Using Collections and Strings in a Reusable Class

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Chapter 9

• Using Collections and Strings in a Reusable
  Class

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Introduction

• Design code with reusability in mind
• Abstract, or separate out, the specific
  functionality of a given class
• Design classes to provide a function regardless
  of the particular application where it is used
• Use Java classes in your program when they can
  provide a needed function

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Developing a Reusable Password Class

• A password class is a good candidate for
  reusability
• Passwords can have numerous characteristics
• Minimum length
• Requirement for numeric characters
• Expiration time
• Restrictions on using previously used passwords
• Encryption
• The passwords functionality is encapsulated in
  the class

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Problem Analysis

• Accumulate a history of recently used passwords
• Verify the format of all passwords
• Validate a given password against the current
  password value
• Determine if the current password is approaching
  expiration, provide expiration information, and
  activate/deactivate expiration
• Encrypt passwords using the given algorithm

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Design the Solution
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Public and Private Methods

• The public interface of an object contains
  methods that should be available for use by other
  programs
• The implementation of a public method can be
  changed without changing the interface, or method
  header
• Helper methods are used internally by the object
  and are declared private
• Use the principle of least privilege to restrict
  access from outside the class to the minimum
  level necessary

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Understanding Class and Instance Variables

• Class variables are shared by all objects of a
  class
• One value regardless of the object using it
• Designated by the keyword, static
• Instance variables exist only when an object
  exists
• Values are not shared among object instances
• Class and instance variables have class scope
• Local variables are defined in a method and are
  in scope only within that method

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• Static variables
• Only one copy in memory
• Shared by all instances of the class
• See StaticDemo.java in the Chapter 9 student
  files.

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static variable
constructor
3 new objects
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• Static methods
• Belong to the Class
• Can be called without any instances of the class
• See TripleThird.java and TripleThirdDemo.java in
  the Chapter 9 student files.

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TripleThird.java
Two static methods
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TripleThirdDemo.java
call static method
call static method
dblTriple TripleThird.TripleNumber(dblInput1)
Class name
Static method name
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• Create a new file in TextPad
• Save as Password.java

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Using the Final Qualifier

• Final variables allow constants to have a name
• Avoids duplicate literals in a program
• By convention, finals use all capital letters
• Blank finals can be left uninitialized when
  defined
• Constructors may supply different values
• Are not often used due to potentially different
  values
• Final variables should be declared static to
  ensure the same value is used for all instances

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Understanding the Collections Framework

• A number of commonly used data structures in the
  Java API
• Interfaces
• Provide a general structure independent of the
  representation
• Implementations
• Provide reusable data structures from interfaces
• For example, an ArrayList implements the list
  interface an an array
• Algorithms
• Provide the means of performing common operations

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Creating an ArrayList

• An ArrayList allow resizing, insertions, and
  deletions
• When the list size reaches capacity, the capacity
  is increased automatically
• The ArrayList constructor can take an integer
  parameter that represents the initial capacity of
  the list
• If the initial capacity is not given, the default
  is a size of 10

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ArrayList class

• Need to import java.util.ArrayList
• Using no-arg constructor ArrayList nameList
  new ArrayList()
• Add objects to the ArrayList nameList.add(James
  ) nameList.add(Catherine) nameList.add(Bil
  l)
• Using size method nameList.size would contain 3

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ArrayList class

• ArrayList get method System.out.println(nameList
  .get(1))
• Built-in toString method System.out.println(name
  List)would display as James, Catherine,
  Bill
• Remove method nameList.remove(1)Bill is now
  nameList(1) James, Bill

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ArrayList class

• Inserting an item (adding to the original
  ArrayList) nameList.add(1, Mary)ArrayList
  now contains
• Index 0 James
• Index 1 Mary
• Index 2 Catherine
• Index 3 Bill
• Replace an item nameList.set(1, Becky)would
  replace Mary with Becky

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ArrayList class

• To assign to another variable, use a
  cast String str (String)nameList.get(0)
• Specify the type of data an ArrayList will
  hold ArrayList ltStringgt nameList new
  ArrayListltStringgt()
• Wont need to cast when retreiving items from the
  list

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Creating Overloaded Constructors

• A constructor has no return data type
• A default constructor has no parameters
• If there are no coded constructors, the compiler
  creates a default constructor
• If a constructor has no arguments, the instance
  variables of the class are given default values
• Java allows multiple constructor methods
• Multiple methods with the same name are said to
  be overloaded
• Overloaded methods must have unique method
  signatures

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Creating Overloaded Constructors

• A combination of the method name and the formal
  parameter list in the method header
• The parameter list must be unique for overloaded
  methods
• Different number of parameters
• Same number of parameters with a different
  ordering of data types
• The return data type is not used in determining
  the method signature

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First and second constructors
Set is a user-defined method, not written yet
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• Third and fourth constructors

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Coding Accessor (Get) and Mutator (Set) Methods

• User programs access private instance variables
  of a class by using instance methods
• Accessor methods provide the value of a private
  variable
• Usually named with the verb, get, followed by the
  instance variable name
• Sometimes named with the verb, is, for boolean
  variables
• Mutator methods accept a proposed value for the
  variable
• Usually named with the verb, set, followed by the
  variable name
• Return type can be boolean to indicate success or
  failure
• Responsible for determining if value is valid

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autoExpires Accessor and Mutator Methods
Local parameter variable
Class-level instance variable
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expired Accessor and Mutator Methods
isExpired could have been named getExpired
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expiresNotifyLimit Accessor and Mutator Methods

• Verify that the new value for the mutator method
  is within the acceptable range

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maxHistory Accessor and Mutator Methods

• Pseudocode for setMaxHistory() verifies the range
  of the new value and then adjusts the history
  list accordingly

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Coding Accessor Methods for Read-Only Attributes

• Read-only attributes only require an accessor
  method, because they cannot be modified by the
  user

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Coding public Instance Methods

• set()
• Trims extra space from a new password
• Verifies format and encrypts
• Checks password history list and adds the new
  password, if not already present
• Sets expiration
• validate()
• Verifies the entered password is the latest entry
  in list and decrements remaining uses, if
  necessary
• Both methods throw exceptions for invalid or
  expired passwords

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Using ArrayList Methods

• All objects in Java inherit Object as the root
  class
• Any object may be referenced as an Object
• ArrayList can maintain a list of any type of
  object by using Object as the parameter or return
  type
• Downcasting must be used when retrieving data
  from ArrayList
• Use the cast operator with the specific class
  type
• If necessary, use the instanceof operator to
  verify the object type before downcasting

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Using the String Class

• The String class supports immutable string values
• String objects are constants
• String literals are implemented as instances of
  the String class
• If you assign a new value to a String, a new
  String is created with that value
• The JVM garbage collector disposes of the old
  value
• String concatenation is implemented with the
  StringBuffer class

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Using the StringBuffer Class

• The StringBuffer class implements a mutable
  sequence of characters
• The length and content may be modified at any
  time
• If the capacity of the internal buffer is
  exceeded, it is expanded automatically
• A StringBuffer may be created with no arguments,
  a String literal, a String variable, or a
  concatenation of multiple strings

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Coding private Helper Methods

• Pseudocode for verifyFormat()

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verifyFormat()
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HashCode( )

• Converts string values to an arbitrary integer
  value
• See sample program TestHashCodes.java in the
  Chapter 9 student files
• Try different values for strNum1 and strNum2
• Note the format strNum1.hashCode()
• Hash codes are stored as integers

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Bitwise operators

• Can only be used on integers and characters
• Operates on individual bits
• Bitwise AND
• Bitwise OR
• Bitwise NOT (complement operator)

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Bitwise operators

• num1 holds 45 00101101
• num2 holds 14 00001110

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The encrypt( ) Method

• Encrypted data is called ciphertext
• Encryption techniques
• Substitution and rotation of letters
• Use substring( ) and reverse() methods
• Bit manipulation
• Use the bitwise AND operator to compare the bits
  in each character of the encrypted and original
  password
• Hash codes
• Use hashcode() to transform a string into a
  shorter, fixed-length value
• Hash codes are not necessarily unique

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Pseudocode for encrypt()
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Encryption

• See sample program TestEncryption.java in the
  Chapter 9 student files
• Output in encrypt, original pswd is Hello
• after swap, encryptPswd is lloHe
• after reverse, encryptPswd is eHoll
• after bitwise AND, encryptPswd is _at__at_lll
• hashCode is 69609650
• in Main, strNewWord is _at__at_lll69609650

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encrypt( )
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Testing a Reusable Class

• The PasswordDemo test program provides a GUI
• The JPassword field provides masking of an
  entered password with alternate characters for
  security
• An asterisk is the default
• Exceptions from the Password class are caught and
  a message box displays the passed String message
• Test the Password class
• Enter new and current passwords
• Change the password
• Test valid and invalid inputs

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Chapter 9 Java Homework

• Learn It Online1. Chapter Reinforcement
  True/False Multiple Choice, page 596
• Only type the answers, not the questions
• All answers must be typed and printed out
• Debugging Assignment
• Page 597
• Programming Assignments
• Pages 598 - 601
• 2, 3, 5
• Due 11/29/07