Horstmann Chapter 12

Big Java 4

Chapter 12 – Object-Oriented Design

Chapter Goals

people designing

  • To learn how to discover new classes and methods
  • To use CRC cards for class discovery
  • To identify inheritance, aggregation, and dependency relationships between classes
  • To describe class relationships using UML class diagrams
  • To apply object-oriented design techniques to building complex programs

Discovering Classes

  • When designing a program, you work from a requirements specification
    • The designer’s task is to discover structures that make it possible to implement the requirements
  • To discover classes, look for nouns in the problem description.
  • Find methods by looking for verbs in the task description.

Example: Invoice

an invoice

Figure 1 An Invoice

Example: Invoice

  • Classes that come to mind:
    • Invoice
    • LineItem
    • Customer
  • Good idea to keep a list of candidate classes.
  • Brainstorm: put all ideas for classes onto the list.
  • Cross not useful ones later.
  • Concepts from the problem domain are good candidates for classes.
  • Not all classes can be discovered from the program requirements:
    • Most programs need tactical classes

The CRC Card Method

lecture hall


In a class scheduling system, potential classes from the problem domain include Class, LectureHall, Instructor, and Student.

The CRC Card Method

  • After you have a set of classes
    • Define the behavior (methods) of each class
  • Look for verbs in the task description
    • Match the verbs to the appropriate objects
  • The invoice program needs to compute the amount due
    • Which class is responsible for this method?
      • Invoice class

The CRC Card Method

  • To find the class responsibilities, use the CRC card method.
  • A CRC card describes a class, its responsibilities, and its collaborating classes.
    • CRC - stands for “classes”, “responsibilities”, “collaborators”
  • Use an index card for each class.
  • Pick the class that should be responsible for each method (verb).
  • Write the responsibility onto the class card.
  • Indicate what other classes are needed to fulfill responsibility (collaborators).
  • CRC index card

    Figure 2 A CRC Card

Self Check 12.1

What is the rule of thumb for finding classes?
  • Answer: Look for nouns in the problem description.

Self Check 12.2

Your job is to write a program that plays chess. Might ChessBoard be an appropriate class? How about MovePiece?
  • Answer: Yes (ChessBoard) and no (MovePiece).

Self Check 12.3

Suppose the invoice is to be saved to a file. Name a likely collaborator.
  • Answer: PrintStream

Self Check 12.4

Looking at the invoice in Figure 1, what is a likely responsibility of the Customer class?
  • Answer: To produce the shipping address of the customer.

Self Check 12.5

What do you do if a CRC card has ten responsibilities?
  • Answer: Reword the responsibilities so that they are at a higher level, or come up with more classes to handle the responsibilities.

Relationships Between Classes

The most common types of relationships:

  • Dependency
  • Aggregation
  • Inheritance


  • A class depends on another class if it uses objects of that class.
    • The “knows about” relationship.
  • Example: CashRegister depends on Coin
  • UML diagram of dependency

    Figure 3 Dependency Relationship Between the CashRegiste and Coin Classes


  • It is a good practice to minimize the coupling (i.e., dependency) between classes.
    high and low coupling
  • When a class changes, coupled classes may also need updating.


  • A class aggregates another if its objects contain objects of the other class.
    • Has-a relationship
  • Example: a Quiz class aggregates a Question class.
  • The UML for aggregation:
    UML diagram of aggregation
  • Aggregation is a stronger form of dependency.
  • Use aggregation to remember another object between method calls.
  • Use an instance variable
    public class Quiz
       private ArrayList<Question> questions;
       . . .
  • A class may use the Scanner class without ever declaring an instance variable of class Scanner.
    • This is dependency NOT aggregation
  • Aggregation

    acar aggregates parts


    A car has a motor and tires. In object-oriented design, this “has-a” relationship is called aggregation.


    • Inheritance is a relationship between a more general class (the superclass) and a more
      specialized class (the subclass).
      • The “is-a” relationship.
      • Example: Every truck is a vehicle.
    • Inheritance is sometimes inappropriately used when the has-a relationship would be more appropriate.
      • Should the class Tire be a subclass of a class Circle? No
        • A tire has a circle as its boundary
        • Use aggregation
          public class Tire
             private String rating;
             private Circle boundary;
             . . .


    • Every car is a vehicle. (Inheritance)
    • Every car has a tire (or four). (Aggregation)
      class Car extends Vehicle
         private Tire[] tires;
         . . .
    • Aggregation denotes that objects of one class contain references to objects of another class.
    • UML diagram of inheritance and aggregation

      Figure 6 UML Notation for Inheritance and Aggregation

    UML Relationship Symbols

    Relationship Symbol Line Style Arrow Tip
    Inheritance inheritance arrow Solid Triangle
    Interface Implementation interface arrow Dotted Triangle
    Aggregation aggregation arrow Solid Diamond
    Dependency dependency arrow Dotted Open

    Self Check 12.6

    Consider the CashRegisterTester class of Section 8.2. On which classes does it depend?
    • Answer: The CashRegisterTester class depends on the CashRegister, Coin, and System classes.

    Self Check 12.7

    Consider the Question and ChoiceQuestion objects of Chapter 9. How are they related?
    • Answer: The ChoiceQuestion class inherits from the Question class.

    Self Check 12.8

    Consider the Quiz class described in Section 12.2.2. Suppose a quiz contains a mixture of Question and ChoiceQuestion objects. Which classes does the Quiz class depend on?
    • Answer: The Quiz class depends on the Question class but probably not ChoiceQuestion, if we assume that the methods of the Quiz class manipulate generic Question objects, as they did in Chapter 9.

    Self Check 12.9

    Why should coupling be minimized between classes?
    • Answer: If a class doesn’t depend on another, it is not affected by interface changes in the other class.

    Self Check 12.10

    In an e-mail system, messages are stored in a mailbox. Draw a UML diagram that shows the appropriate aggregation relationship.
    • Answer:
    • UML

    Self Check 12.11

    You are implementing a system to manage a library, keeping track of which books are checked out by whom. Should the Book class aggregate Patron or the other way around?
    • Answer: Typically, a library system wants to track which books a patron has checked out, so it makes more sense to have Patron aggregate Book. However, there is not always one true answer in design. If you feel strongly that it is important to identify the patron who checked out a particular book (perhaps to notify the patron to return it because it was requested by someone else), then you can argue that the aggregation should go the other way around.

    Self Check 12.12

    In a library management system, what would be the relationship between classes Patron and Author?
    • Answer: There would be no relationship.

    Attributes and Methods in UML Diagrams

    UML attributes and methods


    • any number (zero or more): *
    • one or more: 1..*
    • zero or one: 0..1
    • exactly one: 1
    • multiplicities

    Aggregation and Association, and Composition

    • Association: More general relationship between classes.
    • Use early in the design phase.
    • A class is associated with another if you can navigate from objects of one class to objects of the other.
    • Given a Bank object, you can navigate to Customer objects.

      association UML
    • Composition: one of the classes can not exist without the other.
      composition UML

    Application: Printing an Invoice

    Five-part program development process

    1. Gather requirements
    2. Use CRC cards to find classes, responsibilities, and collaborators
    3. Use UML diagrams to record class relationships
    4. Use javadoc to document method behavior
    5. Implement your program

    Application: Printing an Invoice — Requirements

    • Start the development process by gathering and documenting program requirements.
    • Task: Print out an invoice
    • Invoice: Describes the charges for a set of products in certain quantities.
    • Omit complexities
      • Dates, taxes, and invoice and customer numbers
    • Print invoice
      • Billing address, all line items, amount due
    • Line item
      • Description, unit price, quantity ordered, total price
    • For simplicity, do not provide a user interface.
    • Test program: Adds line items to the invoice and then prints it.

    Application: Printing an Invoice

    • Sample Invoice
                       I N V O I C E
      Sam's Small Appliances
      100 Main Street
      Anytown, CA 98765
      Description                    Price Qty Total
      Toaster                        29.95  3  89.85
      Hair dryer                     24.95  1  24.95
      Car vacuum                     19.99  2  39.98
      AMOUNT DUE: $154.78
    • An invoice lists the charges for each item and the amount due.

    Application: Printing an Invoice — CRC Cards

    • Use CRC cards to find classes, responsibilities, and collaborators.
    • Discover classes
    • Nouns are possible classes:
      Amount Due

    Application: Printing an Invoice — CRC Cards

    • Analyze classes:
      LineItem    // Records the product and the quantity
      Description // Field of the Product class
      Price       // Field of the Product class
      Quantity    // Not an attribute of a Product 
      Total       // Computed — not stored anywhere
      Amount Due  // Computed — not stored anywhere
    • Classes after a process of elimination:

    CRC Cards for Printing Invoice

    Invoice and Address must be able to format themselves:

    invoice CRC 1address CRC

    CRC Cards for Printing Invoice

    Add collaborators to Invoice card:

    invoice CRC 2

    CRC Cards for Printing Invoice

    Product and LineItem CRC cards:

    product CRCline item CRC

    CRC Cards for Printing Invoice

    Invoice must be populated with products and quantities:

    invoice CRC 3

    Application: Printing an Invoice — UML Diagrams

    UML for Invoice classes

    Printing an Invoice — Method Documentation

    • Use javadoc comments (with the method bodies left blank) to record the behavior of the classes.
    • Write a Java source file for each class:
      • Write the method comments for those methods that you have discovered,
      • Leave the body of the methods blank
    • Run javadoc to obtain formatted version of documentation in HTML format.
    • Advantages:
      • Share HTML documentation with other team members
      • Format is immediately useful: Java source files
      • Supply the comments of the key methods

    Method Documentation — Invoice Class

       Describes an invoice for a set of purchased products.
    public class Invoice
          Adds a charge for a product to this invoice.
          @param aProduct the product that the customer ordered
          @param quantity the quantity of the product
       public void add(Product aProduct, int quantity)
          Formats the invoice.
          @return the formatted invoice
       public String format()

    Method Documentation — LineItem Class

       Describes a quantity of an article to purchase and its price.
    public class LineItem
          Computes the total cost of this line item.
          @return the total price
       public double getTotalPrice()
          Formats this item.
          @return a formatted string of this line item
       public String format()

    Method Documentation — Product Class

       Describes a product with a description and a price.
    public class Product
          Gets the product description.
          @return the description
       public String getDescription()
          Gets the product price.
          @return the unit price
       public double getPrice()

    Method Documentation — Address Class

       Describes a mailing address.
    public class Address
          Formats the address.
          @return the address as a string with three lines
       public String format()

    The Class Documentation in the HTML Format


    Figure 8 Class Documentation in HTML Format

    Printing an Invoice — Implementation

    • After completing the design, implement your classes.
    • The UML diagram will give instance variables:
      • Look for aggregated classes
      • They yield instance variables


    • Invoice aggregates Address and LineItem .
    • Every invoice has one billing address.
    • An invoice can have many line items:
      public class Invoice
         . . .
         private Address billingAddress;
         private ArrayList<LineItem> items;


    A line item needs to store a Product object and quantity:
    public class LineItem
       . . .
       private int quantity; 
       private Product theProduct;


    • The methods themselves are now very easy.
    • Example:
      • getTotalPrice of LineItem gets the unit price of the product and multiplies it with the quantity
         Computes the total cost of this line item.
         @return the total price
      public double getTotalPrice()
         return theProduct.getPrice() * quantity;
  • Also supply constructors
  • section_3/InvoicePrinter.java

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    Self Check 12.13

    Which class is responsible for computing the amount due? What are its collaborators for this task?
    • Answer: The Invoice class is responsible for computing the amount due. It collaborates with the LineItem class.

    Self Check 12.14

    Why do the format methods return String objects instead of directly printing to System.out?
    • Answer: This design decision reduces coupling. It enables us to reuse the classes when we want to show the invoice in a dialog box or on a web page.