Introduction to Java 2 Programming

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 implement public 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

Introduction to Java 2 Programming