Objects Classes and Inheritance

1
Objects Classes and Inheritance

• Chapters 4 and 5

2
Vocabulary (1)

• A class is a template or blueprint from which
  objects are created
• All Java code exists inside a class NO
  EXCEPTIONS
• We don't have Module types and other types like
  VB .NET
• Encapsulation combines data and behavior into a
  single package
• Encapsulation supplies black box behavior
• Object is the Java superclass and is analogous to
  the .NET System.Object class
• Classes extend other classes through inheritance

3
Vocabulary (2)

• The data in an object are called instance fields
• Static (shared) fields are shared among multiple
  class instances
• No matter the number of instances of a class
  there exists one and only one copy of a static
  fields
• Functions that act on data and change state are
  called methods

4
What's an Object

• An object has three key characteristics
• Behavior The methods of an object. What the
  object can do
• State When happens when a method gets called
• The current condition of the object
• Identity How to distinguish an object from
  others having the same behavior and state
• Each class instance (object) has a unique identity

5
Designing a Class

• Designing a class or classes that comprise an
  application can be somewhat subjective
• Guidelines
• Look at the nouns to describe a system
• These nouns are likely candidates for classes
• Minimize dependencies between classes
• Look at what a class does (verbs)
• These become the methods of the class
• The data in a class also appear as nouns

6
Class Relationships

• There are three common relationships between
  classes
• Dependence (uses-a)
• One class is dependant upon or uses another class
• Aggregation (has-a)
• An order has items
• An employee has payroll records
• Inheritance (is-a)
• A relationship between a more general and a more
  specialized class (more in Chapter 5)

7
Constructing Objects

• Conceptually .NET and Java constructors are the
  same thing
• In Java, the name of a class' constructor is the
  same as the name of the class (rather than a Sub
  procedure named New in VB 6)
• The case sensitivity rule still applies
• The new keyword calls the constructor
• Date dateToday new Date()

8
Object References

• Classes are reference types
• This is the same as reference types in .NET
• Without the new keyword, a class instance is not
  created
• We just get a null pointer
• Look at the figures on page 97
• Multiple variables can reference the same object
  instance
• Again, remember that classes are reference types

9
A Word about Depreciation

• Java has various depreciated methods
• Depreciated methods will work but replacement
  methods should be used instead
• The compiler will flag depreciated methods with
  compiler warnings
• Avoid the use of depreciated methods because they
  may not work with future Java versions

10
Some Fun with Dates

• There are two problems in representing dates
• One is keeping track of time and representing a
  point in time using either
• Astronomical observations
• Atomic clocks
• They add a leap second every year to two at the
  end of the day on 12/31 or 6/30.
• This is the purpose of the Date class
• The other is attaching a name to a point in time
• This is the purpose of the GregorianCalendar class

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The Date Class

• Time is granular to the millisecond
• Boolean before method tests whether a date is
  before the specified date
• Boolean after method tests whether a date is
  after the specified date
• compareTo compares two dates for ordering
• Most of the methods of the Date class are
  depreciated and replaced by the methods of the
  GregorianCalendar class

12
The GregorianCalendar Class (1)

• GregorianCalendar belongs to java.util and is
  derived from java.util.Calendar
• It is the calendar used by most of the world's
  population
• There are other religious calendars however
• Without arguments, the constructor uses the
  current date and time
• With arguments a specific time can be used

13
The GregorianCalendar Class (2)

• isLeapYear returns true if the given year is a
  leap year
• add adds one or more units to a date value
• First argument contains the units
• Second argument contains the number of units to
  add

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The GregorianCalendar Class (Example 1)

• Create a GregorianCalendar an initialize it to
  the current date
• We could have initialized the date using the
  constructor
• Calendar calendar new GregorianCalendar()
• Date datCurrent new Date()
• calendar.setTime(datCurrent)

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The GregorianCalendar Class (Example 2)

• Get some information about the date
• System.out.println("ERA " calendar.get(Calendar
  .ERA))
• System.out.println("YEAR " calendar.get(Calenda
  r.YEAR))
• System.out.println("MONTH " calendar.get(Calend
  ar.MONTH))

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Java Classes (Introduction)

• In Java, everything is a class
• There is not Module type as in .NET
• A single source file can contain one or more
  classes
• Only one class can be public in the source file
  however
• Classes can have one or more overloaded
  constructors
• Unlike VB .NET, the name of the constructor is
  the same as the name of the class
• Classes can have one or more methods

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Constructors (1)

• Like .NET, Java classes have a constructor
• The name of the constructor must be the same as
  the class name
• Similar to C in this regard
• Constructors can be overloaded
• We discuss overloading in Chapter 5
• A default constructor accepts no arguments
• It is possible for one constructor to call
  another constructor
• Use the this keyword with a different argument
  list
• Constructors can accept 0 or more arguments
• A constructor without arguments is considered the
  default constructor
• It is implicitly supplied

18
Constructors (2)

• A constructor cannot return a value
• A constructor is never called directly
• It is always called with the new operator
• A default constructor is the one with no arguments

19
Constructor Example

• Note constructor name is the same as the class
  name
• The constructor is storing argument values in
  hidden fields
• class Customer
• public Customer(String cName, String
• cAddress, double cLimit)
• customerName cName
• customerAddress cAddress
• creditLimit cLimit

20
Accessors and Mutators

• Java does not support Property Procedures in the
  same way as VB .NET
• We could just use fields but that would violate
  the encapsulation rules
• Accessor methods read the value of hidden data
• Prefix the accessor name with "get" to comply
  with standard naming conventions
• Mutator methods write the value of hidden data
• Prefix the mutator name "set"

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Accessor Example

• Accessor getName returns the value of the hidden
  variable "name"
• We could use a field but that violates the
  encapsulation rules
• public String getName()
• return name

22
Mutator Example

• Store the argument value in a hidden field
• public void setName(String n)
• name n

23
Working with Multiple Source Files

• A Java program may appear in one source file or
  multiple source files
• When compiling the main source file, the compiler
  recognizes dependencies and compiles all of the
  necessary subordinate files
• Timestamps are used to determine whether or not
  files are out of date
• In essence, Java has the UNIX make utility
  built inside of it

24
Methods

• Methods are nothing more than public procedures
  appearing in a class
• Private procedures are part of the implementation
  rather than being part of the interface
• public void raiseSalary(double
• byPercent)
• double raise salary
• byPercent / 100
• salary raise

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What is this?

• The this keyword references the current instance
  of a class
• Its equivalent to the VB .NET Me keyword
• public void raiseSalary(double
• byPercent)
• double raise this.salary
• byPercent / 100
• this.salary raise

26
Working with Mutable Objects

• To adhere to encapsulation rules, never return a
  Mutable (changeable) object
• Side effects occur because of the way object
  references work
• Instead, return a clone of the object by calling
  the clone method
• public Date getHireDay()
• return (Date)hireDay.clone()

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Method Scope

• private methods are hidden inside of a class
• A private method can only be called from the same
  class containing the declaration
• public methods are exposed to developers
• Public methods can be called from any class

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Final Instance Fields

• Final instance fields can only be set once
• Final instance fields must be initialized in the
  constructor
• Use final instance fields with read-only
  properties
• Trying to store a value in a final instance field
  will generate an error
• Just add the final keyword to the variable
  declaration

29
Static Fields

• For a static field, only one copy of the data
  exists no matter the number of class instances
• Similar to the Shared keyword in VB. NET
• Declare a variable with the static keyword
• The static variable exists and can be referenced
  even if there is no instance of the class

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Static Methods

• Static methods do not operate on objects
• Static methods are declared with the static
  keyword in the same way as static variables are
  declared
• Static methods CANNOT operate with instance data
• This is logical. Think about why
• Instance methods can operate with static data
• The methods of the Math class are static, for
  example

31
Primitive Parameters

• As in most languages, parameters are passed by
  value or by reference
• Unlike .NET, primitive type parameters are ALWAYS
  passed by value
• The method cannot modify the value of a parameter
  and have the modified value returned to the
  caller
• See figure 4-6 on page 119

32
Object Parameters

• When passing an object reference as a parameter,
  the called method can modify the object
• A copy of the object reference is passed as an
  argument so we are referencing the same memory
  address from the caller and the callee
• NOTE THAT WE ARE NOT CALLING BY REFERENCE SEE
  PAGE 121

33
Object Destruction

• When utilizing a resource such as a file handle,
  the handle should be released before the object
  is destroyed
• Use the finalize method for this
• finalize is roughly equivalent to a destructor
• finalize gets called before the GC destroys the
  object

34
Packages

• Multiple classes are grouped into packages
• By convention, use a companys Internet domain
  name written in reverse
• Packages exist primarily to guarantee the
  uniqueness of class names
• Packages are roughly equivalent to .NET
  namespaces
• Packages are organized hierarchically just as
  .NET namespaces are organized hierarchically
• Standard Java packages appear in java and javax

35
Importing Packages

• It is possible to import all the classes in a
  package with wildcards
• import java.util.
• It is also possible to import a single class
• import java.util.Date
• Package ambiguity can pose a problem

36
Package Implementation

• Without a package declaration, all packages are
  part of the default package
• When creating a package, create files in a
  directory structure that matches the package
  structure

37
Creating Comments with javadoc

• Javadoc is a neat tool used to document classes
  by embedding comments directly into the class
  module
• These comments are parsed by javadoc and used to
  build a help system
• This is how the Java help system is built
• Simply create a comment with / instead of /
• The comment may contain HTML directives

38
Class Comments

• Must appear AFTER any import statements and
  before the class declaration
• import java.util.
• /
• The ltstronggtEmployeelt/stronggt class
• represents an employee in the organization.
• /
• public class Employee

39
Method Comments

• Method comments are created via parameters
• Describe a method parameter
• The tag must precede the method described by the
  parameter
• _at_param variable description
• Describe a return value
• _at_return variable description

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General Comments

• The program author
• _at_author name
• The program version
• _at_version text
• When the method was introduced
• _at_since text
• Use for depreciated methods
• _at_depreciated text
• A link to another URI
• _at_see uri

41
Introduction to Java Inheritance

• Inheritance utilizes an is-a relationship
• Java, like .NET supports single inheritance
• C supports multiple inheritance
• A derived class is created from a base class
• Derived class is often referred to as a subclass
  or child class
• Base class is often called a parent class or
  superclass
• Use the extends keyword to create a derived class
  from a base class

42
A First Inheritance Example

• The Manager class inherits the Employee class
• public class Manager extends Employee

43
Designing for Inheritance

• Place general methods in the base class
• These methods are automatically inherited in a
  derived class
• Create methods in the derived class to override
  base class members or to create new members
• Use the super keyword to explicitly call a base
  class method
• super is equivalent to MyBase in VB .NET

44
A Word about super and this

• super is not a reference to an object
• It is not possible to store into an object
  variable the value of super or this
• super when called in a constructor calls the base
  class constructor
• Super must appear as the first statement in the
  contstructor

45
Preventing Inheritance

• Classes that cannot be inherited are called
  sealed classes
• Use the final keyword in the class declaration
• Sealed classes are more efficient because dynamic
  binding is used instead of static binding
• Security is improved

46
Polymorphic Behavior and Inheritance

• Object variables are polymorphic
• A variable can refer to a type or a subclass of
  that type
• Through dynamic binding, the JVM figures out the
  correct type

47
Casting Object Variables

• The syntax is no different that casting a
  primitive type
• Casts are commonly used with generic containers
  such as an ArrayList
• Use to convert a generic object type to an
  explicit type

48
Abstract Classes

• An abstract class is one for which it is not
  legal to create an instance (so much for the
  grammar)
• Use the abstract keyword in the class declaration
• Abstract class Person

49
Abstract Classes (2)

• Abstract classes may have both abstract and
  concrete members

50
Protected Access

• In a nutshell protected is somewhere between
  public and private
• Protected members are visible to the current
  packages and any subclasses derived from classes
  in the current package
• The access modifier has the same meaning as
  protected in .net