CS 106Introduction to Computer Science I

04 / 25 / 2008

Instructor: Michael Eckmann

Michael Eckmann - Skidmore College - CS 106 - Spring 2008

Today’s Topics• Comments and/or Questions?• Contact class implementing the Comparable

interface• more on interfaces and polymorphism• ArrayList

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 hierarchy 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 call some of the methods and see what they return or how they affect

the list.

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)!!!

Linked lists• A linked list is a data structure where every node contains data

and reference(s) to other node(s.)

• We, as programmers create linked lists by “attaching” node references to each other.

Linked lists • In a singly linked list every node contains data and one

reference to another node.

– e.g.

class Node

{

public AnyType data; // can have more data than one

public Node next;

}

• A Linked List is maintained by keeping a reference to the head of the list.

• From the head, we are able to get at all the other nodes in the list by following the next reference.

Linked lists class Node

{

public AnyType data; // can have more data than one

public Node next;

// when constructing a new node, we set its data and

// set the next reference to null

public Node(AnyType d)

{

data = d;

next = null;

}

}

Linked lists Let me draw a representation of a node on the board and the

connection of that node to other nodes.

Recall that a reference holds an address and when we want to visualize what is happening, we draw an arrow from the reference to the data at the address stored in the reference.

Linked lists Node head; // this will be a reference to the head of the LL

// make sure this is at a scope high enough to be

// accessible for the life of the linked list.

// elsewhere we can have:

Node n = new Node(somedata);

head = n; // sets n to be the head of the linked list

Node newnode = new Node(somedata2);

// to add a newnode to the beginning of the list:

newnode.next = head;

head = newnode;

Linked lists Node newnode = new Node(somedata2);

// to add a newnode to the end of the list (assuming the list is

// not empty, i.e. head != null:

Node currnode;

currnode = head;

while (currnode != null)

{

savenode = currnode;

currnode = currnode.next;

}

savenode.next = newnode;

Linked lists• Let's figure out how to

– Insert after a particular node

–Delete a particular node

–Let's assume that the node we're looking for is definitely in the linked list.

Linked lists • Let's figure out how to

– Insert after a particular node

// insert newnode after findnode

currnode = head;

while (!((currnode.data).equals(findnode.data)))

{

currnode = currnode.next;

}

newnode.next = currnode.next;

currnode.next = newnode;

Linked lists• Let's figure out how to

–Delete a node

// delete findnode

Node prevnode;

Node currnode = head;

while (!((currnode.data).equals(findnode.data)))

{

prevnode = currnode;

currnode = currnode.next;

}

prevnode.next = currnode.next;

currnode = null;

Linked lists• Now let's reconsider these operations to take into account that the

list might be empty (i.e. that head == null).

• Also, let's reconsider the possibility that if we are looking for some particular node that it may not be in the linked list.

Linked lists• Other options

–Storing link to last node (tail)

–Doubly linked lists (and their operations.)

–An idea to have a "dummy" head and tail

• makes adding new nodes easier because we won't have a special case that when the linked list is empty, head doesn't need to be reset.

Linked lists • A doubly linked list is a data structure where every node

contains

– data

– a reference to previous node

– a reference to next node

• What do you think the value of the next node is for the last element of a linked list?

• What do you think the value of the previous node is for the first element of a linked list?

Linked lists• Let's implement our deck of cards as a linked list of card nodes

instead of as an array of cards.

• Note well:

– A user of the Deck and Card class should not need to know how we decided to store the Deck of Cards.

• i.e. the implementation details should be hidden from the user of these classes.

• Deck was implemented as an array of Cards, now we're going to change the implementation to a linked list of Cards and last time we discussed the possibility of storing the Cards in an ArrayList.

Linked lists• Compare a linked list's behaviors to that of arrays and

ArrayLists.

–Which are dynamic in length?

–How about ease of operations in terms of:

• speed of execution and • programmer ease

–Consider these operations:• Add• Insert• Delete