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Chapter 12: Data Structures

Presentation slides for

Java Software SolutionsFoundations of Program DesignThird Edition

by John Lewis and William LoftusJava Software Solutions is published by Addison-Wesley

Presentation slides are copyright 2002 by John Lewis and William Loftus. All rights reserved.Instructors using the textbook may use and modify these slides for pedagogical purposes.

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Data Structures

Now we explore some convenient techniques fororganizing and managing information Chapter 12 focuses on:

collections Abstract Data Types (ADTs) dynamic structures and linked lists queues and stacks non-linear data structures predefined collection classes

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Collections

A collection is an object that serves as a repository forother objects A collection usually provides services such as adding,removing, and otherwise managing the elements it

contains Sometimes the elements in a collection are ordered,sometimes they are not Sometimes collections are homogeneous, sometimes theare heterogeneous

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Abstract Data Types

Collections can be implemented in many different ways An abstract data type (ADT) is an organized collection ofinformation and a set of operations used to manage thatinformation The set of operations defines the interface to the ADT As long as the ADT fulfills the promises of the interface, itdoesn't really matter how the ADT is implemented Objects are a perfect programming mechanism to createADTs because their internal details are encapsulated

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Abstraction

Our data structures should be abstractions That is, they should hide unneeded details We want to separatethe interface of the structure from its

underlying implementation This helps manage complexity and makes it possible to changethe implementation without changing the interface

What do we mean by makes it possible to change theimplementation without changing the interface?Why is changing the implementation without changing theinterface desirable?

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What about a stack?

Is a stack an Abstract Data Type (ADT) with a collection of dataand operations that are allow on the data?How many operations can we legally perform to manipulate a stack?

Do we care about how these operations are implemented?

We only care about the what, not about the how!

A set of operations defines the interface to the ADT.What are they for a stack?

push & pop

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Static vs. Dynamic Structures

A staticdata structure has a fixed sizeThis meaning is different from the meaning of thestatic modifier (variable shared among all instances of a class)

Arrays are static; once you define the number ofelements it can hold, the number doesnt change A dynamicdata structure grows and shrinks at execution

time as required by its contents A dynamic data structure is implemented using links

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Object References

Recall that an object reference is a variable that storesthe address of an object A reference also can be called a pointer References often are depicted graphically:

student

John Smith

40725

3.58

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References as Links

Object references can be used to create links betweenobjects Suppose a Student class contains a reference toanotherStudent object

John Smith

40725

3.57

Jane Jones

58821

3.72

Note Janes null pointer.

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References as Links

References (pointers) can be used to create a variety oflinked structures, such as a linked list:

info

next

info

next

info

null

info

next

Figure 12.1 A linked list

studentList

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Intermediate Nodes

The objects being stored shouldnot be concerned withthe details of the data structure in which they may bestored For example, the Student class should not have to store

a link to the next Student object in the list Instead, we can use a separate node class with twoparts:

1) a reference to an independent object and 2) a link to the next node in the list

The internal representation becomes a linked list ofnodes

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Magazine Collection

Lets explore an example of a collection ofMagazineobjects The collection is managed by theMagazineList class,which has an private inner class calledMagazineNode Because the inner classMagazineNode is private toMagazineList, theMagazineList methods can directlyaccessMagazineNode data without violating encapsulation 1st - seeMagazineRack.java(page 641) 2nd - seeMagazine.java(page 644) 3rd - seeMagazineList.java(page 642)

http://localhost/var/www/apps/conversion/tmp/examples/chap12/Library.javahttp://localhost/var/www/apps/conversion/tmp/examples/chap12/Book.javahttp://localhost/var/www/apps/conversion/tmp/examples/chap12/BookList.javahttp://localhost/var/www/apps/conversion/tmp/examples/chap12/BookList.javahttp://localhost/var/www/apps/conversion/tmp/examples/chap12/Book.javahttp://localhost/var/www/apps/conversion/tmp/examples/chap12/Library.java

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Fig. 12.2 inserting new node into list

A method called insert could be defined to place anode anywhere in the list, for example to keep it sortedInserting a node into the middle of a list

listinfo

next

info

next

info

next

info

null

info

next

insert this new node

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Fig. 12.3 - deleting a node from a list

A method called delete could be defined to remove anode from the listDeleting a node from a list

listinfo

next

info

next

info

next

info

null

info

next

What must we be careful of when deleting a node?

(Hint: what could we lose?)

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Fig 12.4 - doubly linked list

It may be convenient to implement as list as a doublylinked list, with next andprevious references

A doubly linked list

listinfo

next

info

next

info

next

info

null

info

next

prev prev prev prevnull

What is contents of the first nodes prev pointer ?

What does a doubly linked list allow ?

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Fig. 12.5 - list with front & rear references

It may be convenient to use a separate header node, with acount and references to both the front and rear of the list

info

next

info

next

info

null

info

next

front

rear

count: 4list

What will this structure allow to occur quicker

than without rear pointer?

header node

list with front and rear references

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Other Dynamic List Implementations

A linked list can be circularly linked --last node in the list points to the first node in the list

If the linked list is doubly linked, the first node in the list alsopoints to the last node in the listDiscuss example implementing priority control of resourceallocation

i.e., CPU time sharing via circular queue

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Dynamic Implementations

The representation should facilitatethe intended operations and shouldmake them easy to implement.

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Classic Data Structures

Classic linear data structures include queues and stacks Classic nonlinear data structures include trees, binary

trees, graphs, anddigraphs

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Fig. 12.6 a queue data structure

A queue is similar to a list but adds items only to the rearof the list and removes them only from the front It is called a FIFO data structure: First-In, First-Out Analogy: a line of people at a movie ticket window

enqueue dequeue

first item in,

first item out

last item in,

last item out

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Queues

We can define the operations for a queue enqueue - add an item to the rear of the queue dequeue (or serve) - remove an item from the front of the queue empty - returns true if the queue is empty

As with our linked list example, by storing genericObject references, any object can be stored in thequeue

Queues often are helpful in simulations or any situation inwhich items get backed up while awaiting processing(Jobs waiting their turn to be processed.)

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Queues

A queue can be represented by a singly-linked list.Operationrs: enqueue add an item to reardequeue remove an item from front

empty returns true if queue is emptyIs it more efficient if the references point from front to the rear?

queueinfo4

null

info3

next

rear

info2

next

info1

next

front

queueinfo4

next

info3

next

info2

next

info1

null

rear front

Two

representations

of same queue

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Queues

A queue can be represented by an array What may happen as queue grows via enqueue with noimmediately occurring dequeues?

0 1 2 3 4 5

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Stacks

A stack ADT is also linear, like a list or a queue Items are added and removed from only one end of astack It is therefore LIFO: Last-In, First-Out Analogies:

a stack of plates in a cupboard a stack of bills to be paid a stack of hay bales in a barn

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Fig. 12.7 stack data structure

Stacks often are drawn vertically:

poppush

first item in, last item out

last item in, first item out

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Stacks

Some stack operations: push - add an item to the top of the stack pop - remove an item from the top of the stack peek (or top) - retrieves the top item without removing it empty - returns true if the stack is empty

A stack can be represented by a singly-linked list; it doesntmatter whether the references point from the top toward thebottom or vice versa A stack can be represented by an array, but the new itemshould be placed in the next available place in the array ratherthan at the end of the array

(What can happen with an array implementation?)

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Stacks

The java.util package contains a Stack class LikeArrayList operations, the Stack operationsoperate on Object references See (not during class) Decode.java(page 649) whichreverses the strings in a message. The words in the message are separated by a single

space. The Stack class is used to push the characters ofthe word onto the stack and then pops the characters outin reverse order.

http://localhost/var/www/apps/conversion/tmp/examples/chap12/Decode.javahttp://localhost/var/www/apps/conversion/tmp/examples/chap12/Decode.java

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Decode.java

S

m

a

r

t

push

t r a m S

pop

S m a r t

reverse a string1

2

3

4

5

2

3

4

51

Decode.java reverses the strings in a message

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Fig 12.8 - Trees

A tree is a non-linear data structure that consists of a rootnode and potentially many levels of additional nodes that forma hierarchy

Nodes that have no children are called leaf nodes Non-root and non-leaf nodes are called internal nodes

A tree data structureleaf nodes

root nodeimagine an upsidedown tree internalnodes

Tree can re resent inheritance relationshi between classes.

Organization chart represented via

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Organization chart represented viaa tree data structure

leaf nodes

root nodepresidentVP VP VP VP

mgr mgr

mgr

mgr

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Binary Trees

A binary tree is defined recursively. Either it is empty (thebase case) or it consists of a root and two subtrees, eachof which is a binary tree

Binary trees and trees typically are represented usingreferences as dynamic links, though it is possible to usefixed representations like arrays

leaf nodes

root node

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Fig. 12.9 - graph

A graph is a non-linear structure (also called a network) Unlike a tree or binary tree, a graph does not have a root noprimary entry point. Any node can be connected to any other node by an edge Can have any number of edges and nodes Analogy: the highway system connecting cities on a map

a graph data structure

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Fig. 12.10 - digraphs

Each edge of directed graph or digraph has a specific directiondenoted by arrows. Edges with direction sometimes are called arcs Analogy #1: airline flights between airports (see below) Analogy #2: Solution to a problem (on board miles on arcs)

a directed graph

airline routes represented via digraph

What else could be provided?

A R

B

C

D E

F

LJ

P

Y

WX

N

S

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Graphs and Digraphs

Both graphs and digraphs can be represented usingdynamic links or using arrays.

As always, the representation shouldfacilitate the intended operations andmake them convenient to implement

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Collection Classes

The Java standard library contains several classes that representcollections, often referred to as the Java Collections API.

(APIApplication Programmer Interface) Java Collections API supports the organization and management of data. Their underlying implementation is implied in the class names such asArrayList and LinkedList Several interfaces are used to define operations on the collections, such as

List, Set, SortedSet,Map, and SortedMapSet a collection of items with no duplicates.

Map group of items that can be referenced by a

key value.

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Summary

Chapter 12 has focused on: collections Abstract Data Types (ADTs) dynamic structures and linked lists queues and stacks non-linear data structures predefined collection classes