Inheritance

Chapter Goals

• To learn about inheritance
• To understand how to inherit and override superclass methods
• To be able to invoke superclass constructors
• To learn about protected and package access control
• To understand the common superclass Object and to override its toString and equals methods

An Introduction to Inheritance

• Inheritance: extend classes by adding methods and fields
• Example: Savings account = bank account with interest

  class SavingsAccount extends BankAccount
  {
     new methods
     new instance fields
  }

• SavingsAccount automatically inherits all methods and instance fields of BankAccount

  SavingsAccount collegeFund = new SavingsAccount(10);
  // Savings account with 10% interest
  collegeFund.deposit(500);
  // OK to use BankAccount method with SavingsAccount object

• Extended class = superclass (BankAccount), extending class = subclass (Savings)
• Inheriting from class ≠ implementing interface: subclass inherits behavior and state
• One advantage of inheritance is code reuse

An Inheritance Diagram

• Every class extends the Object class either directly or indirectly
  

An Introduction to Inheritance

• In subclass, specify added instance fields, added methods, and changed or overridden methods

  public class SavingsAccount extends BankAccount
  {
     public SavingsAccount(double rate)
     {
        interestRate = rate;
     }
  
     public void addInterest()
     {
        double interest = getBalance() * interestRate / 100;
        deposit(interest);
     }
  
     private double interestRate;
  }

• Encapsulation: addInterest calls getBalance rather than updating the balance field of the superclass (field is private)
• Note that addInterest calls getBalance without specifying an implicit parameter (the calls apply to the same object)

Layout of a Subclass Object

• SavingsAccount object inherits the balance instance field from BankAccount, and gains one additional instance field: interestRate:
  

Syntax 13.1: Inheritance

public class SavingsAccount extends BankAccount
{
   public SavingsAccount(double rate)
   {
      interestRate = rate;
   }

   public void addInterest()
   {
      double interest = getBalance() * interestRate / 100;
      deposit(interest);
   }

   private double interestRate;
}

Self Check

1. Which instance fields does an object of class SavingsAccount have?
2. Name four methods that you can apply to SavingsAccount objects
3. If the class Manager extends the class Employee, which class is the superclass and which is the subclass?

Answers

1. Two instance fields: balance and interestRate.
2. deposit, withdraw, getBalance, and addInterest.
3. Manager is the subclass; Employee is the superclass.

Inheritance Hierarchies

• Sets of classes can form complex inheritance hierarchies
• Example:
  

Inheritance Hierarchies Example: Swing hierarchy

• Superclass JComponent has methods getWidth, getHeight
• AbstractButton class has methods to set/get button text and icon

A Simpler Example: Hierarchy of Bank Accounts

• Consider a bank that offers its customers the following account types:
  1. Checking account: no interest; small number of free transactions per month, additional transactions are charged a small fee
  2. Savings account: earns interest that compounds monthly
• Inheritance hierarchy:
  
• All bank accounts support the getBalance method
• All bank accounts support the deposit and withdraw methods, but the implementations differ
• Checking account needs a method deductFees; savings account needs a method addInterest

Self Check

4. What is the purpose of the JTextComponent class in Figure 4?
5. Which instance field will we need to add to the CheckingAccount class?

Answers

4. To express the common behavior of text fields and text components.
5. We need a counter that counts the number of withdrawals and deposits.

Inheriting Methods

• Override method:
  • Supply a different implementation of a method that exists in the superclass
  • Must have same signature (same name and same parameter types)
  • If method is applied to an object of the subclass type, the overriding method is executed
• Inherit method:
  • Don't supply a new implementation of a method that exists in superclass
  • Superclass method can be applied to the subclass objects
• Add method:
  • Supply a new method that doesn't exist in the superclass
  • New method can be applied only to subclass objects

Inheriting Instance Fields

• Can't override fields
• Inherit field: All fields from the superclass are automatically inherited
• Add field: Supply a new field that doesn't exist in the superclass
• What if you define a new field with the same name as a superclass field?
  • Each object would have two instance fields of the same name
  • Fields can hold different values
  • Legal but extremely undesirable

Implementing the CheckingAccount Class

• Overrides deposit and withdraw to increment the transaction count:

  public class CheckingAccount extends BankAccount
  {
     public void deposit(double amount) { . . . }
     public void withdraw(double amount) { . . . }
     public void deductFees() { . . . } // new method
     private int transactionCount;      // new instance field
  }

• Each CheckingAccount object has two instance fields:
  • balance (inherited from BankAccount)
  • transactionCount (new to CheckingAccount)
• You can apply four methods to CheckingAccount objects:
  • getBalance() (inherited from BankAccount)
  • deposit(double amount) (overrides BankAccount method)
  • withdraw(double amount) (overrides BankAccount method)
  • deductFees() (new to CheckingAccount)

Inherited Fields are Private

• Consider deposit method of CheckingAccount

  public void deposit(double amount)
  {
     transactionCount++;
     // now add amount to balance
     . . .
  }

• Can't just add amount to balance
• balance is a private field of the superclass
• A subclass has no access to private fields of its superclass
• Subclass must use public interface

Invoking a Superclass Method

• Can't just call
  deposit(amount)
  in deposit method of CheckingAccount
  
• That is the same as
  this.deposit(amount)
• Calls the same method (infinite recursion)
• Instead, invoke superclass method
  super.deposit(amount)
• Now calls deposit method of BankAccount class
• Complete method:

  public void deposit(double amount)
  {
     transactionCount++;
     // Now add amount to balance
     super.deposit(amount);
  }

Syntax 13.2: Calling a Superclass Method

Implementing Remaining Methods

public class CheckingAccount extends BankAccount
{
   . . .
   public void withdraw(double amount)
   {
      transactionCount++;
      // Now subtract amount from balance
      super.withdraw(amount);
   }

   public void deductFees()
   {
      if (transactionCount > FREE_TRANSACTIONS)
      {
         double fees = TRANSACTION_FEE
            * (transactionCount - FREE_TRANSACTIONS);
         super.withdraw(fees);
      }
      transactionCount = 0;
   }
   . . .
   private static final int FREE_TRANSACTIONS = 3;
   private static final double TRANSACTION_FEE = 2.0;
}

Self Check

6. Why does the withdraw method of the CheckingAccount class call super.withdraw?
7. Why does the deductFees method set the transaction count to zero?

Answers

6. It needs to reduce the balance, and it cannot access the balance field directly.
7. So that the count can reflect the number of transactions for the following month.

Common Error: Shadowing Instance Fields

• A subclass has no access to the private instance fields of the superclass
• Beginner's error: "solve" this problem by adding another instance field with same name:

  public class CheckingAccount extends BankAccount
  {
     public void deposit(double amount)
     {
        transactionCount++;
        balance = balance + amount;
     }
     . . .
     private double balance; // Don't
  }

• Now the deposit method compiles, but it doesn't update the correct balance!
  

Subclass Construction

• super followed by a parenthesis indicates a call to the superclass constructor

  public class CheckingAccount extends BankAccount
  {
     public CheckingAccount(double initialBalance)
     {
        // Construct superclass
        super(initialBalance);
        // Initialize transaction count
        transactionCount = 0;
     }
     . . .
  }

• Must be the first statement in subclass constructor
• If subclass constructor doesn't call superclass constructor, default superclass constructor is used
  • Default constructor: constructor with no parameters
  • If all constructors of the superclass require parameters, then the compiler reports an error

Syntax 13.3: Calling a Superclass Constructor

public CheckingAccount(double initialBalance)
{
   super(initialBalance);
   transactionCount = 0;
}

Self Check

8. Why didn't the SavingsAccount constructor in Section 13.1 call its superclass constructor?
9. When you invoke a superclass method with the super keyword, does the call have to be the first statement of the subclass method?

Answers

8. It was content to use the default constructor of the superclass, which sets the balance to zero.
9. No–this is a requirement only for constructors. For example, the SavingsAccount.deposit method first increments the transaction count, then calls the superclass method.

Converting Between Subclass and Superclass Types

• Ok to convert subclass reference to superclass reference

  SavingsAccount collegeFund = new SavingsAccount(10);
  BankAccount anAccount = collegeFund;
  Object anObject = collegeFund;

• The three object references stored in collegeFund, anAccount, and anObject all refer to the same object of type SavingsAccount
  

Converting Between Subclass and Superclass Types

• Superclass references don't know the full story:

  anAccount.deposit(1000); // OK
  anAccount.addInterest();
  // No--not a method of the class to which anAccount belongs

• When you convert between a subclass object to its superclass type:
  • The value of the reference stays the same–it is the memory location of the object
  • But, less information is known about the object
• Why would anyone want to know less about an object?
  • Reuse code that knows about the superclass but not the subclass:

    public void transfer(double amount, BankAccount other)
    {
       withdraw(amount);
       other.deposit(amount);
    }

    Can be used to transfer money from any type of BankAccount

Converting Between Subclass and Superclass Types

• Occasionally you need to convert from a superclass reference to a subclass reference

  BankAccount anAccount = (BankAccount) anObject;

• This cast is dangerous: if you are wrong, an exception is thrown
• Solution: use the instanceof operator
• instanceof: tests whether an object belongs to a particular type

  if (anObject instanceof BankAccount)
  {
     BankAccount anAccount = (BankAccount) anObject;
     . . .
  }

Syntax 13.4: The instanceof Operator

if (anObject instanceof BankAccount)
{
   BankAccount anAccount = (BankAccount) anObject;
   . . .
}

Self Check

10. Why did the second parameter of the transfer method have to be of type BankAccount and not, for example, SavingsAccount?
11. Why can't we change the second parameter of the transfer method to the type Object?

Answers

10. We want to use the method for all kinds of bank accounts. Had we used a parameter of type SavingsAccount, we couldn't have called the method with a CheckingAccount object.
11. We cannot invoke the deposit method on a variable of type Object.

Polymorphism

• In Java, type of a variable doesn't completely determine type of object to which it refers

  BankAccount aBankAccount = new SavingsAccount(1000);
  // aBankAccount holds a reference to a SavingsAccount

• Method calls are determined by type of actual object, not type of object reference

  BankAccount anAccount = new CheckingAccount();
  anAccount.deposit(1000); // Calls "deposit" from CheckingAccount

• Compiler needs to check that only legal methods are invoked

  Object anObject = new BankAccount();
  anObject.deposit(1000); // Wrong!

Polymorphism

• Polymorphism: ability to refer to objects of multiple types with varying behavior
• Polymorphism at work:

  public void transfer(double amount, BankAccount other)
  {
     withdraw(amount); // Shortcut for this.withdraw(amount)
     other.deposit(amount);
  }

• Depending on types of amount and other, different versions of withdraw and deposit are called

File AccountTester.java

File BankAccount.java

File CheckingAccount.java

File SavingsAccount.java

Output

Mom's savings balance = $7035.0
Harry's checking balance = $1116.0

Self Check

12. If a is a variable of type BankAccount that holds a non-null reference, what do you know about the object to which a refers?
13. If a refers to a checking account, what is the effect of calling a.transfer(1000, a)?

Answers

12. The object is an instance of BankAccount or one of its subclasses.
13. The balance of a is unchanged, and the transaction count is incremented twice.

Access Control

• Java has four levels of controlling access to fields, methods, and classes:
  • public access
    • Can be accessed by methods of all classes
  • private access
    • Can be accessed only by the methods of their own class
  • protected access
    • See Advanced Topic 13.3
  • package access
    • The default, when no access modifier is given
    • Can be accessed by all classes in the same package
    • Good default for classes, but extremely unfortunate for fields

Recommended Access Levels

• Instance and static fields: Always private. Exceptions:
  • public static final constants are useful and safe
  • Some objects, such as System.out, need to be accessible to all programs (public)
  • Occasionally, classes in a package must collaborate very closely (give some fields package access); inner classes are usually better
• Methods: public or private
• Classes and interfaces: public or package
  • Better alternative to package access: inner classes
    • In general, inner classes should not be public (some exceptions exist, e.g., Ellipse2D.Double)
• Beware of accidental package access (forgetting public or private)

Self Check

14. What is a common reason for defining package-visible instance fields?
15. If a class with a public constructor has package access, who can construct objects of it?

Answers

14. Accidentally forgetting the private modifier.
15. Any methods of classes in the same package.

Object: The Cosmic Superclass

• All classes defined without an explicit extends clause automatically extend Object
  
• Most useful methods:
  • String toString()
  • boolean equals(Object otherObject)
  • Object clone()
• Good idea to override these methods in your classes

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]"
• 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 "BankAccount@d24606bf"

Overriding the toString Method

• To provide a nicer representation of an object, override toString:
  

  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 tests for equal contents
  
• == tests for equal location
  

Overriding the equals Method

• Define the equals method to test whether two objects have equal state
• When redefining equals method, you cannot change object signature; use a cast instead:

  public class Coin
  {
     . . .
     public boolean equals(Object otherObject)
     {
        Coin other = (Coin) otherObject;
        return name.equals(other.name) && value == other.value;
     }
     . . .
  }

• You should also override the hashCode method so that equal objects have the same hash code

Self Check

16. Should the call x.equals(x) always return true?
17. Can you implement equals in terms of toString? Should you?

Answers

16. It certainly should–unless, of course, x is null.
17. If toString returns a string that describes all instance fields, you can simply call toString on the implicit and explicit parameters, and compare the results. However, comparing the fields is more efficient than converting them into strings.

Overriding the clone Method

• Copying an object reference gives two references to same object
  BankAccount account2 = account;
• Sometimes, need to make a copy of the object
  
• Define clone method to make new object (see Advanced Topic 13.6)
• Use clone:
  BankAccount clonedAccount = (BankAccount) account.clone();
• Must cast return value because return type is Object

The Object.clone method

• Creates shallow copies
  
• Does not systematically clone all subobjects
• Must be used with caution
• It is declared as protected; prevents from accidentally calling x.clone() if the class to which x belongs hasn't redefined clone to be public
• You should override the clone method with care (see Advanced Topic 13.6)

Scripting Languages