Data Structures in Java and Introduction to Collection Framework

DATA STRUCTURES IN JAVA
Arun Seetharaman
March 10, 1998

TOPIC OBJECTIVES
• Understand some useful data structures in the java.util
package
• The Observer-Observable pattern
• Java Collection Framework
• Iterators

DATA STRUCTURES
• Well designed java.util package
• A bunch of dynamic data structures
• You do not have to write linked lists and hash tables
• Could be building blocks for more complicated data
structures

THE “VECTOR” CLASS
• A very generic structure for storing and retrieving
objects
• A very simple structure to use
• Tries to optimize storage, resulting in some extra
capacity at times
• Removal of elements closes the holes created

VECTOR OPERATIONS
public void addElement(Object);
public void clear();
public Object elementAt(int);
public void insertElementAt(
Object, int);
public void removeElementAt(int);
public void setElementAt(
Object, int);
public int size();

THE “HASHTABLE” CLASS
• Useful for searchable data structures
• Stores key-value pairs
• Key-based search mechanism
• Better than Vector for large amount of data that need
to be searched
public Object get(Object);
public Object put(Object,Object);
protected void rehash();

CALENDARS
• Date: mostly deprecated as of JDK1.1
• Calendar: Convert Date objects to fields
• GregorianCalendar: Important class that cater to the
differences between Julian date and Gregorian date.
Also understands timezones
• TimeZone: represents any timezone with the GMT offset

THE BITSET CLASS
• Represents a collection of bits
• Grows dynamically as bits are required
• Bits accessed using 0-based index
• public void and(BitSet);
• public void andNot(BitSet);
• public void or(BitSet);
• public void xor(BitSet);

OBSERVER-OBSERVABLE
PATTERN
• Update multiple objects from a common source of
data
• Displaying data in multiple formats like charts,
worksheet is a common example
• More often implemented using the MVC design pattern

THE “OBSERVABLE”
CLASS
• Represents the data source
• Can have one or more observers
void addObserver(Observer);
void deletebserver(Observer);
void notifyObservers(Object);
void setChanged();
boolean hasChanged();
void clearChanged();

AN OBSERVABLE ENTITY
class MsgObservable extends Observable
implements Runnable {
public void run() {
while(true) {
msg = waitForMsg();
setChanged();
notifyObservers(msg);
}
}
}

THE “OBSERVER”
INTERFACE
• Implemented when changes in an “Observable”
object need to be informed
• update is invoked, when the Observable object calls
the notifyObservers method
class LogServer implements Observer {
public void update(Observable obs,
Object obj) {
logTheMsg(obj);
}
}

WHAT IS A COLLECTION
FRAMEWORK?
• Unified Architecture
• Interfaces : implementation-independence
• Implementations : reusable data structures
• Algorithms : reusable functionality
• Best-known examples
• C++ Standard Template Library (STL)
• Smalltalk

ARCHITECTURE
OVERVIEW
• Core Collection Interfaces
• General-Purpose Implementations
• Wrapper Implementations
• Abstract Implementations
• Algorithms

COLLECTION INTERFACE
public interface Collection {
int size();
boolean isEmpty();
boolean contains(Object element);
Iterator iterator();
Object[] toArray();
Object[] toArray(Object a[]);
}

ITERATOR INTERFACE
• Replacement for Enumeration interface
• Adds remove method
• Improves method names
public interface Iterator {
boolean hasNext();
Object next();
void remove(); // Optional
}

COLLECTION EXAMPLE
public static boolean removeNulls(
Collection c) {
boolean modified = false;
for (Iterator i=c.iterator();i.hasNext();){
if (i.next()==null) {
i.remove();
modified = true;
}
}
return modified;
}

SET INTERFACE
• Adds no methods to Collection!
• Adds stipulation: no duplicate elements
• Mandates equals and hashCode calculation
public interface Set
extends Collection {
}

SET IDIOMS
Set s1, s2;
boolean subset = s1.containsAll(s2);
Set union = new HashSet(s1).addAll(s2);
Set intersection = new
HashSet(s1).retainAll(s2);
Set difference = new
HashSet(s1).removeAll(s2);

LIST INTERFACE
• A sequence of objects
public interface List
extends Collection {
Object get(int);
int lastIndexOf(Object);
int lastIndexOf(Object, int);
ListIterator listIterator(int);
…
}

LIST EXAMPLE
• Reusable algorithms to swap elements and randomize
public static void swap(List a,
int i, int j) {
Object tmp = a.get(i);
a.set(i, a.get(j));
a.set(j, tmp);
}
public static void randomize(List a) {
for (int i=a.size(); i>1; i--)
swap(a, i-1,
(r.nextInt() &~ (1<<31)) % i);
}

MAP INTERFACE
• A key-value mapping
public interface Map {
int size();
boolean isEmpty();
boolean containsKey(Object key);
boolean containsValue(Object value);
Object get(Object key);
public Set keySet();
public Collection values();
public Set entries();
}

MAP IDIOMS
Map m;
for (iterator i=m.keySet().iterator();
i.hasNext(); )
System.out.println(i.next());
Map a, b;
boolean isSubMap =
a.entries().containsAll(b.entries());
Set commonKeys = new
HashSet(a.keySet()).retainAll(b.keySet);

GENERAL-PURPOSE
IMPLEMENTATIONS

COLLECTIONS
• Exclusively static methods
• Contains polymorphic algorithms
• Work on collections
• Implementers can substitute algorithms

SYNCHRONIZATION
WRAPPERS
• Anonymous implementations, one per core interface
• Static factories take collection of appropriate type
• Thread-safety assured if all access via wrapper
• Iteration must be synchronized manually

SYNCHRONIZATION
WRAPPER EXAMPLE
Set s = Collections.synchronizedSet(
newHashSet());
...s.add("wombat");
// Thread-safe ...
synchronized(s) {
Iterator i = s.iterator(); // In synch block!
while (i.hasNext())
System.out.println(i.next());
}

UNMODIFIABLE
WRAPPERS
• Unmodifiable wrappers
• Analogous to synchronization wrappers
• Provide read-only access
• Available for core interfaces

CONVENIENCE
WRAPPERS
• List-view of arrays
• Multiple-copy list
• Singletons
• Empty collections

SOME NEEDS FOR
CUSTOMIZATION
• Persistence
• Highly concurrent collections
• High-performance, special-purpose
• Space-efficient representations
• Fancy data structures
• Convenience classes

REUSABLE ALGORITHMS
• Sorting
• Searching
• Shuffling
• Data Manipulation
• Extreme Values

ALGORITHMS: SORTING
• Uses an optimized merge sort
• Doesn’t reorder equal elements
• static void sort(List);
• static void sort(List, Comparator);

COMPARABLE AND
COMPARATOR
• Comparable: uses the natural ordering for that object
type
• int compareTo(Object);
• Comparator: can define custom ordering for object
• int compare(Object, Object);
• boolean equals(Object);

ALGORITHMS:
SEARCHING
• Works on sorted lists
• Uses the binary search algorithm
• int binarySearch(List, Object);
• int binarySearch(List, Object, Comparator);

ALGORITHMS: SHUFFLING
• Force randomness in the Collection
• A random object defines the randomness for the shuffle
operation
• void shuffle(List);
• void shuffle(List, Random);

OTHER ALGORITHMS
• Data Manipulation:
• void copy(List, List);
• void fill(List, Object);
• void reverse(List);
• Extreme Values:
• Object max/min(Collection);
• Object max/min(Collection, Comparator);

COMPATIBILITY
• Upward Compatibility
• Vector implements List
• Hashtable implements Map
• Arrays.toList(myArray)
• Backward Compatibility
• myCollection.toArray()
• Vector(myCollection)
• Hashtable(myMap)

TOPIC SUMMARY
• The java.util package has a good collection of data
structures
• Classes to support the Observer-Observable pattern
• Collection framework for improved data structures
• Many reusable algorithms implemented

Data Structures in Java and Introduction to Collection Framework

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