Inheritance

One of the biggest advantages of object-oriented

design is that of inheritance. A class may be

derived from another class, the base class. The

derived class inherits the fields and methods of the

class. The derived class may have additional fields

and methods, and may replace the methods that it

inherits.

Benefits of Inheritance

Inheritance allows the programmer to write simpler,

smaller code. If there are two or more classes that

are similar (but not exactly the same), all the

commonalities can be put into a single base class.

The classes are then derived classes of the base

class. Only the differences between the derived

classes and the base classes need to be coded.

Example: Bank Account

Suppose the we have different types of bank

accounts: Checking account Checking account with interest Checking with overdraft protection Savings account Medical spending account

Example (cont'd)

The fields and methods that are common to all

accounts can be put into a single base class. This

might include: Balance Transaction log Deposit Withdraw Check Balance

Example (cont'd)

The various account class would be derived from the

base class and would inherit the common fields

and methods. We could then add additional fields

and methods to customize each derived class. For

example, a checking account with interest might

have an APR field and a method to add interest to

the balance.

Overiding Methods

In addition to new methods, a derived class can

replace the methods of the base class. This is called

overriding a method. For example, the savings

account might have restriction on the number of

withdrawals that can be made in a month. So, the

withdraw method would be different for the

savings account class.

Syntax

To create a derived class, we use the keyword

extends in the class heading followed by the base

class. For example:

public class SavingAcct extends BankAccount {

}

Hierarchy of Classes

A derived class may also be the base class for

another derived class. That is, you can have a chain

of derived class. There is a hierarchy of classes,

that is, a tree of all classes in Java. The root of the

tree, the class from which all other classes are

derived is called Object. All classes inherit the

methods of Object.

Terminology

We often use the term subclass for a derived class

and superclass for the base class. Also some use

the terms child class and parent class. We also use

the terms descendant class and ancestor class for

class related by more than one level of inheritance.

Note that deriving a class is very different from

instantiating a class. We are not creating any

objects, but rather, modifying a blueprint.

Overriding Methods

A derived class may override a method inherited

from the base class. We simply write a new

method with the same name and parameter list

(number and types – names don't matter here) as

the method in the base class. When the method is

called upon an object of the derived class, the

derived class' version of the method is invoked.

Overloading Methods

Note that overriding a method is different from

overloading a method. We overload a method

when the parameter list of the new method is

different (different number or types) than the

parameter list of the original method. Java will call

the appropriate method based on the number and

types of the arguments in the call to the method.

It is a common mistake to overload when we me to

override a method.

The Return Type

In general, you cannot change the return type of an

overridden method. The exception is that if the

base method has a return type which is a class, the

derived class may have a return type which is a

descendant of the base class' return type. For

example, if the BankAccount class has a method

that returns a BankAccount, the SavingsAcct class

could override that method and return a

SavingsAcct.

Restricting Access

Some times we only want to allow methods of a

derived class to call a method in the base class. We

can use the access modifier protected (as opposed

to public or private) to do so. That is, if a method

or field is protected, only the methods of the class

and any derived classes may access that field or

method.

Object Class

All classes are descendants of the Object class. The

Object class has no fields but does have several

useful methods, for example, the toString method.

However, these methods work for an generic

object, not your specific object and may not do

what you want. Thus, it's often useful to override

these methods.

toString Method

The toString method is used to return (as a String) a

human-readable description of the object. It should

create a string with the information from the

pertinent fields formatted in an easily

understandable way. The default toString method

prints out the name of the class and a

representation of the pointer to the data object.

equals Method

The equals method is useful for comparing two

objects to check if they are the same. The default

equals method returns true if and only if the two

objects are the same data object in memory. Usual,

you should override this method to return true if

and only if the two objects represent the same

thing, i.e., the relevant fields are equal.

equals Method (cont'd)

Note the definition of the equals method is:

public boolean equals(Object other)

That is, the argument is of type other. To use the

fields and methods of the actual class of objects

being compared, you will need to typecast other to

the right type, i.e., ((Ship)other).size.

Other Useful Stuff

The instanceOf operator checks if an object is of a

particular type. The syntax is:

<Object> instanceof <ClassName>

The getClass() method of class Object returns the

class that was used to create the object.

Abstract Class

An abstract class is a class that can never been

instantiated. It should be used for a base class

when no generic object will ever be created, but

rather, only objects of the derived classes can be

created. For example, in the Nethack program,

there is no such thing as a generic Creature, only

Dragon, UmberHulk, Human, etc. The keyword

abstract is used to denote an abstract class.

Using an Abstract Class

One time we need to use an abstract class is when we

have an array which contains objects of different

class. The abstract base class is the type of the

object stored in the array. For example, in the

Battleship program, the board had a field which

was a 10x10 array of Thing, where a Thing could

be a Ship, Water, Hit, or Miss. Thing would be an

abstract base class.

Abstract Base Class (cont'd)

An abstract base class may have fields and methods

that are inherited by the derived classes.

Abstract Method

On ocassion, all the derived classes have the same

method, but the method is different for each

derived class. That is, there is no one generic

method that is used by any class. However, we

would like to specific that every derived class

should have such a method (we just don't know

what the code is!)

An abstract method in a base class is one that has no

code, but must be overridden in any derived class.