Lecture 7 Object Oriented Programming in Java Advanced

Lecture 7 Object Oriented Programming in Java Advanced Topics Collection Framework June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 1

Today’s Lecture • Trail: Collections • Lessons: – – Introduction Interfaces Implementations Algorithms June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 2

Collections • Collections simply allow you to group together related objects • Collections provide sophisticated ways to hold and even manipulate these many objects June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 3

History • The Java 2 collection framework represents a thorough redesign of the rather poor showings in Java 1. 0 and 1. 1 – simple arrays are efficient but difficult to use for complex tasks such copying, duplicating, sorting, . . . – Vector and Hashtable classes in JDK 1. x where useful but flawed in design and lacked standard built-in functionality • If you were familiar with the Vector and Hashtable classes you will still find them in Java 2. They still are maintained for backward compatibility but still suffer from some of the same problems June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 4

Purpose of an OO Framework • Reuse and programming- by- difference – Using inheritance and stub class implementations (abstract classes), a new class can be implemented by providing only what is different in this class compared to one which already exists – The effort to develop a new class is proportional to the difference in functionality between the particular class and that in the framework June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 5

Frameworks vs. Class Libraries • Framework and Class Libraries are similar but different – Class libraries have no predefined flow of control, no predefined interactions. There are just a set of instantiated classes by the client – Framework provide for customization by sub-classing, Provide default behaviors, Defines object interactions • The collection framework is a little bit of both (class library and true framework) – The Collection Framework is a good example of the power of object oriented design June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 6

Abstraction of a Framework June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 7

Collection Framework Architecture • • Interfaces Abstract Implementations General Purpose Implementations Legacy Implementations June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 8

Collection Framework Interfaces • Interfaces are the roles a component of object can play • Here they specify the abstract data types which represent collections: June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 9

Collection • Collection – A Collection represents a group of objects, known as its elements • Behaviors: – Basic Operations – Bulk Operations – Array Operations June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 10

Collection Methods public interface Collection { // Basic Operations int size(); boolean is. Empty(); boolean contains(Object element); boolean add(Object element); // Optional boolean remove(Object element); // Optional Iterator iterator(); // Bulk Operations boolean add. All(Collection c); // Optional boolean remove. All(Collection c); // Optional boolean retain. All(Collection c); // Optional …. // Array Operations Object[] to. Array(); Object[] to. Array(Object a[]); } June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 11

Lists • A List is an ordered collection (sometimes called a sequence) • Lists can contain duplicate elements • Examples: – List of first name in the class sorted by alphabetical order: • Eric, Fred, Greg, John, John – List of cars sorted by origin: • Ford, Chevrolet, Jeep, Nissan, Toyota, BMW, VW June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 12

List Interface Methods • Inherits from Collection • Some additions: – – – – void add(int index, Object element); boolean add. All(int index, Collection c); Object get(int index); Object remove(int index); Object set(int index, Object element); int last. Index. Of(Object o); int index. Of(Object o); June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 13

Sets • A Set is a collection that cannot contain duplicate elements • Examples: – Set of cars: • {BMW, Ford, Jeep, Chevrolet, Nissan, Toyota, VW} – Nationalities in the class • {Chinese, American, Canadian, Indian} – Course schedule for John • {95 -707, 90 -203, 95 -405} June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 14

Set Interface Methods • Same as Collection Methods but the contract is different: – No duplicates are maintained June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 15

Map • A Map is an object that maps keys to values. Maps cannot contain duplicate keys. • Each key can map to at most one value • Examples: – Think of a dictionary: • word <-> description – address book • name <-> phone number A 1 B 2 C 3 D Illegal mapping Map June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 16

Map Interface Methods • Basics – – – Object put(Object key, Object value); Object get(Object key); Object remove(Object key) int size(); . . . • Bulk – void put. All(Map t); – void clear(); • Collection Views – public Set key. Set(); – public Collection values(); – public Set entry. Set(); June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 17

Iterator Interface – Similar to the old Enumeration interface of Vector and Hashtable – An Iterator is an object whose job is to move through a sequence of objects and select each object in that sequence without the client programmer knowing or caring about the underlying structure of that sequence – Here is what you can do with an Iterator: – Ask a container to hand you an Iterator using a method called iterator( ). This Iterator will be ready to return the first element in the sequence on your first call to its next( ) method. – Get the next object in the sequence with next( ). – See if there any more objects in the sequence with has. Next( ). – Remove the last element returned by the iterator with remove( ). June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 18

Iterator Interface • The interface definition: public interface Iterator { boolean has. Next(); Object next(); void remove(); // Optional } • Sample code: static void filter(Collection c) { for (Iterator i = c. iterator(); i. has. Next(); ) if (!cond(i. next())) i. remove(); } June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 19

Implementations June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 20

Roll-out your own • Abstract Implementations – – Abstract. Collection Abstract. Set Abstract. List Abstract. Map June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 21

Overall Taxonomy June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 22

Cats and Dogs - I // Simple container with Iterator. import java. util. *; public class Cats. And. Dogs { public static void main(String[] args) { Array. List cats = new Array. List(); for(int i = 0; i < 7; i++) cats. add(new Cat(i)); Iterator e = cats. iterator(); while(e. has. Next()) ((Cat)e. next()). print(); } } June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 23

Cats and Dogs - II // Simple container with Iterator. import java. util. *; public class Cats. And. Dogs { public static void main(String[] args) { Array. List cats = new Array. List(); for(int i = 0; i < 7; i++) cats. add(new Cat(i)); Iterator e = cats. iterator(); while(e. has. Next()) ((Cat)e. next()). print(); } } June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 24

Cats and Dogs - III // Simple container with Iterator. import java. util. *; public class Cats. And. Dogs { public static void main(String[] args) { Array. List cats = new Array. List(); for(int i = 0; i < 7; i++) cats. add(new Cat(i)); Iterator e = cats. iterator(); while(e. has. Next()) ((Cat)e. next()). print(); } } June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 25

Cats and Dogs - IV // Simple container with Iterator. import java. util. *; public class Cats. And. Dogs { public static void main(String[] args) { Array. List cats = new Array. List(); for(int i = 0; i < 7; i++) cats. add(new Cat(i)); Iterator e = cats. iterator(); while(e. has. Next()) ((Cat)e. next()). print(); } } June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 26

Collection. Printer import java. util. *; public class Collection. Printer { static Collection fill(Collection c) { // add elements to the collection containers here return c; } static Map fill(Map m) { // add elements to the map here return m; } public static void main(String[] args) { // fill various collection containers here…. } } June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 27

Danger with Collections: Unknown Type public class Cat { private int cat. Number; Cat(int i) { cat. Number = i; } void print() { System. out. println("Cat #" + cat. Number); }} public class Dog { private int dog. Number; Dog(int i) { dog. Number = i; } void print() { System. out. println(”Dog #" + dog. Number); }} June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 28

Unknown Types public class Cats. And. Dogs { public static void main(String[] args) { Array. List cats = new Array. List(); for(int i = 0; i < 7; i++) cats. add(new Cat(i)); // Not a problem to add a dog to cats: cats. add(new Dog(7)); for(int i = 0; i < cats. size(); i++) ((Cat)cats. get(i)). print(); // Dog is detected only at run-time } } June 1, 2000 Object Oriented Programming in Java (95 -707) Java Language Basics 29