Introduction to Java 2 Programming

Lecture 5

The Collections API

Overview

• Collection classes– Types of collection– Working with Collections– Sorting

Collections

• Implementations of common data structures, such as Linked Lists, Sets, etc.– Part of the java.util package.– Also known as “containers”

• Advantages– Can hold any kind of object– Much more flexible than arrays

• Disadvantages– Not as efficient as arrays (for some uses)– Can’t store primitive types– Not type-safe. Store references to Object

Types of Collection

• Two Types of Containers• Collections

– Group of objects, which may restricted or manipulated in some way

– E.g. ordered to make a List or LinkedList– E.g. a Set, an unordered group which can only contain

one of each item

• Maps– Associative array, Dictionary, Lookup Table, Hash– A group of name-value pairs

Java Collections

Collection Implementations

• Several implementations associated with each of the basic interfaces

• Each has its own advantages/disadvantages• Maps

– HashMap, SortedMap

• Lists– ArrayList, LinkedList

• Sets– HashSet, SortedSet

The Basics

• HashMap and ArrayList are most commonly encountered

• Usual object creation syntax • Always refer to the objects via one of the

Collections interfaces– Take advantage of polymorphism

List myList = new ArrayList();List otherList = new ArrayList(5);Map database = new HashMap();Set things = new HashSet();

Adding Items

• For Collections, use add()List myList = new ArrayList();

myList.add(“A String”);

myList.add(“Other String”);

• For Maps, use put()Map myMap = new HashMap();

myMap.put(“google”, “http://www.google.com”);

mpMap.put(“yahoo”, “http://www.yahoo.com”);

Copying

• Very easy, just use addAll()

List myList = new ArrayList();

//assume we add items to the list

List otherList = new ArrayList();

myList.addAll(myList);

Getting Individual Items

• Use get()• Note that we have to cast the object to its original

type.• Collections…String s = (String)myList.get(1); //get first element

String s2 = (String)myList.get(10); //get tenth element

• Maps…String s = (String)myMap.get(“google”);

String s2 = (String)mpMap.get(“yahoo”);

Getting All items

• For Lists, we could use a for loop, and loop through the list to get() each item

• But this doesn’t work for Maps.

• To allow generic handling of collections, Java defines an object called an Iterator– An object whose function is to walk through a

Collection of objects and provide access to each object in sequence

Getting All items

• Get an iterator using the iterator() method• Iterator objects have three methods:

– next() – gets the next item in the collection– hasNext() – tests whether it has reached the end– remove() – removes the item just returned

• Basic iterators only go forwards– Lists objects have a ListIterator that can go forward and

backward

Getting All items (List)

• Simple example:List myList = new ArrayList();

//we add items

Iterator iterator = myList.iterator();

while (iterator.hasNext())

{

String s = (String)iterator.next();

//do something with it

}

Getting All Items (Map)

• Example of using a Map• Note that we can get a Set of all keys (keySet) or

Collection of all values (values)

Map myMap = new HashMap();//we add items

Iterator iterator = myMap.keySet.iterator();while (iterator.hasNext()) { String theKey = (String)iterator.next(); Object theValue = myMap.get(theKey); //do something useful}

Other Functions

• The java.util.Collections class has many useful methods for working with collections– min, max, sort, reverse, search, shuffle

• Virtually all require your objects to implement an extra interface, called Comparable

Comparable• The Comparable interface labels objects that can be

compared to one another.– Allows sorting algorithms to be written to work on any kind

of object– so long as they support this interface

• Single method to implementpublic int compareTo(Object o);

• Returns – A negative number of parameter is less than the object– Zero if they’re equal– A positive number if the parameter is greater than the object

Comparable Example

public class Person implements Comparable {

private String email;

private String lastName;

public int compareTo(Object object) {

Person other = (Person)object;

//compare based on email address only

return other.getEmail().compareTo(email);

}

}

Comparable Example

Person a = new Person(“me@ldodds.com”, Dodds”);

Person b = new Person(“bob@builder.com”, “Builder”);

List people = new ArrayList();

People.add(a);

People.add(b);

Java.util.Collections.sort(people);

//collection is now sorted by email

Comparator• Like Comparable, but is a stand-alone object used

for comparing other objects– Useful when you want to use your criteria, not that of

the implementor of the object.

– Or altering the behaviour of a system

• Many of the methods in the Collections object also allow a Comparator to be specified

• Again method has single method:public int compare(Object obj1, Object obj2)

Comparator Example

• Java String comparison is lexicographic not alphabetic, I.e. based on the character set, not alphabetic order

public class AlphaComparison implements Comparator{ public int compare(Object obj1, Object obj2) { String s1 = ((String)o1).toLowerCase(); String s2 = ((String)o2).toLowerCase(); return s1.compareTo(s2); }}

Comparator Example

String one = “One”;

String two = “Two”;

String three = “Three”;

List strings = new ArrayList();

strings.add(one);

strings.add(two);

strings.add(three);

Collections.sort(strings, new AlphaComparison());

//now in alphabetical order