M8 - Collections

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Module 8 Core Java API: Collections

Java Course


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Module Objectives

At the end of this module, participants will beable to: Identify the core Collections interfaces and

their behavior. Use implementations of the various

Collections interfaces.


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Overview of the Java Collections API

The Java Collections API is a set of interfaces andimplementations included in the Java standard library. A Collection or a Container is an object that groups together

multiple elements into a single unit. Each collection implements different storage, retrieval and

manipulate behaviors. Found under the java.util package.


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The API contains Interfaces that specifies the abstractbehavior of each collection. The API contains concrete implementations of the interfaces. Each concrete implementation uses different algorithms and

behavior.

Overview of the Java Collections API (cont.)


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Overview of the Java Collections API (cont.)

Collection

Set List Queue

Sorted Set

Map

Sorted Map


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Collection Interface

Root of the collections hierarchy. The Collections interface represents general purpose behavior

for the interfaces in the hierarchy. Does not have a concrete implementation. Used in order to obtain a general reference to any kind of

Collection.

** Refer to the CollectionsSample.java sample code


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Set Interface

A Set is a Collection that does not allow duplicate entries. A Set Has the same methods as the Collection interface. Can contain at least 1 null element. Commonly used implementations are HashSet, LinkedHashSet

and TreeSet. Elements in a HashSet and LinkedHashSet are not sorted. Elements in a Treeset are sorted.

** Refer to the SetSample.java sample code


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List Interface

A List is an ordered collection of elements. May contain duplicates. An elements indexing is similar to arrays, with the first element

in index 0. Contains methods that allow manipulation of elements based on

its position in the sequence. Common implementations are ArrayList and LinkedList.

** Refer to the ListSample.java sample code


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Queue Interface

A Queue is a collection for holding elements typically in a First-In-First-Out order. Has versions for a set of methods inherited from the Collections

interface that return special values instead returning anexception when failing.

** Refer to the QueueSample.java sample code

Throws Exception Returns special value

Insert add() offer()

Remove remove() poll()

Examine element() peek()


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Map Interface

A Map is a collection that pairs a key to an element contained inthe collection. The key and the element can be any Object. A key is assigned to an element when it is added to the map

using the put(, ). The key is used to retrieve an element from the Map using the

get() method.

** Refer to the MapSample.java sample code


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Ordering

A collection can be sorted using the Collections.sort() method. Elements in a collection implement the Comparable interface

which defines the sorting order. Elements that do not implement the Comparable interface

cannot be sorted and will throw an exception.


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Comparable Interface

The Comparable Interface can be implemented by any classthat wants to define its natural ordering. Classes that implement the Comparable interface will need to

define the compareTo(Object) method which defines how aninstance of the class orders against the specified object.

The compareTo(Object) method should return: 0 if the instance and the specified object are equal. A negative integer if the instance is less than the specified object. A positive integer if the instance is greater than the specified object.


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Comparator Interface

The Comparator interface is similar to the Comparable interfaceexcept that it accepts two objects to be compared as parameters The Comparator interface can be implemented to compare two

objects that are not Comparable Classes implementing the Comparator interface will need to

override the compare(Object A, Object B) method to return thefollowing:

0 if A and B are equal. A negative number if A is less than B.

A positive number if A is greater than B.

** Refer to the Car.java and ComparableSample.java sample code


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Questions and Comments


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Activity1) Open your SEF workspace in Eclipse.2) In the package explorer, navigate to the

following package sef.module9.activity.3) Review the interfaces Radar and

RadarContact and note the requirementsof the interfaces.

4) Complete the implementation ofRadarContactImpl based on theRadarContact interface.

5) Test the implementation using theRadarContactTest JUnit driver.

6) Complete the implementation of theDistanceComparator class. 7) Complete the implementation of

RadarImpl based on the Radar interface.8) Test the implementation using RadarTest

JUnit driver.

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M8 - Collections