Horstmann Chapter 9

Big Java 4

Chapter 9 – Inheritance

Chapter Goals

vehicles

  • To learn about inheritance
  • To implement subclasses that inherit and override superclass methods
  • To understand the concept of polymorphism
  • To be familiar with the common superclass Object and its methods

Inheritance Hierarchies

  • Inheritance: the relationship between a more general class (superclass) and a more specialized class (subclass).
  • The subclass inherits data and behavior from the superclass.
  • Cars share the common traits of all vehicles
    • Example: the ability to transport people from one place to another

  • hierarchy of vehicles

    Figure 1 An Inheritance Hierarchy of Vehicle Classes

Inheritance Hierarchies

  • The class Car inherits from the class Vehicle
  • The Vehicle class is the superclass
  • The Car class is the subclass
  • inheritance diagram

    Figure 2 Inheritance Diagram

Inheritance Hierarchies

  • Inheritance lets you can reuse code instead of duplicating it.
  • Two types of reuse
    • A subclass inherits the methods of the superclass
    • Because a car is a special kind of vehicle, we can use a Car object in algorithms that manipulate Vehicle objects
  • The substitution principle:
    • You can always use a subclass object when a superclass object is expected.
  • A method that processes Vehicle objects can handle any kind of vehicle

Inheritance Hierarchies

question hierarchy
Figure 3 Inheritance Hierarchy of Question Types

Example: Computer-graded quiz

  • There are different kinds of questions
  • A question can display its text, and it can check whether a given response is a correct answer.
  • You can form subclasses of the Question class.

section_1/Question.java

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section_1/QuestionDemo1.java

Your browser does not support this feature Program Run:
  • Who was the inventor of Java?
    Your answer: James Gosling
    true

Self Check 9.1

Consider classes Manager and Employee. Which should be the superclass and which should be the subclass?
  • Answer: Because every manager is an employee but not the other way around, the Manager class is more specialized. It is the subclass, and Employee is the superclass.

Self Check 9.2

What are the inheritance relationships between classes BankAccount, CheckingAccount, and SavingsAccount?
  • Answer: CheckingAccount and SavingsAccount both inherit from the more general class BankAccount.

Self Check 9.3

What are all the superclasses of the JFrame class? Consult the Java API documentation or Appendix D.
  • Answer: The classes Frame, Window, and Component in the java.awt package, and the class Object in the java.lang package.

Self Check 9.4

Consider the method doSomething(Car c). List all vehicle classes from Figure 1 whose objects cannot be passed to this method.
vehicle hierarcy

  • Answer: Vehicle, Truck, Motorcycle

Self Check 9.5

Should a class Quiz inherit from the class Question? Why or why not?
  • Answer: It shouldn’t. A quiz isn’t a question; it has questions.

Implementing Subclasses

stretch limo
Like the manufacturer of a stretch limo, who starts with a regular car and modifies it, a programmer makes a subclass by modifying another class.

  • To get a ChoiceQuestion class, implement it as a subclass of Question
    • Specify what makes the subclass different from its superclass.
    • Subclass objects automatically have the instance variables that are declared in the superclass.
    • Only declare instance variables that are not part of the superclass objects.
  • A subclass inherits all methods that it does not override.
  • ChoiceQuestion is subclass of Question

    Figure 4 The ChoiceQuestion Class is a Subclass of the Question Class.

Implementing Subclasses

  • The subclass inherits all public methods from the superclass.
  • You declare any methods that are new to the subclass.
  • You change the implementation of inherited methods if the inherited behavior is not appropriate.
  • Override a method: supply a new implementation for an inherited method

Implementing Subclasses

A ChoiceQuestion object differs from a Question object in three ways:

  • Its objects store the various choices for the answer.
  • There is a method for adding answer choices.
  • The display method of the ChoiceQuestion class shows these choices so that the respondent can choose one of them.

Implementing Subclasses

  • The ChoiceQuestion class needs to spell out the three differences:
    public class ChoiceQuestion extends Question
    {
       // This instance variable is added to the subclass
       private ArrayList<String> choices;
       
       // This method is added to the subclass
       public void addChoice(String choice, boolean correct) { . . . }
       
       // This method overrides a method from the superclass
       public void display() { . . . }
    }
  • The extends reserved word indicates that a class inherits from a superclass.

Implementing Subclasses

  • UML of ChoiceQuestion and Question
    UML of the ChoiceQuestion

Figure 5 The ChoiceQuestion Class Adds an Instance Variable and a Method, and Overrides a Method

Syntax 9.1 Subclass Declaration

Subclass declaration

Implementing Subclasses

  • A ChoiceQuestion object
    ChoiceQuestion Object
  • You can call the inherited methods on a subclass object:
    choiceQuestion.setAnswer("2");
  • The private instance variables of the superclass are inaccessible.
  • The ChoiceQuestion methods cannot directly access the instance variable answer.
  • ChoiceQuestion methods must use the public interface of the Question class to access its private data.

Implementing Subclasses

  • Adding a new method: addChoice
    public void addChoice(String choice, boolean correct)
    {
       choices.add(choice);
       if (correct)
       {
          // Convert choices.size() to string
          String choiceString = "" + choices.size();
          setAnswer(choiceString);
       }
    }
  • addChoice method can not just access the answer variable in the superclass:
  • It must use the setAnswer method
  • Invoke setAnswer on the implicit parameter:
    setAnswer(choiceString);
    OR
    this.setAnswer(choiceString);

Self Check 9.6

Suppose q is an object of the class Question and cq an object of the class ChoiceQuestion. Which of the following calls are legal?

a. q.setAnswer(response)
b. cq.setAnswer(response)
c. q.addChoice(choice, true)
d. cq.addChoice(choice, true)

  • Answer: a, b, d

Self Check 9.7

Suppose the class Employee is declared as follows:
public class Employee
{
   private String name;
   private double baseSalary;
   
   public void setName(String newName) { . . . }
   public void setBaseSalary(double newSalary) { . . . }
   public String getName() { . . . }
   public double getSalary() { . . . }
}
Declare a class Manager that inherits from the class Employee and adds an instance variable bonus for storing a salary bonus. Omit constructors and methods.
  • Answer:
    public class Manager extends Employee
    {
        private double bonus;
       // Constructors and methods omitted
    }

Self Check 9.8

Which instance variables does the Manager class from Self Check 7 have?
  • Answer: name, baseSalary, and bonus

Self Check 9.9

In the Manager class, provide the method header (but not the implementation) for a method that overrides the getSalary method from the class Employee.
  • Answer:
    public class Manager extends Employee
    {
       . . .
       public double getSalary() { . . . }
    }

Self Check 9.10

Which methods does the Manager class from Self Check 9 inherit?
  • Answer: getName, setName, setBaseSalary

Common Error: Replicating Instance Variables from the Superclass

  • A subclass has no access to the private instance variables of the superclass:
    public ChoiceQuestion(String questionText)
    {
       text = questionText; // Error—tries to access private superclass variable
    }
  • Beginner's error: "solve" this problem by adding another instance variable with same name
  • Error!
    public class ChoiceQuestion extends Question
    {
       private ArrayList<String> choices;
       private String text; // Don’t!
    . . .
    }
  • The constructor compiles, but it doesn’t set the correct text!
    shadowing
  • The ChoiceQuestion constructor should call the setText method of the Question class.

Overriding Methods

  • If you are not satisfied with the behavior of an inherited method,
    • you override it by specifying a new implementation in the subclass.
  • An overriding method can extend or replace the functionality of the superclass method.
  • The display method of the ChoiceQuestion class needs to:
    • Display the question text.
    • Display the answer choices.

Overriding Methods

  • Problem: ChoiceQuestion's display method can’t access the text variable of the superclass directly because it is private.
  • Solution: It can call the display method of the superclass, by using the reserved word super
    public void display()
    {
       // Display the question text
       super.display(); // OK
       // Display the answer choices
       . . .
    }
  • super is a reserved word that forces execution of the superclass method.

section_3/QuestionDemo2.java

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section_3/ChoiceQuestion.java

Your browser does not support this feature Program Run:
  • What was the original name of the Java language?
    1: *7
    2: Duke
    3: Oak
    4: Gosling
    Your answer: *7
    false
    In which country was the inventor of Java born?
    1: Australia
    2: Canada
    3: Denmark
    4: United States
    Your answer: 2
    true

Syntax 9.2 Calling a Superclass Method

Calling a super class method

Self Check 9.11

What is wrong with the following implementation of the display method?
public class ChoiceQuestion
{
   . . .
   public void display()
   {
      System.out.println(text);
      for (int i = 0; i < choices.size(); i++)
      {
         int choiceNumber = i + 1;
         System.out.println(choiceNumber + ": " + choices.get(i));
      }
   }
}
  • Answer: The method is not allowed to access the instance variable text from the superclass.

Self Check 9.12

What is wrong with the following implementation of the display method?
public class ChoiceQuestion
{
   . . .
   public void display()
   {
      this.display();
      for (int i = 0; i < choices.size(); i++)
      {
         int choiceNumber = i + 1;
         System.out.println(choiceNumber + ": " + choices.get(i));
      }
   }
}
  • Answer: The type of the this reference is ChoiceQuestion. Therefore, the display method of ChoiceQuestion is selected, and the method calls itself.

Self Check 9.13

Look again at the implementation of the addChoice method that calls the setAnswer method of the superclass. Why don’t you need to call super.setAnswer?
  • Answer: Because there is no ambiguity. The subclass doesn’t have a setAnswer method.

Self Check 9.14

In the Manager class of Self Check 7, override the getName method so that managers have a * before their name (such as *Lin, Sally).
  • Answer:
    public String getName()
    {
       return "*" + super.getName();
    }

Self Check 9.15

In the Manager class of Self Check 9, override the getSalary method so that it returns the sum of the salary and the bonus.
  • Answer:
    public double getSalary()
    {
       return super.getSalary() + bonus;
    }

Common Error: Accidental Overloading

  • Overloading: when two methods have the same name but different parameter types.
  • Overriding: when a subclass method provides an implementation of a superclass method whose parameter variables have the same types.
  • When overriding a method, the types of the parameter variables must match exactly.

Syntax 9.3 Constructor with Superclass Initializer

supper class initializer of constructor

Polymorphism

  • Problem: to present both Question and ChoiceQuestion with the same program.
  • We do not need to know the exact type of the question
    • We need to display the question
    • We need to check whether the user supplied the correct answer
  • The Question superclass has methods for displaying and checking.
  • We can simply declare the parameter variable of the presentQuestion method to have the type Question:
    public static void presentQuestion(Question q)
    {
       q.display();
       System.out.print("Your answer: ");
       Scanner in = new Scanner(System.in);
       String response = in.nextLine();
       System.out.println(q.checkAnswer(response));
    }

Polymorphism - continued

  • We can substitute a subclass object whenever a superclass object is expected:
    ChoiceQuestion second = new ChoiceQuestion();
    . . .
    presentQuestion(second); // OK to pass a ChoiceQuestion
  • When the presentQuestion method executes -
    • The object references stored in second and q refer to the same object
    • The object is of type ChoiceQuestion
  • variables of different type can point to the same object

    Figure 7 Variables of Different Types Referring to the Same Object


Polymorphism - continued

  • The variable q knows less than the full story about the object to which it refers
  • A Question Reference

    Figure 8 A Question Reference Can Refer to an Object of Any Subclass of Question

  • In the same way that vehicles can differ in their method of locomotion, polymorphic objects carry out tasks in different ways.
    snowmobile

Polymorphism - continued

  • When the virtual machine calls an instance method -
    • It locates the method of the implicit parameter’s class.
    • This is called dynamic method lookup
  • Dynamic method lookup allows us to treat objects of different classes in a uniform way.
  • This feature is called polymorphism.
  • We ask multiple objects to carry out a task, and each object does so in its own way.
  • Polymorphism means “having multiple forms”
    • It allows us to manipulate objects that share a set of tasks, even though the tasks are executed in different ways.

section_4/QuestionDemo3.java

Your browser does not support this feature Program Run:
  • Who was the inventor of Java?
    Your answer: Bjarne Stroustrup
    false
    In which country was the inventor of Java born?
    1: Australia
    2: Canada
    3: Denmark
    4: United States
    Your answer: 2
    true
    

Self Check 9.16

Assuming SavingsAccount is a subclass of BankAccount, which of the following code fragments are valid in Java?

a. BankAccount account = new SavingsAccount();
b. SavingsAccount account2 = new BankAccount();
c. BankAccount account = null;
d. SavingsAccount account2 = account;
  • Answer: a only.

Self Check 9.17

If account is a variable of type BankAccount that holds a non-null reference, what do you know about the object to which account refers?
  • Answer: It belongs to the class BankAccount or one of its subclasses.

Self Check 9.18

Declare an array quiz that can hold a mixture of Question and ChoiceQuestion objects.
  • Answer:
    Question[] quiz = new Question[SIZE];

Self Check 9.19

Consider the code fragment
ChoiceQuestion cq = . . .; // A non-null value
cq.display();
Which actual method is being called?
  • Answer: You cannot tell from the fragment—cq may be initialized with an object of a subclass of ChoiceQuestion. The display method of whatever object cq references is invoked.

Self Check 9.20

Is the method call Math.sqrt(2) resolved through dynamic method lookup?
  • Answer: No. This is a static method of the Math class. There is no implicit parameter object that could be used to dynamically look up a method.

Object: The Cosmic Superclass

  • Every class defined without an explicit extends clause automatically extend Object:
  • The class Object is the direct or indirect superclass of every class in Java.
  • Some methods defined in Object:
    • toString - which yields a string describing the object
    • equals - which compares objects with each other
    • hashCode - which yields a numerical code for storing the object in a set

Object: The Cosmic Superclass

Object hierarchy

Figure 9 The Object Class Is the Superclass of Every Java Class

Overriding the toString Method

  • Returns a string representation of the object
  • Useful for debugging:
    Rectangle box = new Rectangle(5, 10, 20, 30);
    String s = box.toString();
       // Sets s to "java.awt.Rectangle[x=5,y=10,width=20,height=30]"
  • toString is called whenever you concatenate a string with an object:
    "box=" + box;
       // Result: "box=java.awt.Rectangle[x=5,y=10,width=20,height=30]"
  • The compiler can invoke the toString method, because it knows that every object has a toString method:
    • Every class extends the Object class which declares toString

Overriding the toString Method

  • Object.toString prints class name and the hash code of the object:
    BankAccount momsSavings = new BankAccount(5000);
    String s = momsSavings.toString();
       // Sets s to something like "[email protected]"
  • Override the toString method in your classes to yield a string that describes the object’s state.
    public String toString()
    {
       return "BankAccount[balance=" + balance + "]";
    }
  • This works better:
    BankAccount momsSavings = new BankAccount(5000);
    String s = momsSavings.toString(); // Sets s to "BankAccount[balance=5000]"

Overriding the equals Method

  • equals method checks whether two objects have the same content:
    if (stamp1.equals(stamp2)) . . .
       // Contents are the same
    two stamps
  • == operator tests whether two references are identical - referring to the same object:
    if (stamp1 == stamp2) . . . 
       // Objects are the same 

Overriding the equals Method


equal objects

Figure 10 Two References to Equal Objects

same object

Figure 11 Two References to the Same Object

 

Overriding the equals Method

  • To implement the equals method for a Stamp class -
    • Override the equals method of the Object class:
    public class Stamp
    {
       private String color;
       private int value;
       . . .
       public boolean equals(Object otherObject)
       {
          . . .
       }
       . . .
    }
  • Cannot change parameter type of the equals method - it must be Object
  • Cast the parameter variable to the class Stamp instead:
    Stamp other = (Stamp) otherObject;
  • Overriding the equals Method

    • After casting, you can compare two Stamps
      public boolean equals(Object otherObject)
      {
         Stamp other = (Stamp) otherObject;
         return color.equals(other.color)
              && value == other.value;
      }
    • The equals method can access the instance variables of any Stamp object.
    • The access other.color is legal.

    The instanceof Operator

    • It is legal to store a subclass reference in a superclass variable:
      ChoiceQuestion cq = new ChoiceQuestion();
      Question q = cq; // OK
      Object obj = cq; // OK
    • Sometimes you need to convert from a superclass reference to a subclass reference.
    • If you know a variable of type Object actually holds a Question reference, you can cast
      Question q = (Question) obj
    • If obj refers to an object of an unrelated type, "class cast" exception is thrown.
    • The instanceof operator tests whether an object belongs to a particular type.
      obj instanceof Question
    • Using the instanceof operator, a safe cast can be programmed as follows:
      if (obj instanceof Question)
      {
         Question q = (Question) obj;
      }

    Self Check 9.21

    Why does the call
    System.out.println(System.out);
    produce a result such as [email protected]?
    • Answer: Because the implementor of the PrintStream class did not supply a toString method.

    Self Check 9.22

    Will the following code fragment compile? Will it run? If not, what error is reported?
    Object obj = "Hello";
    System.out.println(obj.length());
    • Answer: The second line will not compile. The class Object does not have a method length.

    Self Check 9.23

    Will the following code fragment compile? Will it run? If not, what error is reported?
    Object obj = "Who was the inventor of Java?";
    Question q = (Question) obj;
    q.display();
    • Answer: The code will compile, but the second line will throw a class cast exception because Question is not a superclass of String.

    Self Check 9.24

    Why don’t we simply store all objects in variables of type Object?
    • Answer: There are only a few methods that can be invoked on variables of type Object.

    Self Check 9.25

    Assuming that x is an object reference, what is the value of x instanceof Object?
    • Answer: The value is false if x is null and true otherwise.