CS 106Introduction to Computer Science I

04 / 25 / 2007

Instructor: Michael Eckmann

Michael Eckmann - Skidmore College - CS 106 - Spring 2007

Today’s Topics• questions, comments? • more on interfaces• ArrayLists

Interfaces• Notice how the bubbleSort method can now sort any array of

objects as long as those objects are of class types that implement Comparable.

• This is a clear benefit of polymorphism with Interfaces.

instanceof• we saw that instanceof can test whether a reference is

referring to a particular type of object.

Interfaces• So, the bubbleSort method that sorts Comparables now can

sort an array of

– Strings

– Cards

– Contacts

– references to any objects whose class implements the Comparable interface

Interfaces• Interfaces can have hierarchies just like classes have hierarchies

• That is, an interface can inherit from another interface.

• The child interface inherits everything from the parent interface (all the abstract methods and all the constants.)

• Any class that implements an interface

– must implement the interface's methods and all its parents' methods

– gets access to all the constants in the child interface and the parents' interfaces

Interfaces• Example:

– All these interfaces are made up (they are NOT part of the Java API) – assume that they are interfaces we create ourselves.

– Let's say we have interfaces named Liquid, Drinkable, and TastyDrink.

– Let's assume that the TastyDrink interface inherits from Drinkable. And the Drinkable interface inherits from Liquid.

– A class, say Soda, implements the TastyDrink interface.

– Class Soda must provide implementations of all the abstract methods of TastyDrink, Drinkable and Liquid.

– Class Soda also has access to all the constants in the three interfaces.

Interfaces• A big difference between classes and interfaces is in the possible

hierarchies.

• Class hierarchies and interface hierarchies never overlap. They are kept separate.

• Class hierarchies cannot have multiple inheritance, but interface hierarchies can. We saw an example on board last time.

• For the syntax see:

• http://java.sun.com/docs/books/tutorial/java/IandI/interfaceDef.html

• Also, a class can extend only one other class, but can implement any number of interfaces.

• Eg. A class can implement both Comparable and Drinkable.

Interfaces• Interfaces can be used as a reference type. Anywhere you can use

other types (primitives or class names) you can use an interface name.

• Only objects of classes that implement the interface are allowed to be assigned to that reference variable.

• Example:

– We just saw that a method can have as a parameter a Comparable variable. (or in our case, an array of Comparables.)

– This means that the only objects that can be passed in to the method must be of a type (a class) that implements Comparable

• This is extremely useful as it allows use to write more generic code.

Designing classes• Before we discuss designing for polymorphism, I'd like to

discuss my opinions on what a good design for the last assignment is.

• A class to represent a Fraction. The best name for this class is Fraction. Why? because it represents one fraction.

• A class that contains static methods that determine primes, perfects, etc. I'll explain why I believe they should be static.

Designing classes• A class to represent a Fraction contains as instance data

– numerator

– denominator

• and methods to

– multiply

– divide

– add

– subtract

• Since these methods are non-static they'll need to be called by a Fraction object. But these methods all work on 2 Fractions so we'll take the second Fraction in as a parameter.

Designing classes• Further, I would reduce the Fraction to lowest terms in the

Fraction constructor. If there are any other functions that can change the numerator and/or denominator then I'd reduce the fraction to lowest terms there too.

• In the methods to multiply, divide, add and subtract, you should Instantiate a new Fraction which will be the resulting Fraction that will be returned.

Designing classes• Fraction class shouldn't have any user input or output (i.e. not

dialog boxes nor console printing). Its whole job should be to represent a fraction and provide behaviors of fractions (like arithmetic operations).

• We'll want to sometimes print a Fraction in a standard format so Fraction should provide a toString() method that returns a String like:

2 / 5

• This is better than a method in Fraction that actually does the printing to the console or a dialog box. Why?

Designing classes• This is better than a method in Fraction that actually does the

printing to the console or a dialog box. Why?

• Because the program that uses the Fraction class might want to write the fraction to a file or send it as an email message or print it to the console or print it in a dialog box or ...

• Providing a toString() method supplies the programmer with a way to get the Fraction as a String and then s/he can do whatever s/he wants to with that String.

• The Fraction class should be written so that it can be used by any program that needs Fractions regardless of what that program needs to do with them. It should be able to use the Fraction class as written, unaltered.

Designing classes• A class with static methods:

– isPrime• parameter: an int• return: boolean

– isPerfect• parameter: an int• return: boolean

Designing classes• Same class with these static methods:

– printPrimes• parameter: an int (representing the upper limit)• prints all the primes from 1 to upper limit• uses isPrime• returns nothing

– printPerfects• parameter: an int (representing the upper limit)• prints all the perfects from 1 to upper limit• uses isPerfect• returns nothing

• Why would I make these methods static?

Designing for polymorphism• Polymorphism allows a consistent approach to inconsistent but

related behaviors. What does that mean?

• First examine the problem to find opportunities that lend themselves to this before we write code.

• Let's look at some example situations and discuss them.

Designing for polymorphism• Different types of vehicles move in different ways

• All business transactions for a company must be logged

• All products produced by a company must meet certain quality standards

• A hotel needs to plan their remodeling efforts for every room

• A casino wants to analyze the profit margin for their games

• A dispatcher must schedule moving vans and personnel based on the job size

• A drawing program allows the user to draw various shapes

ArrayList• This is a good time to bring up a class in Java API named

ArrayList, because it works on general Objects.

• ArrayList is a class in the Java API

• It can store different types of data into an “array”

• An ArrayList can change size throughout the lifetime of the program

– whereas a regular array is a fixed size

• An ArrayList actually stores references to objects, not the objects themselves

• We cannot store values of primitive types directly -- we have to use the wrapper classes if we want to do that (we'll see what this means later).

ArrayList• Some ArrayList methods

– ArrayList() --- constructor

– boolean add(Object o) – add the object reference to the end of the list

– void add(int index, Object o) – add the object reference to the list at the index

– void clear() -- remove all elements from the list

– Object get(int index) – return the object reference at that index in the list

– int indexOf(Object o) – returns the index of the first occurrence of o

– boolean isEmpty() -- returns true if the list contains no elements, false otherwise

ArrayList• Some ArrayList methods

– Object remove(int index) – returns the object reference at that index in the list and removes it from the list

– int size() – returns the number of elements in the list

– boolean contains(Object o) -- returns true if the list contains o, false otherwise

– void ensureCapacity(int minCapacity) – tells Java to ensure that the list will be able to contain at least minCapacity references

– int lastIndexOf(Object o) – what do you think this does?

– boolean remove(Object o) -- removes a single instance of o from the list

– void removeRange(int fromIndex, int toIndex) --removes from the list all of the elements whose index is between fromIndex, inclusive and toIndex, exclusive.

ArrayList• Some ArrayList methods

– Object set(int index, Object o) -- replaces the element at the specified index in this list with o.

– What if this method wasn't in the class. How might you achieve that functionality?

ArrayList• Let's create an ArrayList of different types and add some elements to it

and remove some, etc...

• When we create one that allows different types, we have to cast any returned Objects from methods to be the type we want.

• Because Java implemented ArrayLists generically, and because of the way Java decided to have the class hierarchy such that all classes have Object as their superclass, this usefulness of ArrayLists is dramatically increased over a class that may have only allowed one type for its elements.

• So, that's a large benefit of good object oriented design --- generality --- so that you (or the Java implementers themselves) don't have to implement multiple classes to work on multiple types.

• If you create your class hierachy well, lots of stuff can be done generically (e.g. Have code that works on all Pets instead of just Dog or Cat objects for instance.)

ArrayList• If we stored several different types of data in a list, we may not know at

some point what type of object is actually stored in a particular index of the ArrayList.

• How do we find out what type a particular object actually is?

ArrayList• If we stored several different types of data in a list, we may not know at

some point what type of object is actually stored in a particular index of the ArrayList.

• How do we find out what type a particular object actually is?

• We can use the instanceof operator to check if some reference is an instance of some type.

e.g.

Object o = somearraylist.get(0);

if (o instanceof Contact)

{

//then cast o to be a Contact and use it

}

ArrayList• Let's look at how each method affected the list and what they returned.

– ArrayList() , boolean add(Object o) ,

– void add(int index, Object o) , void clear() , Object get(int index) ,

– int indexOf(Object o) , boolean isEmpty() ,

– Object remove(int index) , int size() , boolean contains(Object o) ,

– void ensureCapacity(int minCapacity) ,

– int lastIndexOf(Object o) , boolean remove(Object o) ,

– protected void removeRange(int fromIndex, int toIndex)

Polymorphism via Inheriance• An ArrayList uses polymorphism because it holds

references to objects of Object.

• An Object reference can be used to refer to any object (of any type)!!!