Collections

Collections

Using Generics in Collections

Examining LISTS

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Chapter Objectives

Define the concept and terminology related to collections

Explore the basic structure of the Java Collections API

Discuss the abstract design of collections

Define a set collection

Use a set collection to solve a problem

Examine an array implementation of a set

Define and examine concepts of a LIST

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Collections A collection is an object that gathers and organizes other

objects (elements)

Many types of fundamental collections have been defined: stack, queue, list, tree, graph, etc.

They can be broadly categorized as linear (organizes the elements in a straight line) or nonlinear

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FIGURE 3.1 A linear and a nonlinear collection

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Collections The elements within a collection are usually organized

based on:

the order in which they were added to a collection, or some inherent relationship among the elements themselves

For example, a list of people may be kept in alphabetical order by name or in the order in which they were added to the list

Which type of collection you use depends on what you are trying to accomplish

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Abstraction

An abstraction hides certain details at certain times

It provides a way to deal with the complexity of a large system

A collection, like any well-defined object, is an abstraction

We want to separate the interface of the collection (how we interact with it) from the underlying details of how we choose to implement it

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FIGURE 3.2 A well-defined interface masks the implementation of the collection.

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Issues with Collections For each collection we examine, we will consider:

How does the collection operate conceptually?

How do we formally define its interface?

What kinds of problems does it help us solve?

What ways might we implement it?

What are the benefits and costs of each implementation?

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Terms Various terms are used in the study of collections – defined in various

ways Recall our definitions:

data type – a group of values and the operations defined on those values

abstract data type – a data type whose values and operations are not inherently defined in a programming language -

data structure – the programming constructs used to implement a collection

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The Java Collections API The Java Collections API is a set of classes that represent some specific

collection types, implemented in various ways

It is part of the large class library that can be used by any Java program

API stands for Application Programming Interface

As we explore various collections, we will examine the appropriate classes in the Java Collections API

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A Set Collection Let's look at an example of a collection

A set collection groups elements without regard to their relationship to each other

It's as if you threw them all in a box

You can reach into a box and pull out an element, and are equally likely to get any one

It is a nonlinear collection, but could be implemented with a linear data structure

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The conceptual view of a set collection

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Collection Operations Every collection has a set of operations that define how we

interact with it They usually include ways for the user to:

add and remove elements determine if the collection is empty determine the collection's size

They also may include:

iterators, to process each element in the collection operations that interact with other collections

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FIGURE 3.4 The operations on a set collection

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Java Interfaces The programming construct in Java called an interface is a convenient

way to define the operations on a collection

A Java interface lists the set of abstract methods (no bodies) that a class implements

It provides a way to establish a formal declaration that a class will respond to a particular set of messages (method calls)

GENERIC TYPES

Generic Data Type A type for which the operations are defined but the types of the items being manipulated are not.

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Listing 3.1

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Listing 3.1 (cont.)

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FIGURE 3.5 UML description of the SetADT<T> interface

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Iterators An iterator is an object that allows the user to acquire and

use each element in a collection in turn

The program design determines:

the order in which the elements are delivered the way the iterator is implemented

In the case of a set, there is no particular order to the elements, so the iterator order will be arbitrary (random)

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Iterators Collections that support iterators often have a method

called iterator that returns an Iterator object

Iterator is actually an interface defined in the Java standard class library

Iterator methods:

hasNext – returns true if there are more elements in the iteration

next – returns the next element in the iteration

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Set Exceptions Collections must always manage problem situations

carefully

For example: attempting to remove an element from an empty set

The designer of the collection determines how it might be handled

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Implementing a Set We use a set collection without any regard to how that

collection was implemented

We use the set collection for its functionality – how it is implemented is fundamentally irrelevant

It could be implemented in various ways

Let's examine how we can use an array to implement it

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An array of object references

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Managing Capacity An array has a particular number of cells when it is created

– its capacity So the array's capacity is also the set’s capacity

What do we do when the set is full and a new element is added?

We could throw an exception

We could return some kind of status indicator

We could automatically expand the capacity- This is the best solution

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The ArraySet Class

In the Java Collections API, class names indicate both the underlying data structure and the collection

Thus, the ArraySet class represents an array implementation of a set collection

Set elements are kept contiguously at one end of the array

An integer (count) represents: the number of elements in the set the next empty index in the array

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FIGURE 3.9 An array implementation of a set

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ArraySet - Constructors

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ArraySet - size and isEmpty

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ArraySet - add

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ArraySet - expandCapacity

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ArraySet - addAll

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ArraySet - removeRandom

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ArraySet - remove

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ArraySet - union

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ArraySet - contains

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ArraySet - equals

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ArraySet - equals (continued)

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ArraySet - iterator

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Listing 3.4

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Listing 3.4 (cont.)

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Listing 3.4 (cont.)

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FIGURE 3.10 UML description of the bingo system

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Analysis of ArraySet If the array is not full, adding an element to the set is O(1)

Expanding the capacity is O(n)

Removing a particular element, because it must be found, is O(n)

Removing a random element is O(1)

Adding all elements of another set is O(n)

The union of two sets is O(n+m), where m is the size of the second set

Chapter 8 LISTS

Using Arrays with LISTS -

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Chapter 8 Objectives Examine list processing and various ordering techniques

Define a list abstract data type

Demonstrate how a list can be used to solve problems

Examine various list implementations

Compare list implementations

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Lists Lists are linear collections that are noted for their

flexibility

Adding and removing elements in lists are not restricted by the collection structure

That is, they don't have to operate on one end or the other

We will examine three types of list collections: ordered lists unordered lists indexed lists

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Ordered Lists The elements in an ordered list are ordered by some

inherent characteristic of the elements

names in alphabetical order

scores in ascending order

Therefore, the elements themselves determine where they are stored in the list

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A conceptual view of an ordered list

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Unordered Lists

There is an order to the elements in an unordered list, but that order is not based on element characteristics

The user of the list determines the order of the elements

A new element can be put on the front or the rear of the list, or it can be inserted after a particular element already in the list

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A conceptual view of an unordered list

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Indexed Lists In an indexed list, elements are referenced by their numeric

position in the list

Like an unordered list, there is no inherent relationship among the elements

The user can determine the order

Every time the list changes, the indexes are updated

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A conceptual view of an indexed list

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List Operations There are several operations common to all three list types

These include removing elements in various ways and checking the status of the list

The key differences between the list types involve the way elements are added

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The common operations on a list

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The operation particular to an ordered list

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FIGURE 8.6 The operations particular to an unordered list

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List Operations As with other collections, we use Java interfaces to define

the collection operations

Recall that interfaces can be defined via inheritance (derived from other interfaces)

We define the common list operations in one interface, then derive three others from it that define the interfaces of the three list types

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Listing 8.1

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Listing 8.1 (cont.)

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Listing 8.2

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Listing 8.3

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Listing 8.4 (cont.)

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Implementing Lists with Arrays

An array implementation of a list could follow strategies similar to those used for a queue – which we will look at later

We could fix one end of the list at index 0 and shift as needed when an element is added or removed

Or we could use a circular array to avoid the shift at one end

However, there is no avoiding a shift when an element in the middle is added or removed

Let's examine the fixed version

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FIGURE 8.13 An array implementation of a list

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ArrayList - the remove Operation

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ArrayList - the find Operation

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ArrayList - the contains Operation

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ArrayOrderedList - the add Operation

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Analysis of List Implementations

In both array and linked implementations, many operations are similar in efficiency

Most are O(1), except when shifting or searching need to occur, in which case they are order O(n)

In particular situations, the frequency of the need for particular operations may guide the use of one approach over another