Chapter 10 – 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
To use inheritance for customizing user interfaces

Inheritance Hierarchies

Often categorize concepts into hierarchies:

Inheritance Hierarchies

Set of classes can form an inheritance hierarchy
- Classes representing the most general concepts are near the root, more specialized classes towards the branches:

Inheritance Hierarchies

Superclass: more general class
Subclass: more specialized class that inherits from the superclass
- Example: JPanel is a subclass of JComponent

Inheritance Hierarchies

Example: Different account types:
1. Checking account:
  - No interest
  - Small number of free transactions per month
  - Charges transaction fee for additional transactions
2. Savings account:
  - Earns interest that compounds monthly
Superclass: BankAccount
Subclasses: CheckingAccount & SavingsAccount

Inheritance Hierarchies

Behavior of account classes:
- All support getBalance method
- Also support deposit and withdraw methods, but implementation details differ
- Checking account needs a method deductFees to deduct the monthly fees and to reset the transaction counter
- Checking account must override deposit and withdraw methods to count the transactions

Self Check 10.1

What is the purpose of the JTextComponent class in Figure 2?

Answer: To express the common behavior of text fields and text components.

Self Check 10.2

Why don’t we place the addInterest method in the BankAccount class?

Answer: Not all bank accounts earn interest.

Inheritance Hierarchies

Inheritance is a mechanism for extending existing classes by adding instance variables and methods:

public class SavingsAccount extends BankAccount 
{ 
   added instance variables 
   new methods 
}

A subclass inherits the methods of its superclass:

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

Inheritance Hierarchies

In subclass, specify added instance variables and methods, and changed or overridden methods:

public class SavingsAccount extends BankAccount
{
   private double interestRate;
   
   public SavingsAccount(double rate)
   {
      Constructor implementation
   }

   public void addInterest()
   {
      Method implementation
   }
}

Inheritance Hierarchies

Instance variables declared in the superclass are present in subclass objects
SavingsAccount object inherits the balance instance variable from BankAccount, and gains one additional instance variable, interestRate:

Inheritance Hierarchies

Implement the new addInterest method:

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

Inheritance Hierarchies

A subclass has no access to private instance variables of its superclass
Encapsulation: addInterest calls getBalance rather than updating the balance variable of the superclass (variable is private)
Note that addInterest calls getBalance without specifying an implicit parameter (the calls apply to the same object)
Inheriting from a class differs from implementing an interface: the subclass inherits behavior from the superclass

ch10/accounts/SavingsAccount.java

Syntax 10.1 Inheritance

Self Check 10.3

Which instance variable does an object of class SavingsAccount have?

Answer: Two instance variables: balance and interestRate.

Self Check 10.4

Name four methods that you can apply to SavingsAccount objects.

Answer: deposit, withdraw, getBalance, and addInterest.

Self Check 10.5

If the class Manager extends the class Employee, which class is the superclass and which is the subclass?

Answer: Manager is the subclass; Employee is the superclass.

Common Error: Shadowing Instance Variables

A subclass has no access to the private instance variables of the superclass:

public class SavingsAccount extends BankAccount 
{  
   public void addInterest() 
   {  
      double interest = getBalance() * interestRate / 100; 
      balance = balance + interest; // Error 
   } 
   . . . 
}

Common Error: Shadowing Instance variables

Beginner's error: solve this problem by adding another instance variable with same name:

public class SavingsAccount extends BankAccount 
{  
   private double balance; // Don’t 
   . . . 
   public void addInterest() 
   {  
      double interest = getBalance() * interestRate / 100; 
      balance = balance + interest; // Compiles but doesn’t update the correct balance 
   } 
}

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

Overriding Methods

A subclass method overrides a superclass method if it has the same name and parameter types as a superclass method
- When such a method is applied to a subclass object, the overriding method is executed

Example: deposit and withdraw methods of the CheckingAccount class override the deposit and withdraw methods of the BankAccount class to handle transaction fees:

public class BankAccount 
{  
   . . . 
   public void deposit(double amount) { . . . } 
   public void withdraw(double amount) { . . . } 
   public double getBalance() { . . . } 
} 

public class CheckingAccount extends BankAccount 
{  
   . . . 
   public void deposit(double amount) { . . . } 
   public void withdraw(double amount) { . . . } 
   public void deductFees() { . . . } 
}

Overriding Methods

Problem: Overriding method deposit can't simply add amount to balance:

public class CheckingAccount extends BankAccount 
{  
   . . . 
   public void deposit(double amount) 
   {  
      transactionCount++; 
      // Now add amount to balance 
      balance = balance + amount; // Error 
   } 
}

If you want to modify a private superclass instance variable, you must use a public method of the superclass
deposit method of CheckingAccount must invoke the deposit method of BankAccount

Overriding Methods

Idea:

public class CheckingAccount extends BankAccount 
{  
   public void deposit(double amount) 
   {  
      transactionCount++; 
      // Now add amount to balance 
      deposit(amount); // Not complete 
   } 
   . . . 
}

Won't work because compiler interprets
```
deposit(amount);
```
as
```
this.deposit(amount);
```
which calls the method we are currently writing ⇒ infinite recursion

Overriding Methods

Use the super reserved word to call a method of the superclass:

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

Overriding Methods

Remaining methods of CheckingAccount also invoke a superclass method:

public class CheckingAccount extends BankAccount 
{  
   private static final int FREE_TRANSACTIONS = 3; 
   private static final double TRANSACTION_FEE = 2.0; 
   private int transactionCount; 
   . . . 
   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; 
   } 
   . . . 
}

Syntax 10.2 Calling a Superclass Method

Animation 10.1: Inheritance

Link to Flash animation

Self Check 10.6

Categorize the methods of the SavingsAccount class as inherited, new, and overridden.

Answer: The SavingsAccount class inherits the deposit, withdraw, and getBalance methods. The addInterest method is new. No methods override superclass methods.

Self Check 10.7

Why does the withdraw method of the CheckingAccount class call super.withdraw?

Answer: It needs to reduce the balance, and it cannot access the balance variable directly.

Self Check 10.8

Why does the deductFees method set the transaction count to zero?

Answer: So that the count can reflect the number of transactions for the following month.

Subclass Construction

To call the superclass constructor, use the super reserved word in the first statement of the subclass constructor:

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

When subclass constructor doesn't call superclass constructor, the superclass must have a constructor with no parameters
- If, however, all constructors of the superclass require parameters, then the compiler reports an error

ch10/accounts/CheckingAccount.java

Syntax 10.3 Calling a Superclass Constructor

Self Check 10.9

Why didn't the SavingsAccount constructor in Section 10.2 call its superclass constructor?

Answer: It was content to use the default constructor of the superclass, which sets the balance to zero.

Self Check 10.10

When you invoke a superclass method with the super keyword, does the call have to be the first statement of the subclass method?

Answer: 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 type of the anAccount variable

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 10.4 The `instanceof` Operator

Self Check 10.11

Why did the second parameter of the transfer method have to be of type BankAccount and not, for example, SavingsAccount?

Answer: 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.

Self Check 10.12

Why can't we change the second parameter of the transfer method to the type Object?

Answer: We cannot invoke the deposit method on a variable of type Object.

Polymorphism and Inheritance

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

BankAccount anAccount = new CheckingAccount();
anAccount.deposit(1000);

Which deposit method is called?

Dynamic method lookup: When the virtual machine calls an instance method, it locates the method of the implicit parameter's class

Polymorphism and Inheritance

Example:

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

When you call

anAccount.transfer(1000, anotherAccount);

two method calls result:

anAccount.withdraw(1000); 
anotherAccount.deposit(1000);

Polymorphism and Inheritance

Polymorphism: Ability to treat objects with differences in behavior in a uniform way
The first method call
```
withdraw(amount);
```
is a shortcut for
```
this.withdraw(amount);
```
this can refer to a BankAccount or a subclass object

ch10/accounts/AccountTester.java

Program Run:

Mom's savings balance: 7035.0
Expected: 7035
Harry's checking balance: 1116.0
Expected: 1116

Self Check 10.13

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?

Answer: The object is an instance of BankAccount or one of its subclasses.

Self Check 10.14

If a refers to a checking account, what is the effect of calling a.transfer(1000, a)?

Answer: The balance of a is unchanged, and the transaction count is incremented twice.

Protected Access

Protected features can be accessed by all subclasses and by all classes in the same package
Solves the problem that CheckingAccount methods need access to the balance instance variable of the superclass BankAccount:
```
public class BankAccount 
{  
   . . . 
   protected double balance; 
}
```

Protected Access

The designer of the superclass has no control over the authors of subclasses:
- Any of the subclass methods can corrupt the superclass data
- Classes with protected instance variables are hard to modify — the protected variables cannot be changed, because someone somewhere out there might have written a subclass whose code depends on them
Protected data can be accessed by all methods of classes in the same package
It is best to leave all data private and provide accessor methods for the data

`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 same contents:

if (coin1.equals(coin2)) . . . 
   // Contents are the same

Overriding the `equals` Method

== tests for references to the same object:

if (coin1 == coin2) . . . 
   // Objects are the same

Overriding the `equals` Method

Need to override the equals method of the Object class:

public class Coin 
{  
   . . . 
   public boolean equals(Object otherObject) 
   {  
      . . . 
   } 
   . . . 
}

Cannot change parameter type; 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

The `clone` Method

Copying an object reference gives two references to same object:

BankAccount account = new BankAccount(1000); 
BankAccount account2 = account; 
account2.deposit(500); 
   // Now both account and account2 refer to a bank account with a balance of 1500

Sometimes, need to make a copy of the object:

The `clone` Method

Implement clone method to make a new object with the same state as an existing object

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 Special Topic 10.6)

Self Check 10.15

Should the call x.equals(x) always return true?

Answer: It certainly should — unless, of course, x is null.

Self Check 10.16

Can you implement equals in terms of toString? Should you?

Answer: If toString returns a string that describes all instance variables, you can simply call toString on the implicit and explicit parameters, and compare the results. However, comparing the variables is more efficient than converting them into strings.

Scripting Languages

Using Inheritance to Customize Frames

Use inheritance for complex frames to make programs easier to understand
Design a subclass of JFrame
Store the components as instance variables
Initialize them in the constructor of your subclass
If initialization code gets complex, simply add some helper methods

Example: Investment Viewer Program

public class InvestmentFrame extends JFrame 
{ 
   private JButton button; 
   private JLabel label; 
   private JPanel panel; 
   private BankAccount account;
   
   public InvestmentFrame() 
   {  
      account = new BankAccount(INITIAL_BALANCE); 
      // Use instance variables for components 
      label = new JLabel("balance: " + account.getBalance()); 
      // Use helper methods 
      createButton(); 
      createPanel(); 
      setSize(FRAME_WIDTH, FRAME_HEIGHT); 
   } 
   
   private void createButton() 
   {
         button = new JButton("Add Interest"); 
      ActionListener listener = new AddInterestListener(); 
      button.addActionListener(listener); 
   } 
   
   private void createPanel() 
   { 
      panel = new JPanel(); 
      panel.add(button); 
      panel.add(label);      
      add(panel); 
   } 
}

Example: Investment Viewer Program

Of course, we still need a class with a main method:

public class InvestmentViewer2 
{  
   public static void main(String[] args) 
   {  
      JFrame frame = new InvestmentFrame(); 
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
      frame.setVisible(true);      
   } 
}

Self Check 10.17

How many Java source files are required by the investment viewer application when we use inheritance to define the frame class?

Answer: Three: InvestmentFrameViewer, InvestmentFrame, and BankAccount.

Self Check 10.18

Why does the InvestmentFrame constructor call setSize(FRAME_WIDTH, FRAME_HEIGHT), whereas the main method of the investment viewer class in Chapter 9 called frame.setSize(FRAME_WIDTH, FRAME_HEIGHT)?

Answer: The InvestmentFrame constructor adds the panel to itself.

Chapter 10 – Inheritance

Chapter Goals

Inheritance Hierarchies

Inheritance Hierarchies

Inheritance Hierarchies

Inheritance Hierarchies

Inheritance Hierarchies

Self Check 10.1

Self Check 10.2

Inheritance Hierarchies

Inheritance Hierarchies

Inheritance Hierarchies

Inheritance Hierarchies

Inheritance Hierarchies

ch10/accounts/SavingsAccount.java

Syntax 10.1 Inheritance

Self Check 10.3

Self Check 10.4

Self Check 10.5

Common Error: Shadowing Instance Variables

Common Error: Shadowing Instance variables

Overriding Methods

Overriding Methods

Overriding Methods

Overriding Methods

Overriding Methods

Syntax 10.2 Calling a Superclass Method

Animation 10.1: Inheritance

Self Check 10.6

Self Check 10.7

Self Check 10.8

Subclass Construction

ch10/accounts/CheckingAccount.java

Syntax 10.3 Calling a Superclass Constructor

Self Check 10.9

Self Check 10.10

Converting Between Subclass and Superclass Types

Converting Between Subclass and Superclass Types

Converting Between Subclass and Superclass Types

Syntax 10.4 The instanceof Operator

Self Check 10.11

Self Check 10.12

Polymorphism and Inheritance

Polymorphism and Inheritance

Polymorphism and Inheritance

ch10/accounts/AccountTester.java

Self Check 10.13

Self Check 10.14

Protected Access

Protected Access

Object: The Cosmic Superclass

Overriding the toString Method

Overriding the toString Method

Overriding the equals Method

Overriding the equals Method

Overriding the equals Method

The clone Method

The clone Method

The Object.clone method

Self Check 10.15

Self Check 10.16

Scripting Languages

Using Inheritance to Customize Frames

Example: Investment Viewer Program

Example: Investment Viewer Program

Self Check 10.17

Self Check 10.18

Syntax 10.4 The `instanceof` Operator

`Object`: The Cosmic Superclass

Overriding the `toString` Method

Overriding the `toString` Method

Overriding the `equals` Method

Overriding the `equals` Method

Overriding the `equals` Method

The `clone` Method

The `clone` Method

The `Object.clone` method