Introduction to the Collections Framework
This module takes you on an extended tour of the Collections Framework, first introduced with the Java 2 platform, Standard Edition, version 1.2. The Collections Framework provides a well-designed set of interfaces and classes for storing and manipulating groups of data as a single unit, a collection. The framework provides a convenient API to many of the abstract data types familiar from computer science data structure curriculum: maps, sets, lists, trees, arrays, hashtables and other collections. Because of their object-oriented design, the Java classes in the Collections Framework encapsulate both the data structures and the algorithms associated with these abstractions. The framework provides a standard programming interface to many of the most common abstractions, without burdening the programmer with too many procedures and interfaces. The operations supported by the collections framework nevertheless permit the programmer to easily define higher level data abstractions, such as stacks, queues, and thread-safe collections.

One thing worth noting early on is that while the framework is included with the Java 2 platform, a subset form is available for use with Java 1.1 runtime environments. The framework subset is discussed in a later section.

Before diving into the Collections Framework, it helps to understand some of the terminology and set theory involved when working with the framework.

Mathematical Background
In common usage a collection is the same as the intuitive, mathamatical concept of a set. A set is just a group of unique items, meaning that the group contains no duplicates. The Collections Framework in fact includes a Set interface, and a number of concrete Set classes. But the formal notion of a set predates Java technology by a century, when the British mathematician George Boole defined it in formal logic. Most people learned some set theory in elementary school when introduced to “set intersection” and “set union” through the familiar Venn Diagrams:

Some real-world examples of sets include the following:

The set of uppercase letters ‘A’ through ‘Z’
The set of nonnegative integers {0, 1, 2 …}
The set of reserved Java programming language keywords {‘import’, ‘class’, ‘public’, ‘protected’…}
A set of people (friends, employees, clients, …)
The set of records returned by a database query
The set of Component objects in a Container
The set of all pairs
The empty set {}
These examples show the basic properties of sets:

Sets contains only one instance of each item
Sets may be finite or infinite
Sets can define abstract concepts
Sets are fundamental to logic, mathematics, and computer science, but also practical in everyday applications in business and systems. The idea of a “connection pool” is a set of open connections to a database server. Web servers have to manage sets of clients and connections. File descriptors provide another example of a set in the operating system.

A map is a special kind of set. It is a set of pairs, each pair representing a one-directional “mapping” from one element to another. Some examples of maps are:

The map of IP addresses to domain names (DNS)
A map from keys to database records
A dictionary (words mapped to meanings)
The conversion from base 2 to base 10
Like sets, the idea behind a map is much older than the Java programming language, older even than computer science. Sets and maps are important tools in mathematics and their properties are well-understood. People also long recognized the usefulness of solving programming problems with sets and maps. A language called SETL (Set Language) invented in 1969 included sets as one of its only primitive data types (SETL also included garbage collection–not widely accepted until Java technology developed in the 1990’s). Although sets and maps appear in many languages including C++, the Collections Framework is perhaps the best designed set and map package yet written for a popular language. Users of C++ Standard Template Library (STL) and Smalltalk’s collection hierarchy might argue that last point.

Also because they are sets, maps can be finite or infinite. An example of an infinite map is the conversion from base 2 to base 10. Unfortunately, the Collections Framework does not support infatuate maps–sometimes a mathematical function, formula or algorithm is preferred. But when a problem can be solved with a finite map, the Collections Framework provides the Java programmer with a useful API.

Because the Collections Framework has formal definitions for the classes Set, Collection, and Map, you’ll notice the lower case words set, collection and map to distinguish the implementation from the concept.

Collection Interfaces and Classes
Now that you have some set theory under your belt, you should be able to understand with the Collections Framework more easily. The Collections Framework is made up of a set of interfaces for working with groups of objects. The different interfaces describe the different types of groups. For the most part, once you understand the interfaces, you understand the framework. While you always need to create specific implementations of the interfaces, access to the actual collection should be restricted to the use of the interface methods, thus allowing you to change the underlying data structure, without altering the rest of your code. The following diagrams shows the framework interface hierarchy.

One might think that Map would extend Collection. In mathematics, a map is just a collection of pairs. In the Collections Framework, however, the interfaces Map and Collection are distinct with no lineage in the hierarchy. The reasons for this distinction have to do with the ways that Set and Map are used in the Java technology libraries. The typical application of a Map is to provide access to values stored by keys. The set of collection operations are all there, but you work with a key-value pair, instead of an isolated element. Map is therefore designed to support the basic operations of get() and put() which are not required by Set. Moreover, there are methods that return Set views of Map objects:

Set set = aMap.keySet();
When designing software with the Collections Framework, it is useful to remember the following hierarchical relationships of the four basic interfaces of the framework:

The Collection interface is a group of objects, with duplicates allowed
Set extends Collection but forbids duplicates
List extends Collection also, allows duplicates and introduces positional indexing
Map extends neither Set nor Collection
Moving on to the framework implementations, the concrete collection classes follow a naming convention, combining the underlying data structure with the framework interface. The following table shows the six collection implementations introduced with the Java 2 framework, in addition to the four historical collection classes. For information on how the historical collection classes changed, like how Hashtable was reworked into the framework, see the Historical Collection Classes section, below.

Interface Implementation Historical
Set HashSet TreeSet
List ArrayList LinkedList Vector
Stack
Map HashMap TreeMap Hashtable
Properties
There are no implementations of the Collection interface. The historical collection classes are called such because they have been around since the 1.0 release of the Java class libraries.

If you are moving from the historical collection classes to the new framework classes, one of the primary differences is that all operations are unsynchronized with the new classes. While you can add synchronization to the new classes, you cannot remove it from the old.

Collection Interface
The Collection interface is used to represent any group of objects, or elements. You use the interface when you wish to work with a group of elements in as general a manner as possible. Here is a list of the public methods of Collection in Unified Modeling Language (UML) notation.

The interface supports basic operations like adding and removing. When you try to remove an element, only a single instance of the element in the collection is removed, if present.

boolean add(Object element)
boolean remove(Object element)
The Collection interface also supports query operations:

int size()
boolean isEmpty()
boolean contains(Object element)
Iterator iterator()
Iterator Interface
The iterator() method of the Collection interface returns an Iterator. An Iterator is similar to the Enumeration interface, which you may already be familiar with, and will be described later. With the Iterator interface methods, you can traverse a collection from start to finish and safely remove elements from the underlying Collection:

The remove() method is optionally supported by the underlying collection. When called, and supported, the element returned by the last next() call is removed. To demonstrate, the following shows the use of the Iterator interface for a general Collection:

Collection collection = …;
Iterator iterator = collection.iterator();
while (iterator.hasNext()) {
Object element = iterator.next();
if (removalCheck(element)) {
iterator.remove();
}
}

Group Operations
Other operations the Collection interface supports are tasks done on groups of elements or the entire collection at once:

boolean containsAll(Collection collection)
boolean addAll(Collection collection)
void clear()
void removeAll(Collection collection)
void retainAll(Collection collection)
The containsAll() method allows you to discover if the current collection contains all the elements of another collection, a subset. The remaining methods are optional, in that a specific collection might not support the altering of the collection. The addAll() method ensures all elements from another collection are added to the current collection, usually a union. The clear() method removes all elements from the current collection. The removeAll() method is like clear() but only removes a subset of elements. The retainAll() method is similar to the removeAll() method, but does what might be perceived as the opposite. It removes from the current collection those elements not in the other collection, an intersection.

The remaining two interface methods, which convert a Collection to an array, will be discussed later.

AbstractCollection Class
The AbstractCollection class provides the basis for the concrete collection framework classes. While you are free to implement all the methods of the Collection interface yourself, the AbstractCollection class provides implementations for all the methods, except for the iterator() and size() methods, which are implemented in the appropriate subclass. Optional methods like add() will throw an exception if the subclass doesn’t override the behavior.

Collection Framework Design Concerns
In the creation of the Collections Framework, the Sun development team needed to provide flexible interfaces that manipulated groups of elements. To keep the design simple, instead of providing separate interfaces for optional capabilities, the interfaces define all the methods an implementation class may provide. However, some of the interface methods are optional. Because an interface implementation must provide implementations for all the interface methods, there needed to be a way for a caller to know if an optional method is not supported. The manner the framework development team chose to signal callers when an optional method is called was to thrown an UnsupportedOperationException. If in the course of using a collection, an UnsupportedOperationException is thrown, like trying to add to a read-only collection, then the operation failed because it is not supported. To avoid having to place all collection operations within a try-catch block, the UnsupportedOperationException class is an extension of the RuntimeException class.

In addition to handling optional operations with a runtime exception, the iterators for the concrete collection implementations are fail-fast. That means that if you are using an Iterator to traverse a collection while underlying collection is being modified by another thread, then the Iterator fails immediately by throwing a ConcurrentModificationException (another RuntimeException). That means the next time an Iterator method is called, and the underlying collection has been modified, the ConcurrentModificationException exception gets thrown.

Set Interface
The Set interface extends the Collection interface and, by definition, forbids duplicates within the collection. All the original methods are present and no new methods are introduced. The concrete Set implementation classes rely on the equals() method of the object added to check for equality.

HashSet, TreeSet Classes
The Collections Framework provides two general-purpose implementations of the Set interface: HashSet and TreeSet. More often than not, you will use a HashSet for storing your duplicate-free collection. For efficiency, objects added to a HashSet need to implement the hashCode() method in a manner that properly distributes the hash codes. While most system classes override the default hashCode() implementation in Object, when creating your own classes to add to a HashSet remember to override hashCode(). The TreeSet implementation is useful when you need to extract elements from a collection in a sorted manner. In order to work property, elements added to a TreeSet must be sortable. The Collections Framework adds support for Comparable elements and will be covered in detail later. For now, just assume a tree knows how to keep elements of the java.lang wrapper classes sorted. It is generally faster to add elements to a HashSet, then convert the collection to a TreeSet for sorted traversal.

To optimize HashSet space usage, you can tune the initial capacity and load factor. The TreeSet has no tuning options, as the tree is always balanced, ensuring log(n) performance for insertions, deletions, and queries.

Both HashSet and TreeSet implement the Cloneable interface.

Set Usage Example
To demonstrate the use of the concrete Set classes, the following program creates a HashSet and adds a group of names, including one name twice. The program than prints out the list of names in the set, demonstrating the duplicate name isn’t present. Next, the program treats the set as a TreeSet and displays the list sorted.

import java.util.*;

public class SetExample {
public static void main(String args[]) {
Set set = new HashSet();
set.add(“Bernadine”);
set.add(“Elizabeth”);
set.add(“Gene”);
set.add(“Elizabeth”);
set.add(“Clara”);
System.out.println(set);
Set sortedSet = new TreeSet(set);
System.out.println(sortedSet);
}
}

Running the program produces the following output. Notice that the duplicate entry is only present once, and the second list output is sorted.

[Gene, Clara, Bernadine, Elizabeth]
[Bernadine, Clara, Elizabeth, Gene]
AbstractSet Class
The AbstractSet class overrides the equals() and hashCode() methods to ensure two equal sets return the same hash code. Two sets are equal if they are the same size and contain the same elements. By definition, the hash code for a set is the sum of the hash codes for the elements of the set. Thus, no matter what the internal ordering of the sets, two equal sets will report the same hash code.

Magercises
Using a HashSet for a Sparse Bit Set
Using a TreeSet to provide a sorted JList
List Interface
The List interface extends the Collection interface to define an ordered collection, permitting duplicates. The interface adds position-oriented operations, as well as the ability to work with just a part of the list.

The position-oriented operations include the ability to insert an element or Collection, get an element, as well as remove or change an element. Searching for an element in a List can be started from the beginning or end and will report the position of the element, if found.

void add(int index, Object element)
boolean addAll(int index, Collection collection)
Object get(int index)
int indexOf(Object element)
int lastIndexOf(Object element)
Object remove(int index)
Object set(int index, Object element)
The List interface also provides for working with a subset of the collection, as well as iterating through the entire list in a position friendly manner:

ListIterator listIterator()
ListIterator listIterator(int startIndex)
List subList(int fromIndex, int toIndex)
In working with subList(), it is important to mention that the element at fromIndex is in the sub list, but the element at toIndex is not. This loosely maps to the following for-loop test cases:

for (int i=fromIndex; i<toIndex; i++) {
// process element at position i
}
In addition, it should be mentioned that changes to the sublist [like add(), remove(), and set() calls] have an effect on the underlying List.

ListIterator Interface
The ListIterator interface extends the Iterator interface to support bi-directional access, as well as adding or changing elements in the underlying collection.

The following source code demonstrates the looping backwards through a list. Notice that the ListIterator is originally positioned beyond the end of the list [list.size()], as the index of the first element is 0.

List list = …;
ListIterator iterator = list.listIterator(list.size());
while (iterator.hasPrevious()) {
Object element = iterator.previous();
// Process element
}
Normally, one doesn't use a ListIterator to alternate between going forward and backward in one iteration through the elements of a collection. While technically possible, one needs to know that calling next() immediately after previous() results in the same element being returned. The same thing happens when you reverse the order of the calls to next() and previous().

The add() operation requires a little bit of explanation, also. Adding an element results in the new element being added immediately prior to the implicit cursor. Thus, calling previous() after adding an element would return the new element and calling next() would have no effect, returning what would have been the next element prior to the add operation.

ArrayList, LinkedList Classes
There are two general-purpose List implementations in the Collections Framework: ArrayList and LinkedList. Which of the two List implementations you use depends on your specific needs. If you need to support random access, without inserting or removing elements from any place other than the end, than ArrayList offers the optimal collection. If, however, you need to frequently add and remove elements from the middle of the list and only access the list elements sequentially then LinkedList offers the better implementation.

Both ArrayList and LinkedList implement the Cloneable interface. In addition, LinkedList adds several methods for working with the elements at the ends of the list (only the new methods are shown in the following diagram):

By using these new methods, you can easily treat the LinkedList as a stack, queue, or other end-oriented data structure.

LinkedList queue = …;
queue.addFirst(element);
Object object = queue.removeLast();
LinkedList stack = …;
stack.addFirst(element);
Object object = stack.removeFirst();
The Vector and Stack classes are historical implementations of the List interface. They will be discussed later.

List Usage Example
The following program demonstrates the use of the concrete List classes. The first part creates a List backed by an ArrayList. After filling the list, specific entries are retrieved. The LinkedList part of the example treats the LinkedList as a queue, adding things at the beginning of the queue and removing them from the end.

import java.util.*;

public class ListExample {
public static void main(String args[]) {
List list = new ArrayList();
list.add("Bernadine");
list.add("Elizabeth");
list.add("Gene");
list.add("Elizabeth");
list.add("Clara");
System.out.println(list);
System.out.println("2: " + list.get(2));
System.out.println("0: " + list.get(0));
LinkedList queue = new LinkedList();
queue.addFirst("Bernadine");
queue.addFirst("Elizabeth");
queue.addFirst("Gene");
queue.addFirst("Elizabeth");
queue.addFirst("Clara");
System.out.println(queue);
queue.removeLast();
queue.removeLast();
System.out.println(queue);
}
}

Running the program produces the following output. Notice that, unlike Set, List permits duplicates.

[Bernadine, Elizabeth, Gene, Elizabeth, Clara]
2: Gene
0: Bernadine
[Clara, Elizabeth, Gene, Elizabeth, Bernadine]
[Clara, Elizabeth, Gene]
AbstractList and AbstractSequentialList Classes
There are two abstract List implementations classes: AbstractList and AbstractSequentialList. Like the AbstractSet class, they override the equals() and hashCode() methods to ensure two equal collections return the same hash code. Two sets are equal if they are the same size and contain the same elements in the same order. The hashCode() implementation is specified in the List interface definition and implemented here.

Besides the equals() and hashCode() implementations, AbstractList and AbstractSequentialList provide partial implementations of the remaining List methods. They make the creation of concrete list implementations easier, for random-access and sequential-access data sources, respectively. Which set of methods you need to define depends on the behavior you wish to support. The following table should help you remember which methods need to be implemented. One thing you'll never need to provide yourself is an implementation of the Iterator iterator() method.

AbstractList AbstractSequentialList
unmodifiable Object get(int index)
int size() ListIterator listIterator(int index)
– boolean hasNext()
– Object next()
– int nextIndex()
– boolean hasPrevious()
– Object previous()
– int previousIndex()
int size()
modifiable unmodifiable +
Object set(int index, Object element) unmodifiable +
ListIterator
– set(Object element)
variable-size
and modifiable modifiable +
add(int index, Object element)
Object remove(int index) modifiable +
ListIterator
– add(Object element)
– remove()
As the Collection interface documentation states, you should also provide two constructors, a no-argument one and one that accepts another Collection.

Magercise
Using an ArrayList with a JComboBox
Map Interface
The Map interface is not an extension of the Collection interface. Instead, the interface starts off its own interface hierarchy, for maintaining key-value associations. The interface describes a mapping from keys to values, without duplicate keys, by definition.

The interface methods can be broken down into three sets of operations: altering, querying, and providing alternative views.

The alteration operations allow you to add and remove key-value pairs from the map. Both the key and value can be null. However, you should not add a Map to itself as a key or value.

Object put(Object key, Object value)
Object remove(Object key)
void putAll(Map mapping)
void clear()
The query operations allow you to check on the contents of the map:

Object get(Object key)
boolean containsKey(Object key)
boolean containsValue(Object value)
int size()
boolean isEmpty()
The last set of methods allow you to work with the group of keys or values as a collection.

public Set keySet()
public Collection values()
public Set entrySet()
Since the collection of keys in a map must be unique, you get a Set back. Since the collection of values in a map may not be unique, you get a Collection back. The last method returns a Set of elements that implement the Map.Entry interface, described next.

Map.Entry Interface
The entrySet() method of Map returns a collection of objects that implement Map.Entry interface. Each object in the collection is a specific key-value pair in the underlying Map.

Iterating through this collection, you can get the key or value, as well as change the value of each entry. However, the set of entries becomes invalid, causing the iterator behavior to be undefined, if the underlying Map is modified outside the setValue() method of the Map.Entry interface.

HashMap, TreeMap Classes
The Collections Framework provides two general-purpose Map implementations: HashMap and TreeMap . As with all the concrete implementations, which implementation you use depends on your specific needs. For inserting, deleting, and locating elements in a Map, the HashMap offers the best alternative. If, however, you need to traverse the keys in a sorted order, then TreeMap is your better alternative. Depending upon the size of your collection, it may be faster to add elements to a HashMap, then convert the map to a TreeMap for sorted key traversal. Using a HashMap requires that the class of key added have a well-defined hashCode() implementation. With the TreeMap implementation, elements added to the map must be sortable. Again, more on sorting later.

To optimize HashMap space usage, you can tune the initial capacity and load factor. The TreeMap has no tuning options, as the tree is always balanced.

Both HashMap and TreeMap implement the Cloneable interface.

The Hashtable and Properties classes are historical implementations of the Map interface. They will be discussed later.

Map Usage Example
The following program demonstrates the use of the concrete Map classes. The program generates a frequency count of words passed from the command line. A HashMap is initially used for data storage. Afterwards, the map is converted to a TreeMap to display the key list sorted.

import java.util.*;

public class MapExample {
public static void main(String args[]) {
Map map = new HashMap();
Integer ONE = new Integer(1);
for (int i=0, n=args.length; i<n; i++) {
String key = args[i];
Integer frequency = (Integer)map.get(key);
if (frequency == null) {
frequency = ONE;
} else {
int value = frequency.intValue();
frequency = new Integer(value + 1);
}
map.put(key, frequency);
}
System.out.println(map);
Map sortedMap = new TreeMap(map);
System.out.println(sortedMap);
}
}

Running the program with the the text from Article 3 of the Bill of Rights produces the following output. Notice how much more useful the sorted output looks.

Unsorted

{prescribed=1, a=1, time=2, any=1, no=1, shall=1, nor=1, peace=1, owner=1, soldier=1, to=1, the=2, law=1, but=1, manner=1, without=1, house=1, in=4, by=1, consent=1, war=1, quartered=1, be=2, of=3}

and sorted

{a=1, any=1, be=2, but=1, by=1, consent=1, house=1, in=4, law=1, manner=1, no=1, nor=1, of=3, owner=1, peace=1, prescribed=1, quartered=1, shall=1, soldier=1, the=2, time=2, to=1, war=1, without=1}

AbstractMap Class
Similar to the other abstract collection implementations, the AbstractMap class overrides the equals() and hashCode() methods to ensure two equal maps return the same hash code. Two maps are equal if they are the same size, contain the same keys, and each key maps to the same value in both maps. By definition, the hash code for a map is the sum of the hash codes for the elements of the map, where each element is an implementation of the Map.Entry interface. Thus, no matter what the internal ordering of the maps, two equal maps will report the same hash code.

WeakHashMap Class
A WeakHashMap is a special-purpose implementation of Map for storing only weak references to the keys. This allows for the key-value pairs of the map to be garbage collected when the key is no longer referenced outside of the WeakHashMap. Using WeakHashMap is beneficial for maintaining registry-like data structures, where the utility of an entry vanishes when its key is no longer reachable by any thread.

The Java 2 SDK, Standard Edition, version 1.3 adds a constructor to WeakHashMap that accepts a Map. With version 1.2 of the Java 2 platform, the available constructors only permit overriding the default load factor and initial capacity setting, not initializing the map from another map (as recommended by the Map interface documentation).

Sorting
There have been many changes to the core Java libraries to add support for sorting with the addition of the Collections Framework to the Java 2 SDK, version 1.2. Classes like String and Integer now implement the Comparable interface to provide a natural sorting order. For those classes without a natural order, or when you desire a different order than the natural order, you can implement the Comparator interface to define your own.

To take advantage of the sorting capabilities, the Collections Framework provides two interfaces that use it: SortedSet and SortedMap.

Comparable Interface
The Comparable interface, in the java.lang package, is for when a class has a natural ordering. Given a collection of objects of the same type, the interface allows you to order the collection into that natural ordering.

The compareTo() method compares the current instance with an element passed in as an argument. If the current instance comes before the argument in the ordering, a negative value is returned. If the current instance comes after, then a positive value is returned. Otherwise, zero is returned. It is not a requirement that a zero return value signifies equality of elements. A zero return value just signifies that two objects are ordered at the same position.

There are fourteen classes in the Java 2 SDK, version 1.2, that implements the Comparable interface. The following table shows their natural ordering. While some classes share the same natural ordering, you can only sort classes that are mutually comparable. For the current release of the SDK, that means the same class.

Class Ordering
BigDecimal
BigInteger
Byte
Double
Float
Integer
Long
Short Numerical
Character Numerical by Unicode value
CollationKey Locale-sensitive string ordering
Date Chronological
File Numerical by Unicode value of characters in fully-qualified, system-specific pathname
ObjectStreamField Numerical by Unicode value of characters in name
String Numerical by Unicode value of characters in string
The documentation for the compareTo() method of String defines the ordering lexicographically. What this means is the comparison is of the numerical values of the characters in the text, which is not necessarily alphabetically in all languages. For locale-specific ordering, use Collator with CollationKey.

The following demonstrates the use of Collator with CollationKey to do a locale-specific sorting:

import java.text.*;
import java.util.*;

public class CollatorTest {
public static void main(String args[]) {
Collator collator =
Collator.getInstance();
CollationKey key1 =
collator.getCollationKey("Tom");
CollationKey key2 =
collator.getCollationKey("tom");
CollationKey key3 =
collator.getCollationKey("thom");
CollationKey key4 =
collator.getCollationKey("Thom");
CollationKey key5 =
collator.getCollationKey("Thomas");

Set set = new TreeSet();
set.add(key1);
set.add(key2);
set.add(key3);
set.add(key4);
set.add(key5);
printCollection(set);
}
static private void printCollection(
Collection collection) {
boolean first = true;
Iterator iterator = collection.iterator();
System.out.print("[");
while (iterator.hasNext()) {
if (first) {
first = false;
} else {
System.out.print(", ");
}
CollationKey key =
(CollationKey)iterator.next();
System.out.print(key.getSourceString());
}
System.out.println("]");
}
}

Running the program produces the following output:

[thom, Thom, Thomas, tom, Tom]
If the names were stored directly, without using Collator, then the lowercase names would appear apart from the uppercase names:

[Thom, Thomas, Tom, thom, tom]
Making your own class Comparable is just a matter of implementing the compareTo() method. It usually involves relying on the natural ordering of several data members. Your own classes should also override equals() and hashCode() to ensure two equal objects return the same hash code.

Comparator Interface
When a class wasn't designed to implement java.lang.Comparable, you can provide your own java.util.Comparator. Providing your own Comparator also works if you don't like the default Comparable behavior.

The return values of the compare() method of Comparator are similar to the compareTo() method of Comparable. In this case, if the first element comes before the second element in the ordering, a negative value is returned. If the first element comes after, then a positive value is returned. Otherwise, zero is returned. Similar to Comparable, a zero return value does not signify equality of elements. A zero return value just signifies that two objects are ordered at the same position. Its up to the user of the Comparator to determine how to deal with it. If two unequal elements compare to zero, you should first be sure that is what you want and second document the behavior. Using a Comparator that is not compatible to equals can be deadly when used with a TreeSet or TreeMap. With a set, only the first will be added. With a map, the value for the second will replace the value for the second (keeping the key of the first).

To demonstrate, you may find it easier to write a new Comparator that ignores case, instead of using Collator to do a locale-specific, case-insensitive comparison. The following is one such implementation:

class CaseInsensitiveComparator implements
Comparator {
public int compare(Object element1,
Object element2) {
String lowerE1 = (
(String)element1).toLowerCase();
String lowerE2 = (
(String)element2).toLowerCase();
return lowerE1.compareTo(lowerE2);
}
}

Since every class subclasses Object at some point, it is not a requirement that you implement the equals() method. In fact, in most cases you won't. Do keep in mind the equals() method checks for equality of Comparator implementations, not the objects being compared.

The Collections class has one predefined Comparator available for reuse. Calling Collections.reverseOrder() returns a Comparator that sorts objects that implement the Comparable interface in reverse order.

Magercise
Using a Map to Count Words
SortedSet Interface
The Collections Framework provides a special Set interface for maintaining elements in a sorted order: SortedSet.

The interface provides access methods to the ends of the set as well as to subsets of the set. When working with subsets of the list, changes to the subset are reflected in the source set. In addition, changes to the source set are reflected in the subset. This works because subsets are identified by elements at the end points, not indices. In addition, if the fromElement is part of the source set, it is part of the subset. However, if the toElement is part of the source set, it is not part of the subset. If you would like a particular to-element to be in the subset, you must find the next element. In the case of a String, the next element is the same string with a null character appended (string+"").;

The elements added to a SortedSet must either implement Comparable or you must provide a Comparator to the constructor of its implementation class: TreeSet. (You can implement the interface yourself. But the Collections Framework only provides one such concrete implementation class.)

To demonstrate, the following example uses the reverse order Comparator available from the Collections class:

Comparator comparator = Collections.reverseOrder();
Set reverseSet = new TreeSet(comparator);
reverseSet.add("Bernadine");
reverseSet.add("Elizabeth");
reverseSet.add("Gene");
reverseSet.add("Elizabeth");
reverseSet.add("Clara");
System.out.println(reverseSet);

Running the program produces the following output:

[Gene, Elizabeth, Clara, Bernadine]
Because sets must hold unique items, if comparing two elements when adding an element results in a zero return value (from either the compareTo() method of Comparable or the compare() method of Comparator), then the new element is not added. If the elements are equal, then that is okay. However, if they are not, then you should modify the comparison method such that the comparison is compatible with equals().

Using the prior CaseInsensitiveComparator to demonstrate this problem, the following creates a set with three elements: thom, Thomas, and Tom, not five elements as might be expected.

Comparator comparator =
new CaseInsensitiveComparator();
Set set = new TreeSet(comparator);
set.add("Tom");
set.add("tom");
set.add("thom");
set.add("Thom");
set.add("Thomas");

SortedMap Interface
The Collections Framework provides a special Map interface for maintaining keys in a sorted order: SortedMap.

The interface provides access methods to the ends of the map as well as to subsets of the map. Working with a SortedMap is just like a SortedSet, except the sort is done on the map keys. The implementation class provided by the Collections Framework is a TreeMap.

Because maps can only have one value for every key, if comparing two keys when adding a key-value pair results in a zero return value (from either the compareTo() method of Comparable or the compare() method of Comparator), then the value for the original key is replaced with the new value. If the elements are equal, then that is okay. However, if they are not, then you should modify the comparison method such that the comparison is compatible with equals().

Special Collection Implementations
To keep the Collections Framework simple, added functionality is provided by wrapper implementations (also known as the Decorator design pattern–see the Design Patterns book for more information on patterns). These wrappers delegate the collections part to the underlying implementation class, but they add functionality on top of the collection. These wrappers are all provided through the Collections class. The Collections class also provides support for creating special-case collections.

Read-Only Collections
After you've added all the necessary elements to a collection, it may be convenient to treat that collection as read-only, to prevent the accidental modification of the collection. To provide this capability, the Collections class provides six factory methods, one for each of Collection, List, Map, Set, SortedMap, and SortedSet.

Collection unmodifiableCollection(Collection collection)
List unmodifiableList(List list)
Map unmodifiableMap(Map map)
Set unmodifiableSet(Set set)
SortedMap unmodifiableSortedMap(SortedMap map)
SortedSet unmodifiableSortedSet(SortedSet set)
Once you've filled the collection, the best way to make the collection read-only is to replace the original reference with the read-only reference. If you don't replace the original reference, then the collection is not read-only, as you can still use the original reference to modify the collection. The following program demonstrates the proper way to make a collection read-only. In addition, it shows what happens when you try to modify a read-only collection.

import java.util.*;

public class ReadOnlyExample {
public static void main(String args[]) {
Set set = new HashSet();
set.add("Bernadine");
set.add("Elizabeth");
set.add("Gene");
set.add("Elizabeth");
set = Collections.unmodifiableSet(set);
set.add("Clara");
}
}

When run and the last add() operation is attempted on the read-only set, an UnsupportedOperationException is thrown.

Thread-Safe Collections
The key difference between the historical collection classes and the new implementations within the Collections Framework is the new classes are not thread-safe. This was done such that if you don't need the synchronization, you don't use it, and everything works much faster. If, however, you are using a collection in a multi-threaded environment, where multiple threads can modify the collection simultaneously, the modifications need to be synchronized. The Collections class provides for the the ability to wrap existing collections into synchronized ones with another set of six methods:

Collection synchronizedCollection(Collection collection)
List synchronizedList(List list)
Map synchronizedMap(Map map)
Set synchronizedSet(Set set)
SortedMap synchronizedSortedMap(SortedMap map)
SortedSet synchronizedSortedSet(SortedSet set)
Unlike when making a collection read-only, you synchronize the collection immediately after creating it. You also must make sure you do not retain a reference to the original collection, or else you can access the collection unsynchronized. The simplest way to make sure you don't retain a reference is to never create one:

Set set = Collection.synchronizedSet(new HashSet());
Making a collection unmodifiable also makes a collection thread-safe, as the collection can't be modified. This avoids the synchronization overhead.

Singleton Collections
The Collections class provides for the ability to create single element sets fairly easily. Instead of having to create the Set and fill it in separate steps, you can do it all at once. The resulting Set is immutable.

Set set = Collection.singleton("Hello");
The Java 2 SDK, Standard Edition, version 1.3 adds the ability to create singleton lists and maps, too:

List singletonList(Object element)
Map singletonMap(Object key, Object value)
Multiple Copy Collections
If you need an immutable list with multiple copies of the same element, the nCopies(int n, Object element) method of the Collections class returns just such the List:

List fullOfNullList = Collection.nCopies(10, null);
By itself, that doesn't seem too useful. However, you can then make the list modifiable by passing it along to another list:

List anotherList = new ArrayList(fullOfNullList);
This now creates 10 element ArrayList, where each element is null. You can now modify each element at will, as it becomes appropriate.

Empty Collections
The Collections class also provides constants for empty collections:

List EMPTY_LIST
Set EMPTY_SET
The Java 2 SDK, Standard Edition, version 1.3 has a predefined empty map constant:

Map EMPTY_MAP
Historical Collection Classes
While this course is about the new Collections Framework of the Java 2 SDK, there are times when you still need to use some of the original collections capabilities. The following reviews some of the capabilities of working with arrays, vectors, hashtables, enumerations, and other historical capabilities.

Arrays
One learns about arrays fairly early on when learning the Java programming language. Arrays are defined to be fixed-size collections of the same datatype. They are the only collection that supports storing primitive datatypes. Everything else, including arrays, can store objects. When creating an array, you specify both the number and type of object you wish to store. And, over the life of the array, it can neither grow nor store a different type (unless it extends the first type).

To find out the size of an array, you ask its single public instance variable, length, as in array.length.

To access a specific element, either for setting or getting, you place the integer argument within square brackets ([int]), either before or after the array reference variable. The integer index is zero-based, and accessing beyond either end of the array will thrown an ArrayIndexOutOfBoundsException at runtime. If, however, you use a long variable to access an array index, you'll get a compiler-time error.

Arrays are full-fledged subclasses of java.lang.Object. They can be used with the various Java programming language constructs excepting an object:

Object obj = new int[5];
if (obj instanceof int[]) {
// true
}
if (obj.getClass().isArray()) {
// true
}

When created, arrays are automatically initialized, either to false for a boolean array , null for an Object array, or the numerical equivalent of 0 for everything else.

To make a copy of an array, perhaps to make it larger, you use the arraycopy() method of System. You need to preallocate the space in the destination array.

System.arraycopy(Object sourceArray, int sourceStartPosition, Object destinationArray, int destinationStartPosition, int length)
Vector and Stack Classes
A Vector is an historical collection class that acts like a growable array, but can store heterogeneous data elements. With the Java 2 SDK, version 2, the Vector class has been retrofitted into the Collections Framework hierarchy to implement the List interface. However, if you are using the new framework, you should use ArrayList, instead.

When transitioning from Vector to ArrayList, one key difference is the arguments have been reversed to positionally change an element's value:

From original Vector class
void setElementAt(Object element, int index)
From List interface
void set(int index, Object element)
The Stack class extends Vector to implement a standard list-in-first-out (LIFO) stack, with push() and pop() methods. Be careful though. Since the Stack class extends the Vector class, you can still access or modify a Stack with the inherited Vector methods.

Enumeration Interface
The Enumeration interface allows you to iterator through all the elements of a collection. In the Collections Framework, this interface has been superceded by the Iterator interface. However, not all libraries supports the newer interface, so you may find yourself using Enumeration from time to time.

Iterating through an Enumeration is similar to iterating through an Iterator, though some people like the method names better with Iterator. However, there is no removal support with Enumeration.

Enumeration enum = …;
while (enum.hasNextElement()) {
Object element = iterator.nextElement();
// process element
}
Dictionary, Hashtable, Properties Classes
The Dictionary class is completely full of abstract methods. In other words, it should have been an interface. It forms the basis for key-value pair collections in the historical collection classes, with its replacement being Map, in the new framework. Hashtable and Properties are the two specific implementations of Dictionary available.

The Hashtable implementation is a generic dictionary that permits storing any object as its key or value (besides null). With the Java 2 SDK, version 1.2, The class has been reworked into the Collections Framework to implement the Map interface. So, when using, you can use the original Hashtable methods or the newer Map methods. If you need a synchronized Map, using Hashtable is slightly faster than using a synchronized HashMap.

The Properties implementation is a specialized Hashtable for working with text strings. While you have to cast valued retrieved from a Hashtable to your desired class, the Properties class allows you to get text values without casting. The class also supports loading and saving property settings from and input stream or to a output stream. The most commonly used set of properties is the system properties list, retrieved by System.getProperties().

BitSet Class
A BitSet represents an alternate representation of a set. Given a finite number of n objects, you can associate a unique integer with each object. Then each possible subset of the objects corresponds to an n-bit vector, with each bit "on" or "off" depending on whether the object is in the subset. For small values of n a bit vector might be an extremely compact representation. However, for large values of n an actual bit vector might be inefficient, when most of the bits are off.

There is no replacement to BitSet in the new framework.

Magercise
Using a HashSet for a Sparse Bit Set
Algorithm Support
The Collections and Arrays classes, available as part of the Collections Framework, provide support for various algorithms with the collection classes, both new and old. The different operations, starting with sorting and searching, are described next.

Sorting Arrays
While the TreeSet and TreeMap classes offer sorted version of sets and maps, there is no sorted List collection implementation. Also, prior to the collections framework, there was no built in support for sorting arrays. As part of the framework, you get both support for sorting a List, as well as support for sorting arrays of anything, including primitives. With any kind of sorting, all items must be comparable to each other (mutually comparable). If they are not, a ClassCastException will be thrown.

Sorting of a List is done with one of two sort() methods in the Collections class. If the element type implements Comparable then you would use the sort(List list) version. Otherwise, you would need to provide a Comparator and use sort(List list, Comparator comparator). Both versions are destructive to the List and guarantee O(n log2n) performance (or better), including when sorting a LinkedList, using a merge sort variation.

Sorting of arrays is done with one of eighteen different methods. There are two methods for sorting each of the seven primitive types (besides boolean), one for sorting the whole array and one for sorting part of the array. The remaining four are for sorting object arrays (Object[ ]).

The sorting of primitive arrays involving just calling Arrays.sort() with your array as the argument, and letting the compiler determine which of the following methods to pick:

void sort(byte array[ ])
void sort(byte array[ ], int fromIndex, int toIndex)
void sort(short array[ ])
void sort(short array[ ], int fromIndex, int toIndex)
void sort(int array[ ])
void sort(int array[ ], int fromIndex, int toIndex)
void sort(long array[ ])
void sort(long array[ ], int fromIndex, int toIndex)
void sort(float array[ ])
void sort(float array[ ], int fromIndex, int toIndex)
void sort(double array[ ])
void sort(double array[ ], int fromIndex, int toIndex)
void sort(char array[ ])
void sort(char array[ ], int fromIndex, int toIndex)
The sorting of object arrays is a little more involved, as the compiler doesn't check everything for you. If the object in the array implements Comparable, then you can just sort the array directly, in whole or in part. Otherwise, you must provide a Comparator to do the sorting for you. You can also provide a Comparator implementation if you don't like the default ordering.

void sort(Object array[ ])
void sort(Object array[ ], int fromIndex, int toIndex)
void sort(Object array[ ], Comparator comparator)
void sort(Object array[ ], int fromIndex, int toIndex, Comparator comparator)
Searching
Besides sorting, the Collections and Arrays classes provide mechanisms to search a List or array, as well as to find the minimum and maximum values within a Collection.

While you can use the contains() method of List to find if an element is part of the list, it assumes an unsorted list. If you've previously sorted the List, using Collections.sort(), then you can do a much quicker binary search using one of the two overridden binarySearch() methods. If the objects in the List implement Comparable, then you don't need to provide a Comparator. Otherwise, you must provide a Comparator. In addition, if you sorted with a Comparator, you must use the same Comparator when binary searching.

public static int binarySearch(List list, Object key)
public static int binarySearch(List list, Object key, Comparator comparator)
If the List to search subclasses the AbstractSequentialList class (like LinkedList), then a sequential search is actually done.

Array searching works the same way. After using one of the Arrays.sort() methods, you can take the resulting array and search for an element. There are seven overridden varieties of binarySearch() to search for a primitive (all but boolean), and two to search an Object array, both with and without a Comparator.

If the original List or array is unsorted, the result is non-deterministic.

Besides searching for specific elements within a List, you can search for extreme elements within any Collection: the minimum and maximum. If you know your collection is already sorted, just get the first or last element. However, for unsorted collections, you can use one of the min() or max() methods of Collections. If the object in the collection doesn't implement Comparable, then you must provide a Comparator.

Object max(Collection collection)
Object max(Collection collection, Comparator comparator)
Object min(Collection collection)
Object min(Collection collection, Comparator comparator)
Checking Equality
While the MessageDigest class always provided an isEqual() method to compare two byte arrays, it never felt right to use it to compare byte arrays unless they were from message digests. Now, with the help of the Arrays class, you can check for equality of any array of primitive or object type. Two arrays are equal if they contain the same elements in the same order. Checking for equality with arrays of objects relies on the equals() method of each object to check for equality.

byte array1[] = …;
byte array2[] = …;
if (Arrays.equals(array1, array2) {
// They're equal
}
Manipulating
The Collections and Arrays classes offer several ways of manipulating the elements within a List or array. There are no additional ways to manipulate the other key framework interfaces (Set and Map).

With a List, the Collections class lets you replace every element with a single element, copy an entire list to another, reverse all the elements, or shuffle them around. When copying from one list to another, if the destination list is larger, the remaining elements are untouched.

void fill(List list, Object element)
void copy(List source, List destination)
void reverse(List list)
void shuffle(List list)
void shuffle(List list, Random random)
The Arrays class allows you to replace an entire array or part of an array with one element via eighteen overridden versions of the fill() method. All the methods are of the form fill(array, element) or fill(array, fromIndex, toIndex, element).

Big-O Notation
Performance of sorting and searching operations with collections of size n is measured using Big-O notation. The notation describes the complexity of the algorithm in relation to the maximum time in which an algorithm operates, for large values of n. For instance, if you iterate through an entire collection to find an element, the Big-O notation is referred to as O(n), meaning that as n increases, time to find an element in a collection of size n increases linearly. This demonstrates that Big-O notation assumes worst case performance. It is always possible that performance is quicker.

The following table shows the Big-O values for different operations, with 65,536 as the value for n. In addition, the operation count shows that if you are going to perform multiple search operations on a collection, it is faster to do a quick sort on the collection, prior to searching, versus doing a linear search each time. (And, one should avoid bubble sorting, unless n is really small!)

Description Big-O # Operations Example
Constant O(1) 1 Hash table lookup (ideal)
Logarithmic O(log2n) 16 Binary search on sorted collection
Linear O(n) 65,536 Linear search
Linear-logarithmic O(n log2n) 1,048,576 Quick sort
Quadratic O(n2) 4,294,967,296 Bubble sort
Legend: n = 65536
Usage Issues
The Collections Framework was designed such that the new framework classes and the historical data structure classes can interoperate. While it is good if you can have all your new code use the new framework, sometimes you can't. The framework provides much support for intermixing the two sets of collections. In addition, you can develop with a subset of the capabilities with JDK 1.1.

Converting from Historical Collections to New Collections
There are convenience methods for converting from many of the original collection classes and interfaces to the newer framework classes and interfaces. They serve as bridges when you need a new collection but have an historical collection. You can go from an array or Vector to a List, a Hashtable to a Map, or an Enumeration to any Collection.

For going from any array to a List, you use the asList(Object array[]) method of the Arrays class. Changes to the List pass through to the array, but you cannot adjust the size of the array.

String names[] = {"Bernadine",
"Elizabeth", "Gene", "Clara"};
List list = Arrays.asList(names);
Because the original Vector and Hashtable classes have been retrofitted into the new framework, as a List and Map respectively, there is no work to treat either of these historical collections as part of the new framework. Treating a Vector as a List automatically carries to its subclass Stack. Treating a Hashtable as a Map automatically caries to its subclass Properties.

Moving an Enumeration to something in the new framework requires a little more work, as nothing takes an Enumeration in its constructor. So, to convert an Enumeration, you create some implementation class in the new framework and add each element of the enumeration.

Enumeration enumeration = …;
Collection collection = new LinkedList();
while (e.hasMoreElements()) {
collection.add(e.nextElement());
}
// Operate on collection

Converting from New Collections to Historical Collections
In addition to supporting the use of the old collection classes within the new collection framework, there is also support for using the new framework and still using libraries that only support the original collections. You can easily convert from Collection to array, Vector, or Enumeration, as well as from Map to Hashtable.

There are two ways to go from Collection to array, depending upon the type of array you need. The simplest way involves going to an Object array. In addition, you can also convert the collection into any other array of objects. However, you cannot directly convert the collection into an array of primitives, as collections must hold objects.

To go from a collection to an Object[], you use the toArray() method of Collection:

Collection collection = …;
Object array[] = collection.toArray();
The toArray() method is overridden to accept an array for placing the elements of the collection: toArray(Object array[]). The datatype of the argument determines the type of array used to stored the collection and returned by the method. If the array isn't large enough, a new array of the appropriate type will be created.

Collection collection = …;
int size = collection.size();
Integer array[] =
collection.toArray(new Integer[size]);
To go from Collection to Vector, the Vector class now includes a constructor that accepts a Collection. As with all these conversions, if the element in the original conversion is mutable, then no matter from where it is retrieved and modified, its changed everywhere.

Dimension dims[] = {new Dimension (0,0),
new Dimension (0,0)};
List list = Arrays.asList(dims);
Vector v = new Vector(list);
Dimension d = (Dimension)v.get(1);
d.width = 12;

Going from Collection to Enumeration is much easier than going from Enumeration to Collection. The Collections class includes a static method to do the conversion for you:

Collection collection = …;
Enumeration enum =
Collections.enumeration(collection);
The conversion from Map to Hashtable is similar to Collection to Vector, just pass the new framework class to the constructor. After the conversion, changing the value for the key in one does not alter the value for the key in the other.

Map map = …;
Hashtable hashtable = new Hashtable(map);
Working with the Collections Framework Support in JDK 1.1
If you are still using JDK 1.1, you can start taking advantage of the Collections Framework today. Sun Microsystems provides a subset of the collections API for use with JDK 1.1. The interfaces and classes of the framework have been moved from the java.lang and java.util package to the non-core com.sun.java.util.collections package. This is not a complete set of classes changed to support the framework, but only copies of those introduced. Basically, what that means is none of the system classes are sortable by default, you must provide your own Comparator.

The following table lists the classes available in the Collections Framework release for JDK 1.1. In some cases, there will be two different implementations of the same class, like with Vector, as the 1.2 framework version implements List and the core 1.1 version doesn't.

AbstractCollection AbstractList
AbstractMap AbstractSequentialList
AbstractSet ArrayList
Arrays Collection
Collections Comparable
Comparator ConcurrentModificationException
HashMap HashSet
Hashtable Iterator
LinkedList List
ListIterator Map
NoSuchElementException Random
Set SortedMap
SortedSet TreeMap
TreeSet UnsupportedOperationException
Vector
Alternative Collections
Because the Collection Framework was not available prior to the introduction of the Java 2 platform, several alternative collection libraries became available. Two such libraries are Doug Lea's Collections Package and ObjectSpace's JGL.

Doug Lea's Collections Package.
The collections package from Doug Lea (author of Concurrent Programming in Java), was first available in October 1995 and last updated in April 1997. It probably offered the first publicly available collections library. While no longer supported, the library shows the complexity added to the class hierarchy when you try to provide updateable and immutable collections, without optional methods in interfaces or wrapper implementations. While a good alternative at the time, its use is no longer recommended. (Doug also helped author some of the Collections Framework.)

ObjectSpace's JGL
In addition to Doug Lea's collections library, the Generic Collection Library for Java (JGL) from ObjectSpace was an early collection libraries available for the Java platform. Following the design patterns of the Standard Template Library (STL) for C++, the library provides algorithmic support, in addition to a data structure library. While the JGL is a good alternative collection framework, it didn't meet the design goals of the Collections Framework team: "The main design goal was to produce an API that was reasonably small, both in size, and, more importantly, in conceptual weight." With that in mind, the team came up with the Collections Framework.

While not adopted by Sun Microsystems, the JGL has been included with many IDE tool. Due to its early availability, the JGL is available to well over 100,000 developers.

For a comparison of JGL versus the Collections Framework, see The battle of the container frameworks: which should you use? article in JavaWorld. [If you are curious of how the library name maps to the acronym, it doesn't. The name of the first version of the library infringed on Sun's Java trademark. ObjectSpace changed the name, but the original acronym stuck.]

Conclusion
The Collections Framework provides a well-designed set of interfaces, implementations, and algorithms for representing and manipulating groups of elements. Understanding all the capabilities of this framework reduces the effort required to design and implement a comparable set of APIs, as was necessary prior to their introduction. Now that you have completed this module, you can effectively manage groups of data elements.

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