Java Collections Parag Shah Adaptive Software Solutions http://www.adaptivelearningonline.net http://www.adaptivesoftware.biz

Parag Shah

Adaptive Software Solutions

http://www.adaptivelearningonline.net

http://www.adaptivesoftware.biz

Agenda Collection classes in Java New Legacy Iterating through collections

Collection classes in Java

New

Legacy

Iterating through collections

Introduction The need for holding objects Collection support in Java Array Vector Hashmap List Set Map

The need for holding objects

Collection support in Java

Array

Vector

Hashmap

List

Set

Map

Array Arrays are first class objects Advantages of using Arrays Efficient Type of objects determined at compile time Can store primitives directly Disadvantages of using Arrays The size is fixed The type of objects it can store is fixed

Arrays are first class objects

Advantages of using Arrays

Efficient

Type of objects determined at compile time

Can store primitives directly

Disadvantages of using Arrays

The size is fixed

The type of objects it can store is fixed

The Java Collections Library Basic Collection Interfaces Collection List Set Map

Basic Collection Interfaces

Collection

List

Set

Map

Basic Collection Classes Basic Collection Classes

Basic Collection Classes

Collection Classes (contd.) Classes that work with Collections

Classes that work with Collections

Collection Classes Overview Collection: Stores a collection of objects, such that a rule is applied to the objects List: Stores objects in a certain order Implemented By ArrayList LinkedList Set: Stores only unique objects Implemented By HashSet TreeSet Map: Stores key-value pairs Implemented By HashMap TreeMap WeakHashMap

Collection: Stores a collection of objects, such that a rule is applied to the objects

List: Stores objects in a certain order

Implemented By

ArrayList

LinkedList

Set: Stores only unique objects

Implemented By

HashSet

TreeSet

Map: Stores key-value pairs

Implemented By

HashMap

TreeMap

WeakHashMap

Collection Interface java.util.Collection Root Interface for the Collection hierarchy in Java Holds a group of objects which adhere to some rule Classes implementing this Interface are suggested to have atleast 2 constructors: A no-arg constructor A constructor which takes a collection of objects

java.util.Collection

Root Interface for the Collection hierarchy in Java

Holds a group of objects which adhere to some rule

Classes implementing this Interface are suggested to have atleast 2 constructors:

A no-arg constructor

A constructor which takes a collection of objects

List java.util.List A List is an ordered collection of objects Implemented by ArrayList, and LinkedList Users have control on where in the List, new elements should be added Lists can contain duplicates as determined by the equals() method Use caution while adding a List element to a List equals(), and hashCode() may not work properly

java.util.List

A List is an ordered collection of objects

Implemented by ArrayList, and LinkedList

Users have control on where in the List, new elements should be added

Lists can contain duplicates as determined by the equals() method

Use caution while adding a List element to a List

equals(), and hashCode() may not work properly

ArrayList See [SimpleArrayList.java] java.util.ArrayList Resizable array implementation of the List Interface Adding elements require O(n) time Other operations are performed linear time The capacity of ArrayLists can be increased before adding a large number of elements ArrayList is NOT synchronized Inserting elements in the list is inefficient

See [SimpleArrayList.java]

java.util.ArrayList

Resizable array implementation of the List Interface

Adding elements require O(n) time

Other operations are performed linear time

The capacity of ArrayLists can be increased before adding a large number of elements

ArrayList is NOT synchronized

Inserting elements in the list is inefficient

LinkedList See [SimpleLinkedList.java] java.util.LinkedList Doubly Linked List implementation of the List Interface Provides special methods to get, remove, and insert elements at the beginning and the end of the list Enables the usage of a LinkedList as: Queues Stacks Deque Inefficient for random access Very efficient for sequential access and insertions in the middle of the list LinkedList's are not synchronized

See [SimpleLinkedList.java]

java.util.LinkedList

Doubly Linked List implementation of the List Interface

Provides special methods to get, remove, and insert elements at the beginning and the end of the list

Enables the usage of a LinkedList as:

Queues

Stacks

Deque

Inefficient for random access

Very efficient for sequential access and insertions in the middle of the list

LinkedList's are not synchronized

Set java.util.Set A Set is a Collection that does not contain any duplicate elements Elements that are put in a Set must override the equals() method to establish uniqueness A Set cannot contain itself as an element Caution must be exercised when adding a mutable element to a set

java.util.Set

A Set is a Collection that does not contain any duplicate elements

Elements that are put in a Set must override the equals() method to establish uniqueness

A Set cannot contain itself as an element

Caution must be exercised when adding a mutable element to a set

HashSet See [SimpleHashSet.java] java.util.HashSet A Set backed by a HashMap Can contain nulls Basic operations take constant time Iteration order may not always be the same Used for Sets where lookup time has to be optimized User defined classes must override the hashCode() method Operations not synchronized

See [SimpleHashSet.java]

java.util.HashSet

A Set backed by a HashMap

Can contain nulls

Basic operations take constant time

Iteration order may not always be the same

Used for Sets where lookup time has to be optimized

User defined classes must override the hashCode() method

Operations not synchronized

TreeSet See [SimpleTreeSet.java] java.util.TreeSet A Set backed by a TreeMap The sorted Set is gauranteed to be in ascending order, sorted by the natural order of the elements or by the comparator provided User defined classes must implement the Comparable Interface A TreeSet is not synchronized

See [SimpleTreeSet.java]

java.util.TreeSet

A Set backed by a TreeMap

The sorted Set is gauranteed to be in ascending order, sorted by the natural order of the elements or by the comparator provided

User defined classes must implement the Comparable Interface

A TreeSet is not synchronized

Map java.util.Map It is an associative array Used to lookup objects based on keys Implemented by the HashMap and TreeMap classes

java.util.Map

It is an associative array

Used to lookup objects based on keys

Implemented by the HashMap and TreeMap classes

HashMap See [SimpleHashMap.java] java.util.HashMap Implementation is based on a Hashtable Ideal for fast lookups Provides constant time performance for inserting and locating pairs Allows null for keys and values Methods are not synchronized

See [SimpleHashMap.java]

java.util.HashMap

Implementation is based on a Hashtable

Ideal for fast lookups

Provides constant time performance for inserting and locating pairs

Allows null for keys and values

Methods are not synchronized

TreeMap See [SimpleTreeMap.java] java.util.TreeMap Implemented based on Red-Black trees The keys appear sorted Basic operations take log(n) time The ordering maintained by HashMap must be consistent with equals() Methods are not synchronized

See [SimpleTreeMap.java]

java.util.TreeMap

Implemented based on Red-Black trees

The keys appear sorted

Basic operations take log(n) time

The ordering maintained by HashMap must be consistent with equals()

Methods are not synchronized

Iterators Iterators provide the ability to traverse through a sequence of elements without knowing the underlying data structure of the sequence Iterator support in Java Interface Iterator Allows unidirectional traversal and removal of elements from the underlying collection Interface ListIterator Allows bi-directional traversal as well as addition, removal, & substitution of elements in the underlying collection

Iterators provide the ability to traverse through a sequence of elements without knowing the underlying data structure of the sequence

Iterator support in Java

Interface Iterator

Allows unidirectional traversal and removal of elements from the underlying collection

Interface ListIterator

Allows bi-directional traversal as well as addition, removal, & substitution of elements in the underlying collection

Using Iterators Always use iterators and not for loops while iterating through collections Use Iterator to iterate and remove elements from a collection ListIterator provides more features Allows insertion & replacement of elements Allows bi-directional navigation See [ListIterationExample.java] See [MapIteration.java]

Always use iterators and not for loops while iterating through collections

Use Iterator to iterate and remove elements from a collection

ListIterator provides more features

Allows insertion & replacement of elements

Allows bi-directional navigation

See [ListIterationExample.java]

See [MapIteration.java]

Summary Revise various collection interfaces and their implementations Discuss pros and cons of choosing the collection type Discuss legacy collection types

Revise various collection interfaces and their implementations

Discuss pros and cons of choosing the collection type

Discuss legacy collection types

Where to Get More Information Bruce Eckel's – Thinking in Java Java tutorial

Bruce Eckel's – Thinking in Java

Java tutorial