parameters
• Comparing Class Parameters and Primitive-Type
  Parameters

59
Variables

• Variables of a class type name objects, which is
  different from how primitive variables store
  values.
• All variables are implemented as memory
  locations.
• The value of a variable of a primitive type is
  stored in the assigned location.
• The value of a variable of a class type is the
  address where a named object of that class is
  stored.

60
Variables, cont.

• A value of any particular primitive type always
  requires the same amount of storage.
• example a variable of type int always requires 4
  bytes.
• An object of a class type might be arbitrarily
  large.
• An object of type String might be empty, or might
  contain 1, 120, 5280, or more characters.

61
Variables, cont.

• However, there is always a limit on the size of
  an address.
• The memory address where an object is stored is
  called a reference to the object.
• Variables of a class type behave differently from
  variables of a primitive type.

62
Variables of a Primitive Type

• Only one variable of a primitive type is
  associated with the memory location where the
  value of the variable is stored.

63
Variables of a Class Type

• However, multiple variables of a class type can
  contain the memory location where an object is
  stored.
• The object can be accessed and potentially
  changed using any of these variables.
• All these variables contain the same reference
  and name the same object.
• Variables of a class type referencing the same
  memory location are called aliases.

64
Variables of a Class Type, cont.

• When one variable of a class type is assigned to
  another variable of a compatible class type, the
  memory address is copied from one variable to
  another and the two variables become aliases of
  one another.
• The numeric values of these memory addresses
  generally are not available to the programmer nor
  are they needed by the programmer.

65
Allocating Memory for a Reference and an Object

• A declaration such as
• SpeciesFourthTry s
• creates a variable s that can hold a memory
  address.
• A statement such as
• s new SpeciesFourthTry()
• allocates memory for an object of type
  SpeciesFourthTry.

66
with Variables of a Class Type
67
with Variables of a Class Type, cont.

• When used with variables of a class type,
  tests if the variables are aliases of each other,
  not if they reference objects with identical
  data.
• To test for equality of objects in the intuitive
  sense, define and use an appropriate equals
  method.

68
Method equals

• The definition of method equals depends on the
  circumstances.
• In some cases, two objects may be equal when
  the values of only one particular instance
  variable match.
• In other cases, two objects may be equal only
  when the values of all instance variables match.
• Always name the method equals.

69
Class Parameters

• Recall
• When the assignment operator is used with objects
  of a class type, a memory address is copied,
  creating an alias.
• When the assignment operator is used with a
  primitive type, a copy of the primitive type is
  created.

70
Class Parameters, cont.

• When a parameter in a method invocation is a
  primitive type, the corresponding formal
  parameter is a copy of the primitive type.

71
Class Parameters, cont.

• When a parameter in a method invocation is a
  reference to a class type (i.e. a named object),
  the corresponding formal parameter is a copy of
  that reference (i.e. an identically valued
  reference to the same memory location).

72
Class Parameters, cont.

• Example
• if (s1.equals(s2))
• public boolean equals (Species otherObject)
• causes otherObject to become an alias of s2,
  referring to the same memory location, which is
  equivalent to
• otherObject s2

73
Class Parameters, cont.

• Any changes made to the object named otherObject
  will be done to the object named s2, and vice
  versa, because they are the same object.
• If otherObject is a formal parameter of a method,
  the otherObject name exists only as long as the
  method is active.

74
Comparing Class Parameters and Primitive-Type
Parameters

• A method cannot change the value of a variable of
  a primitive type passed into the method.
• A method can change the value(s) of the instance
  variable(s) of a class type passed into the
  method.

75
Comparing Class Parameters and Primitive-Type
Parameters, cont.
76
Comparing Class Parameters and Primitive-Type
Parameters, cont.
77
Summary

• You have become familiar with the concept of a
  class and an object that instantiates the class.
• You have learned how to define classes in Java.
• You have learned how to define and use methods in
  Java.
• You have learned how to create objects in Java

78
Summary, cont.

• You have learned how parameters work in Java.
• You have learned about information hiding and
  encapsulation.
• You have become familiar with the notion of a
  reference (to understand class variables and
  class parameters).