InheritancePart II

Lecture Objectives

• 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

Implementing Remaining Methods

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

Implementing Remaining Methods (Cont’d)

public void deductFees() {

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

Overriding vs. Overloading

• When you override a method, the new method definition in the derived class has the same name and the same number of types of parameters as the method definition in the base class.

• When the name is the same, but the number or types of the parameters differs, whether in the base class or in the derived class, the method is overloaded in the derived class.

Overriding vs. Overloading (Cont’d)

• Example:

in the class SavingAccount and

in class BankAccount overloads the method getName() since the two methods have different parameter lists.

It should be noted that both methods [getName(String title) and String getName()] are available in class SavingAccount!

public String getName(String title)

public String getName()

A Subtle Point About Overloading and Overriding

• Even when two methods have the same number of parameters, a difference in parameter type is sufficient to qualify for overloading.

• But, a reference to an ancestor class type can refer to a descendant type.

• Hence an overridden method in an ancestor class sometimes needs to be invoked explicitly using super.

The final Modifier

• You can prevent a method definition from being overridden by adding the word final to the method heading.

• Example:

• This is used rarely, but it produces more efficient code.

public final void someMethod() {…

}

The final Modifier (Cont’d)

• An entire class can be declared final, in which case it cannot be used as a superclass to derive another class.

• Example:

• Therefore, the class Math can NOT be extended or used as a superclass.

package java.lang;

public final class Math {…

}

Private Instance Variables in the Base Class

• Private instance variables inherited from a base class cannot be accessed directly.

• Instead, they must be accessed using a method that is not declared private.

• While this may seem inconvenient, it provides an important mechanism for controlling access and changes to instance variables in the base class.

Private Methods in the Base Class

• Like private instance variables, private method inherited from a base class cannot be accessed directly.

• Instead, they, too, must be accessed using a method that is not declared private.

• This, too, provides an important mechanism for controlling access to methods in the base class.

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 { private double balance; // Don't

public void deposit(double amount) { transactionCount++; balance = balance + amount; } . . . }

Common Error: Shadowing Instance Fields (Cont’d)

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

Figure 1: Shadowing Instance Fields

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; } . . . }

Subclass Construction (Cont’d)

• 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: Calling a Superclass Constructor

ClassName(parameters){   super(parameters);   . . .}

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

Purpose:To invoke a constructor of the superclass. Note that this statement must be the first statement of the subclass constructor.

Converting Between Subclass and Superclass Types

• It is OK to convert subclass reference to superclass reference

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

Converting Between Subclass and Superclass Types (Cont’d) • The three object references stored in collegeFund, anAccount, and anObject all refer to the same object of type SavingsAccount

Figure 2: Variables of Different Types Refer to the Same Object

Converting Between Subclass and Superclass Types (Cont’d)

• Superclass references don't know the full story:

• 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

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

Converting Between Subclass and Superclass Types (Cont’d)

• Why would anyone want to know less about an object? Reuse code that knows about the superclass but not

the subclass:

• Can be used to transfer money from any type of BankAccount

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

Converting Between Subclass and Superclass Types (Cont’d)

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

• This cast is dangerous: if you are wrong, an exception is thrown

BankAccount anAccount = (BankAccount) anObject;

Converting Between Subclass and Superclass Types (Cont’d)

• Solution: use the instanceof operator

• instanceof: tests whether an object belongs to a particular type

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

Syntax: The InstanceOf Operator

 object instanceof TypeName

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

Purpose:To return true if the object is an instance of TypeName (or one of its subtypes), and false otherwise

The class Object: The Cosmic Superclass

• All classes defined without an explicit extends clause automatically extend Object

Figure 3: The Object Class is the Superclass of Every Java Class

The class Object: The Cosmic Superclass (Cont’d)

• Most useful methods: String toString() boolean equals(Object otherObject) Object clone()

• Good idea to override these methods in your own 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]"

Overriding the tostring() Method (Cont’d)

• The toString() method is called whenever you concatenate a string with an object:

• The Object.toString() method prints class name and the hash code of the object

"box=" + box; // Result: "box=java.awt.Rectangle[x=5,y=10,width=20,height=30]"

BankAccount momsSavings = new BankAccount(5000); String s = momsSavings.toString(); // Sets s to something like "BankAccount@d24606bf"

Overriding the tostring() Method (Cont’d)

• To provide a nicer representation of an object, override the toString() method:

• This works better:

public String toString() { return "BankAccount[balance=" + balance + "]"; }

BankAccount momsSavings = new BankAccount(5000); String s = momsSavings.toString(); // Sets s to "BankAccount[balance=5000]"