Transcript Lecture 7 Object Oriented Programming in Java Advanced Topics Collection Framework

Lecture 7
Object Oriented Programming in Java
Advanced Topics
Collection Framework
June 1, 2000
Object Oriented Programming in Java (95-707)
Java Language Basics
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Today’s Lecture
• Trail: Collections
• Lessons:
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Introduction
Interfaces
Implementations
Algorithms
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Collections
• Collections simply allow you to group together
related objects
• Collections provide sophisticated ways to hold
and even manipulate these many objects
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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
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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
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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
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Abstraction of a Framework
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Collection Framework Architecture
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Interfaces
Abstract Implementations
General Purpose Implementations
Legacy Implementations
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Collection Framework Interfaces
• Interfaces are the roles a component of
object can play
• Here they specify the abstract data types
which represent collections:
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Collection
• Collection
– A Collection represents a group of objects, known as
its elements
• Behaviors:
– Basic Operations
– Bulk Operations
– Array Operations
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Collection Methods
public interface Collection {
// Basic Operations
int size();
boolean isEmpty();
boolean contains(Object element);
boolean add(Object element); // Optional
boolean remove(Object element); // Optional
Iterator iterator();
// Bulk Operations
boolean addAll(Collection c); // Optional
boolean removeAll(Collection c); // Optional
boolean retainAll(Collection c); // Optional
….
// Array Operations
Object[] toArray();
Object[] toArray(Object a[]);
}
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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, Fred, Greg, John, John, John
– List of cars sorted by origin:
• Ford, Chevrolet, Jeep, Nissan, Toyota, BMW, VW
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List Interface Methods
• Inherits from Collection
• Some additions:
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void add(int index, Object element);
boolean addAll(int index, Collection c);
Object get(int index);
Object remove(int index);
Object set(int index, Object element);
int lastIndexOf(Object o);
int indexOf(Object o);
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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}
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Set Interface Methods
• Same as Collection Methods but the contract is
different:
– No duplicates are maintained
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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
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Map Interface Methods
• Basics
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Object put(Object key, Object value);
Object get(Object key);
Object remove(Object key)
int size();
...
• Bulk
– void putAll(Map t);
– void clear();
• Collection Views
– public Set keySet();
– public Collection values();
– public Set entrySet();
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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 are any more objects in the sequence with
hasNext( ).
– Remove the last element returned by the iterator with
remove( ).
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Iterator Interface
• The interface definition:
public interface Iterator {
boolean hasNext();
Object next();
void remove(); // Optional
}
• Sample code:
static void filter(Collection c) {
for (Iterator i = c.iterator(); i.hasNext(); )
if (!cond(i.next()))
i.remove();
}
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Implementations
Hash
Table
Set
HashSet
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Balanced
Tree
HashMap
Linked
List
TreeSet
ArrayList
List
Map
Resizable
Array
LinkedList
TreeMap
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Roll-out your own
• Abstract Implementations
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AbstractCollection
AbstractSet
AbstractList
AbstractMap
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Overall Taxonomy
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Cats and Dogs - I
// Simple container with Iterator.
import java.util.*;
public class CatsAndDogs {
public static void main(String[] args) {
ArrayList cats = new ArrayList();
for(int i = 0; i < 7; i++)
cats.add(new Cat(i));
Iterator e = cats.iterator();
while(e.hasNext())
((Cat)e.next()).print();
}
}
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Cats and Dogs - II
// Simple container with Iterator.
import java.util.*;
public class CatsAndDogs {
public static void main(String[] args) {
ArrayList cats = new ArrayList();
for(int i = 0; i < 7; i++)
cats.add(new Cat(i));
Iterator e = cats.iterator();
while(e.hasNext())
((Cat)e.next()).print();
}
}
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Cats and Dogs - III
// Simple container with Iterator.
import java.util.*;
public class CatsAndDogs {
public static void main(String[] args) {
ArrayList cats = new ArrayList();
for(int i = 0; i < 7; i++)
cats.add(new Cat(i));
Iterator e = cats.iterator();
while(e.hasNext())
((Cat)e.next()).print();
}
}
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Cats and Dogs - IV
// Simple container with Iterator.
import java.util.*;
public class CatsAndDogs {
public static void main(String[] args) {
ArrayList cats = new ArrayList();
for(int i = 0; i < 7; i++)
cats.add(new Cat(i));
Iterator e = cats.iterator();
while(e.hasNext())
((Cat)e.next()).print();
}
}
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CollectionPrinter
import java.util.*;
public class CollectionPrinter {
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….
}
}
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Danger with Collections: Unknown Type
public class Cat {
private int catNumber;
Cat(int i) { catNumber = i; }
void print() {
System.out.println("Cat #" +
catNumber);
}}
public class Dog {
private int dogNumber;
Dog(int i) { dogNumber = i;
}
void print() {
System.out.println(”Dog #"
+ dogNumber);
}}
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Unknown Types
public class CatsAndDogs {
public static void main(String[] args) {
ArrayList cats = new ArrayList();
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
}
}
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