Transcript Chapter 12 Collections

Chapter 12
Collections
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Java Collection Frameworks
The Java collection framework is a set
of utility classes and interfaces.
Designed for working with collections of
objects
A collection is a group of objects, each of
which is known as an element in the
collection.
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Types of Collections
Simple Collections
Sets have no duplicate elements.
Lists are ordered collections that can have
duplicate elements.
Maps
Map uses key/value pairs to associate an object
(the value) with a key.
Both sets and maps can be sorted or
unsorted.
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The Collection Interface
Root interface of
collection classes
Common methods
defined in Collection:
add()
contains()
isEmpty()
iterator()
remove()
size()
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The ArrayList
ArrayList is a concrete
implementation of the List interface.
A concrete implementation is a class that
can be instantiated.
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ArrayList vs. an Array
An ArrayList has several advantages:
Can be resized dynamically as more
objects are added
Elements can be removed from an
ArrayList
Simpler to use than an array
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Controlling ArrayList size
Important methods:
size() returns the number of elements
ensureCapacity() sets the capacity of
the ArrayList to a certain number of
elements
trimToSize() trims an ArrayList to
only the number of elements it currently
holds to reduce memory requirements
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Using Collections
Although objects in a collection do not
have to be of the same type:
It is not good practice to mix object types
in a collection.
You must check object type and cast to
appropriate class if heterogeneous
collections are used.
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Adding Elements to a Collection
add() method is overloaded
myList.add(myObject) adds myObject
to end of collection
myList.add(3, myObject) inserts
myObject at offset 3 in a collection
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Replacing and Removing Elements
To replace and remove elements in an
ArrayList:
set() method replaces an element at a
specified position
remove() method removes an element at
a specified position