Chapter 22 Java Collections Framework

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 1

Chapter 22 Java Collections Framework


Objectives To describe the Java Collections Framework hierarchy (§§22.1-22.2). To use the common methods defined in the Collection interface for operating sets and

lists (§22.3). To use the Iterator interface to traverse a collection (§22.4). To use the for-each loop to simplify traversing a collection (§22.4). To explore how and when to use HashSet (§22.4.1), LinkedHashSet (§22.4.2), or TreeSet

(§22.4.3) to store elements. To compare elements using the Comparable interface and the Comparator interface

(§22.5). To explore how and when to use ArrayList or LinkedList to store elements (§22.6). To use the static utility methods in the Collections class for sorting, searching, shuffling

lists, and finding the largest and smallest element in collections (§22.7). To compare performance of sets and lists (§22.8). To distinguish between Vector and ArrayList, and to use the Stack class for creating

stacks (§22.9). To explore the relationships among Collection, Queue, LinkedList, and PriorityQueue

and to create priority queues using the PriorityQueue class (§22.10). To tell the differences between Collection and Map, and describe when and how to use

HashMap, LinkedHashMap, and TreeMap to store values associated with keys (§22.11). To obtain singleton sets, lists, and maps, and unmodifiable sets, lists, and maps, using the

static methods in the Collections class (§22.12).


Java Collection Framework hierarchy

In object-oriented thinking, a data structure, also known as a container, is an object that stores other objects, referred to as data or elements. Some people refer to data structures as container objects.

A collection is a container object that represents a group of objects, often referred to as elements.

The Java Collections Framework supports two types of containers:One for storing a collection of elements, simply called a

collection.The other for storing key/value pairs, called a map.


Java Collection Framework hierarchy, cont.

Set and List are subinterfaces of Collection.

Set

SortedSet

AbstractSet

Collection

TreeSet

HashSet

List AbstractList

AbstractSequentialList

ArrayList

LinkedList

AbstractCollection

Vector Stack

LinkedHashSet

Interfaces Abstract Classes Concrete Classes

Queue AbstractQueue

Deque

PriorityQueue


Java Collection Framework hierarchy, cont.

An instance of Map represents a group of objects, each of which is associated with a key. You can get the object from a map using a key, and you have to use a key to put the object into the map.

SortedMap

Map

TreeMap

HashMap AbstractMap LinkedHashMap

Interfaces Abstract Classes Concrete Classes


The Collection Interface

«interface»

java.util.Collection<E>

+add(o: E): boolean

+addAll(c: Collection<? extends E>): boolean

+clear(): void

+contains(o: Object): boolean

+containsAll(c: Collection<?>):boolean

+equals(o: Object): boolean

+hashCode(): int

+isEmpty(): boolean

+iterator(): Iterator

+remove(o: Object): boolean

+removeAll(c: Collection<?>): boolean

+retainAll(c: Collection<?>): boolean

+size(): int

+toArray(): Object[]

Adds a new element o to this collection.

Adds all the elements in the collection c to this collection.

Removes all the elements from this collection.

Returns true if this collection contains the element o.

Returns true if this collection contains all the elements in c.

Returns true if this collection is equal to another collection o.

Returns the hash code for this collection.

Returns true if this collection contains no elements.

Returns an iterator for the elements in this collection.

Removes the element o from this collection.

Removes all the elements in c from this collection.

Retains the elements that are both in c and in this collection.

Returns the number of elements in this collection.

Returns an array of Object for the elements in this collection.

«interface»

java.util.Iterator<E>

+hasNext(): boolean

+next(): E

+remove(): void

Returns true if this iterator has more elements to traverse.

Returns the next element from this iterator.

Removes the last element obtained using the next method.

The Collection interface is the root interface for

manipulating a collection of objects.


The Set Interface

The Set interface extends the Collection interface. It does not introduce new methods or constants, but it stipulates that an instance of Set contains no duplicate elements. The concrete classes that implement Set must ensure that no duplicate elements can be added to the set. That is no two elements e1 and e2 can be in the set such that e1.equals(e2) is true.


The Set Interface Hierarchy

java.util.AbstractSet<E>

java.util.HashSet<E> +HashSet()

+HashSet(c: Collection<? extends E>)

+HashSet(initialCapacity: int)

+HashSet(initialCapacity: int, loadFactor: float)

java.util.LinkedHashSet<E> +LinkedHashSet()

+LinkedHashSet(c: Collection<? extends E>)

+LinkedHashSet(initialCapacity: int)

+LinkedHashSet(initialCapacity: int, loadFactor: float)

java.util.TreeSet<E> +TreeSet()

+TreeSet(c: Collection<? extends E>)

+TreeSet(comparator: Comparator<? super E>)

+TreeSet(s: SortedSet<E>)

«interface» java.util.SortedSet<E>

+first(): E

+last(): E

+headSet(toElement: E): SortedSet<E>

+tailSet(fromElement: E): SortedSet<E>

«interface» java.util.Set<E>

«interface» java.util.Collection<E>

«interface» java.util.NavigableSet<E>

+pollFirst(): E

+pollLast(): E

+lower(e: E): E

+higher(e: E):E

+floor(e: E): E

+ceiling(e: E): E


The AbstractSet Class

The AbstractSet class is a convenience class that extends AbstractCollection and implements Set. The AbstractSet class provides concrete implementations for the equals method and the hashCode method. The hash code of a set is the sum of the hash code of all the elements in the set. Since the size method and iterator method are not implemented in the AbstractSet class, AbstractSet is an abstract class.


The HashSet Class

The HashSet class is a concrete class that implements Set. It can be used to store duplicate-free elements. For efficiency, objects added to a hash set need to implement the hashCode method in a manner that properly disperses the hash code.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807

hashCode

A HashSet can be used to store duplicate-free elements. For efficiency, objects added to a hash set need to implement the hashCode method in a manner that properly disperses the hash code.

Recall that hashCode is defined in the Object class. The hash codes of two objects must be the same if the two objects are equal. Two unequal objects may have the same hash code, but you should implement the hashCode method to avoid too many such cases.

The hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.)

Most of the classes in the Java API implement the hashCode method. For example, the hashCode in the Integer class returns its int value. The hashCode in the Character class returns the Unicode of the character. The hashCode in the String class returns s0*31n–1 + s1* 31n–2 + ... + sn–1, where si is s.charAt(i).

11


Example: Using HashSet and Iterator

This example creates a hash set filled with strings, and uses an iterator to traverse the elements in the list.

TestHashSetTestHashSet RunRun


TIP: for-each loop

You can simplify the code in Lines 21-26 using a JDK 1.5 enhanced for loop without using an iterator, as follows:

for (Object element: set) System.out.print(element.toString() + " ");


Example: Using LinkedHashSet LinkedHashSet extends HashSet with a linked-list

İmplementation that supports an ordering of the elements in the set.

The elements in a HashSet are not ordered, but the elements in a LinkedHashSet can be retrieved in the order in which they were inserted into the set.

This example creates a hash set filled with strings, and uses an iterator to traverse the elements in the list.

TestLinkedHashSetTestLinkedHashSet RunRun


SortedSet

SortedSet is a subinterface of Set, which guarantees that the elements in the set are sorted. Additionally, it provides the methods first() and last() for returning the first and last elements in the set, and headSet(toElement) and tailSet(fromElement) for returning a portion of the set whose elements are less than toElement and greater than or equal to fromElement.

NavigableSet extends SortedSet to provide navigation methods lower(e), floor(e), ceiling(e), and higher(e) that return elements respectively less than, less than or equal, greater than or equal, and greater than a given element and return null if there is no such element. The pollFirst() and pollLast() methods remove and return the first and last element in the tree set, respectively.

15


TreeSet A treeSet organizes data in a tree through use of Comparator (natural

ordering) and the hashSet organizes data in a hash table (using a hash function).

TreeSet stores objects in a sorted manner. TreeSet stores its elements in a tree and they are automatically arranged in a sorted order.

TreeSet is a concrete class that implements the SortedSet interface. To create a TreeSet, use its no-arg constructor or use new TreeSet(Collection). You can add objects into a tree set as long as they can be compared with each other.

There are two ways to compare objects. Use the Comparable interface. Since the objects added to the set are instances of

Comparable, they can be compared using the compareTo method. Several classes in the Java API, such as String, Date, Calendar, and all the wrapper classes for the primitive types, implement the Comparable interface. This approach is referred to as natural order.

If the class for the elements does not implement the Comparable interfaces, or if you don't want to use the compareTo method in the class that implements the Comparable interface, specify a comparator for the elements in the set. This approach is referred to as order by comparator.

What is the difference between a treeset and hashset in Java? The difference between these two sets is simply the order/algorithm in which items are added to/accessed/removed from the map. The circumstances determine which particular implementation is better/faster.

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The SortedSet Interface and the TreeSet Class

SortedSet is a subinterface of Set, which guarantees that the elements in the set are sorted. TreeSet is a concrete class that implements the SortedSet interface. You can use an iterator to traverse the elements in the sorted order. The elements can be sorted in two ways.


The SortedSet Interface and the TreeSet Class, cont.

One way is to use the Comparable interface.

The other way is to specify a comparator for the elements in the set if the class for the elements does not implement the Comparable interface, or you don’t want to use the compareTo method in the class that implements the Comparable interface. This approach is referred to as order by comparator.


Example: Using TreeSet to Sort Elements in a Set

This example creates a hash set filled with strings, and then creates a tree set for the same strings. The strings are sorted in the tree set using the compareTo method in the Comparable interface. The example also creates a tree set of geometric objects. The geometric objects are sorted using the compare method in the Comparator interface.

RunRunTestTreeSetTestTreeSet


The Comparator Interface

Sometimes you want to insert elements of different types into a tree set. The elements may not be instances of Comparable or are not comparable. You can define a comparator to compare these elements. To do so, create a class that implements the java.util.Comparator interface. The Comparator interface has two methods, compare and equals.


The Comparator Interface

public int compare(Object element1, Object element2)

Returns a negative value if element1 is less than element2, a positive value if element1 is greater than element2, and zero if they are equal.

public boolean equals(Object element)

Returns true if the specified object is also a comparator and imposes the same ordering as this comparator.

GeometricObjectComparatorGeometricObjectComparator


Example: The Using Comparator to Sort Elements in a Set

Write a program that demonstrates how to sort elements in a tree set using the Comparator interface. The example creates a tree set of geometric objects. The geometric objects are sorted using the compare method in the Comparator interface.

TestTreeSetWithComparatorTestTreeSetWithComparator RunRun


The List Interface

A set stores non-duplicate elements. To allow duplicate elements to be stored in a collection, you need to use a list. A list can not only store duplicate elements, but can also allow the user to specify where the element is stored. The user can access the element by index.


The List Interface, cont.

«interface» java.util.List<E>

+add(index: int, element:E): boolean

+addAll(index: int, c: Collection<? extends E>) : boolean

+get(index: int): E

+indexOf(element: Object): int

+lastIndexOf(element: Object): int

+listIterator(): ListIterator<E>

+listIterator(startIndex: int): ListIterator<E>

+remove(index: int): E

+set(index: int, element: E): E

+subList(fromIndex: int, toIndex: int): List<E>


Adds a new element at the specified index.

Adds all the elements in c to this list at the specified index.

Returns the element in this list at the specified index.

Returns the index of the first matching element.

Returns the index of the last matching element.

Returns the list iterator for the elements in this list.

Returns the iterator for the elements from startIndex.

Removes the element at the specified index.

Sets the element at the specified index.

Returns a sublist from fromIndex to toIndex.


The List Iterator

«interface» java.util.ListIterator<E>

+add(o: E): void

+hasPrevious(): boolean

+nextIndex(): int

+previous(): E

+previousIndex(): int

+set(o: E): void

«interface» java.util.Iterator<E>

Adds the specified object to the list.

Returns true if this list iterator has more elements when traversing backward.

Returns the index of the next element.

Returns the previous element in this list iterator.

Returns the index of the previous element.

Replaces the last element returned by the previous or next method with the specified element.


ArrayList and LinkedListThe ArrayList class and the LinkedList class are concrete implementations of the List interface. Which of the two classes you use depends on your specific needs. If you need to support random access through an index without inserting or removing elements from any place other than the end, ArrayList offers the most efficient collection. If, however, your application requires the insertion or deletion of elements from any place in the list, you should choose LinkedList. A list can grow or shrink dynamically. An array is fixed once it is created. If your application does not require insertion or deletion of elements, the most efficient data structure is the array.


java.util.ArrayList


Creates an empty list with the default initial capacity.

Creates an array list from an existing collection.

Creates an empty list with the specified initial capacity.

Trims the capacity of this ArrayList instance to be the list's current size.

+ArrayList()

+ArrayList(c: Collection<? extends E>)

+ArrayList(initialCapacity: int)

+trimToSize(): void


java.util.ArrayList<E>


java.util.LinkedList


Creates a default empty linked list.

Creates a linked list from an existing collection.

Adds the object to the head of this list.

Adds the object to the tail of this list.

Returns the first element from this list.

Returns the last element from this list.

Returns and removes the first element from this list.

Returns and removes the last element from this list.

+LinkedList()

+LinkedList(c: Collection<? extends E>)

+addFirst(o: E): void

+addLast(o: E): void

+getFirst(): E

+getLast(): E

+removeFirst(): E

+removeLast(): E


java.util.LinkedList<E>


Example: Using ArrayList and LinkedList

This example creates an array list filled with numbers, and inserts new elements into the specified location in the list. The example also creates a linked list from the array list, inserts and removes the elements from the list. Finally, the example traverses the list forward and backward.

RunRunTestListTestList


The Collections ClassThe Collections class contains various static methods for operating on collections and maps, for creating synchronized collection classes, and for creating read-only collection classes.


Performance of Sets and Lists

RunRunSetListPerformanceTestSetListPerformanceTest


The Collections Class UML Diagram java.util.Collections

+sort(list: List): void

+sort(list: List, c: Comparator): void

+binarySearch(list: List, key: Object): int

+binarySearch(list: List, key: Object, c: Comparator): int

+reverse(list: List): void

+reverseOrder(): Comparator

+shuffle(list: List): void

+shuffle(list: List): void

+copy(des: List, src: List): void

+nCopies(n: int, o: Object): List

+fill(list: List, o: Object): void

+max(c: Collection): Object

+max(c: Collection, c: Comparator): Object

+min(c: Collection): Object

+min(c: Collection, c: Comparator): Object

+disjoint(c1: Collection, c2: Collection): boolean

+frequency(c: Collection, o: Object): int

Sorts the specified list.

Sorts the specified list with the comparator.

Searches the key in the sorted list using binary search.

Searches the key in the sorted list using binary search with the comparator.

Reverses the specified list.

Returns a comparator with the reverse ordering.

Shuffles the specified list randomly.

Shuffles the specified list with a random object.

Copies from the source list to the destination list.

Returns a list consisting of n copies of the object.

Fills the list with the object.

Returns the max object in the collection.

Returns the max object using the comparator.

Returns the min object in the collection.

Returns the min object using the comparator.

Returns true if c1 and c2 have no elements in common.

Returns the number of occurrences of the specified element in the collection.

List

Collection


Example: Using the Collections Class

This example demonstrates using the methods in the Collections class. The example creates a list, sorts it, and searches for an element. The example wraps the list into a synchronized and read-only list.

RunRunTestCollectionsTestCollections


The Vector and Stack Classes

The Java Collections Framework was introduced with Java 2. Several data structures were supported prior to Java 2. Among them are the Vector class and the Stack class. These classes were redesigned to fit into the Java Collections Framework, but their old-style methods are retained for compatibility. This section introduces the Vector class and the Stack class.


The Vector Class

In Java 2, Vector is the same as ArrayList, except that Vector contains the synchronized methods for accessing and modifying the vector. None of the new collection data structures introduced so far are synchronized. If synchronization is required, you can use the synchronized versions of the collection classes. These classes are introduced later in the section, “The Collections Class.”


The Vector Class, cont.

java.util.Vector<E>

+Vector()

+Vector(c: Collection<? extends E>)

+Vector(initialCapacity: int)

+Vector(initCapacity:int, capacityIncr: int)

+addElement(o: E): void

+capacity(): int

+copyInto(anArray: Object[]): void

+elementAt(index: int): E

+elements(): Enumeration<E>

+ensureCapacity(): void

+firstElement(): E

+insertElementAt(o: E, index: int): void

+lastElement(): E

+removeAllElements(): void

+removeElement(o: Object): boolean

+removeElementAt(index: int): void

+setElementAt(o: E, index: int): void

+setSize(newSize: int): void

+trimToSize(): void


Creates a default empty vector with initial capacity 10.

Creates a vector from an existing collection.

Creates a vector with the specified initial capacity.

Creates a vector with the specified initial capacity and increment.

Appends the element to the end of this vector.

Returns the current capacity of this vector.

Copies the elements in this vector to the array.

Returns the object at the specified index.

Returns an enumeration of this vector.

Increases the capacity of this vector.

Returns the first element in this vector.

Inserts o to this vector at the specified index.

Returns the last element in this vector.

Removes all the elements in this vector.

Removes the first matching element in this vector.

Removes the element at the specified index.

Sets a new element at the specified index.

Sets a new size in this vector.

Trims the capacity of this vector to its size.


The Stack Class

The Stack class represents a last-in-first-out stack of objects. The elements are accessed only from the top of the stack. You can retrieve, insert, or remove an element from the top of the stack.

java.util.Stack<E>

+Stack()

+empty(): boolean

+peek(): E

+pop(): E

+push(o: E) : E

+search(o: Object) : int

java.util.Vector<E>

Creates an empty stack.

Returns true if this stack is empty.

Returns the top element in this stack.

Returns and removes the top element in this stack.

Adds a new element to the top of this stack.

Returns the position of the specified element in this stack.


Queues and Priority Queues

A queue is a first-in/first-out data structure. Elements are appended to the end of the queue and are removed from the beginning of the queue. In a priority queue, elements are assigned priorities. When accessing elements, the element with the highest priority is removed first.


The Queue Interface

«interface» java.util.Queue<E>

+offer(element: E): boolean

+poll(): E

+remove(): E

+peek(): E

+element(): E


Inserts an element to the queue.

Retrieves and removes the head of this queue, or null if this queue is empty.

Retrieves and removes the head of this queue and throws an exception if this queue is empty.

Retrieves, but does not remove, the head of this queue, returning null if this queue is empty.

Retrieves, but does not remove, the head of this queue, throwing an exception if this queue is empty.


The PriorityQueue Class

java.util.PriorityQueue<E>

+PriorityQueue()

+PriorityQueue(initialCapacity: int)

+PriorityQueue(c: Collection<? extends E>)

+PriorityQueue(initialCapacity: int, comparator: Comparator<? super E>)

«interface» java.util.Queue<E>

Creates a default priority queue with initial capacity 11.

Creates a default priority queue with the specified initial capacity.

Creates a priority queue with the specified collection.

Creates a priority queue with the specified initial capacity and the comparator.

RunRunPriorityQueueDemoPriorityQueueDemo


The Map Interface

The Map interface maps keys to the elements. The keys are like indexes. In List, the indexes are integer. In Map, the keys can be any objects.

Search keys

Corresponding values

…

.

.

Entry


The Map Interface UML Diagram java.util.Map<K, V>

+clear(): void

+containsKey(key: Object): boolean

+containsValue(value: Object): boolean

+entrySet(): Set

+get(key: Object): V

+isEmpty(): boolean

+keySet(): Set<K>

+put(key: K, value: V): V

+putAll(m: Map): void

+remove(key: Object): V

+size(): int

+values(): Collection<V>

Removes all mappings from this map.

Returns true if this map contains a mapping for the specified key.

Returns true if this map maps one or more keys to the specified value.

Returns a set consisting of the entries in this map.

Returns the value for the specified key in this map.

Returns true if this map contains no mappings.

Returns a set consisting of the keys in this map.

Puts a mapping in this map.

Adds all the mappings from m to this map.

Removes the mapping for the specified key.

Returns the number of mappings in this map.

Returns a collection consisting of the values in this map.


Concrete Map Classes

java.util.AbstractMap<K, V>

java.util.HashMap<K, V>

+HashMap()

+HashMap(m: Map)

java.util.LinkedHashMap<K, V>

+LinkedHashMap()

+LinkedHashMap(m: Map)

+LinkedHashMap(initialCapacity: int, loadFactor: float, accessOrder: boolean)

java.util.TreeMap<K, V> +TreeMap()

+TreeMap(m: Map)

+TreeMap(c: Comparator<? super K>)

«interface» java.util.SortedMap<K, V>

+firstKey(): K

+lastKey(): K

+comparator(): Comparator<? super K>)

+headMap(toKey: K): SortedMap

+tailMap(fromKey: K): SortedMap

«interface» java.util.Map<K, V>


HashMap and TreeMap

The HashMap and TreeMap classes are two concrete implementations of the Map interface. The HashMap class is efficient for locating a value, inserting a mapping, and deleting a mapping. The TreeMap class, implementing SortedMap, is efficient for traversing the keys in a sorted order.


LinkedHashMap

LinkedHashMap was introduced in JDK 1.4. It extends HashMap with a linked list implementation that supports an ordering of the entries in the map. The entries in a HashMap are not ordered, but the entries in a LinkedHashMap can be retrieved in the order in which they were inserted into the map (known as the insertion order), or the order in which they were last accessed, from least recently accessed to most recently (access order). The no-arg constructor constructs a LinkedHashMap with the insertion order. To construct a LinkedHashMap with the access order, use the LinkedHashMap(initialCapacity, loadFactor, true).


Example: Using HashMap and TreeMap

This example creates a hash map that maps borrowers to mortgages. The program first creates a hash map with the borrower’s name as its key and mortgage as its value. The program then creates a tree map from the hash map, and displays the mappings in ascending order of the keys.

RunRunTestMapTestMap


Example: Counting the Occurrences of Words in a Text

This program counts the occurrences of words in a text and displays the words and their occurrences in ascending order of the words. The program uses a hash map to store a pair consisting of a word and its count. For each word, check whether it is already a key in the map. If not, add the key and value 1 to the map. Otherwise, increase the value for the word (key) by 1 in the map. To sort the map, convert it to a tree map.

RunRunCountOccurrenceOfWordsCountOccurrenceOfWords


The Arrays Class

The Arrays class contains various static methods for sorting and searching arrays, for comparing arrays, and for filling array elements. It also contains a method for converting an array to a list.


The Arrays Class UML Diagram

Arrays

+asList(a: Object[]): List

Overloaded binarySearch method for byte, char, short, int, long, float, double, and Object.

+binarySearch(a: xType[], key: xType): int

Overloaded equals method for boolean, byte, char, short, int, long, float, double, and Object.

+equals(a: xType[], a2: xType[]): boolean

Overloaded fill method for boolean char, byte, short, int, long, float, double, and Object.

+fill(a: xType[], val: xType): void

+fill(a: xType[], fromIndex: int, toIndex: xType, val: xType): void

Overloaded sort method for char, byte, short, int, long, float, double, and Object.

+sort(a: xType[]): void

+sort(a: xType[], fromIndex: int, toIndex: int): void

Returns a list from an array of objects

Overloaded binary search method to search a key in the array of byte, char, short, int, long, float, double, and Object

Overloaded equals method that returns true if a is equal to a2 for a and a2 of the boolean, byte, char, short, int, long, float, and Object type

Overloaded fill method to fill in the specified value into the array of the boolean, byte, char, short, int, long, float, and Object type

Overloaded sort method to sort the specified array of the char, byte, short, int, long, float, double, and Object type


Example: Using the Arrays Class

This example demonstrates using the methods in the Arrays class. The example creates an array of int values, fills part of the array with 50, sorts it, searches for an element, and compares the array with another array.

RunRunTestArraysTestArrays

Chapter 22 Java Collections Framework

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