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: instanceof 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.