Transcript session3 - Advanced Java 2

Advanced Java
Session 3
New York University
School of Continuing and Professional Studies
Objectives
• Backlog - jar, Synchronized keyword and
Class Constructors
• Collection Interfaces
• Concrete Collections
• Collection Framework
• Algorithms
• Legacy Collections
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Interfaces
• You cannot instantiate an object of type Interface
with new.
• You can declare that an object variable will be of
that interface type.
• You can use “instanceof” to check if an object
implements an interface.
• You cannot put instance fields, or static methods
in interfaces.
• You can put constants in interfaces.
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Implementations
• A class may implement one or more Interfaces
using the keyword “implements”
----public interface Comparable
{
public int compareTo(Object b)
}
-----Class Employee extends Person implements Comparable
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Queue Example
Interface Queue
{
void add(Object obj);
Object remove();
int size();
}
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tail
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Circular Array Implementation
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Linked List Implementation
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tail
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Sort program
import java.util.*;
public class sort {
public static void main(String args[]) {
List l = Arrays.asList(args);
Collections.sort(l);
System.out.println(l);
}
}
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Collection Interface
• Has three fundamental methods
– boolean add (Object obj)
– boolean remove (Object obj)
– Iterator iterator()
• Other methods
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boolean
boolean
boolean
boolean
boolean
contains(Object obj)
size()
isEmpty()
retainAll(Collection c)
containsAll(Collection c)
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Iterator Interface
• Has three fundamental methods
– Object next()
– boolean hasNext()
– void remove()
Element
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Element
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Element
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next()
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Concrete Collections
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LinkedList
ArrayList
HashSet
TreeSet
EnumSet
LinkedHashSet
PriorityQueue
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HashMap
TreeMap
EnumMap
LinkedHashMap
WeakHashMap
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LinkedList
Head
Data – elem1
Next
Previous
Data – elem2
Next
Previous
Data – elem3
Next
Previous
Null
Null
Data – new
Next
Previous
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An example of LinkedList
public class LinkedListExample {
public static void main(String args[]) {
LinkedList staff = new LinkedList();
staff.add(“Bob”); staff.add(“Angela”);
staff.add(“John”);
ListIterator iter = staff.iterator();
while( iter.hasNext() ) { System.out.println(iter.next()); }
while( iter.hasPrevious() ) { System.out.println(iter.previous()); }
System.out.println(staff.get(2));
}
}
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ArrayList
• Similar to a Vector
• Methods of ArrayList are not synchronized
• Methods of Vector are synchronized
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HashSet
• Implements a “hash table” data structure
• Can supply initial capacity
• Class must provide hashCode() method
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TreeSet
• Similar to HashSet, except it sorts the values as
you insert, and when you traverse the values,
they are presented back in a sorted order.
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HashMap
• Implements a data structure to store key,value
pairs
• Entries are indexed by unique key
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TreeMap
• Sorts the keys
• Comparison is applied to keys – not values
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An example of Map
public class MapTest
{ public static void main(String[] args)
{ Map staff = new HashMap();
staff.put("144-25-5464", new Employee("Angela Hung"));
staff.put("567-24-2546", new Employee("Harry Hacker"));
staff.put("157-62-7935", new Employee("Gary Cooper"));
staff.put("456-62-5527", new Employee("Francesca Cruz"));
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An example of Map
// remove an entry
staff.remove("567-24-2546");
// replace an entry
staff.put("456-62-5527", new Employee("Francesca Miller"));
// look up a value
System.out.println(staff.get("157-62-7935"));
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An example of Map
// iterate through all entries
Set entries = staff.entrySet();
Iterator iter = entries.iterator();
while (iter.hasNext())
{ Map.Entry entry = (Map.Entry)iter.next();
Object key = entry.getKey();
Object value = entry.getValue();
System.out.println("key=" + key + ", value=" + value);
}
}
}
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Algorithms/Utility methods in
Collections class
• Common set of methods that work on all types
of Collections and Lists
• min, max – can work on any Collection
• sort, binarySearch, shuffe, reverse, copy, fill –
can work on any List
• unmodifiable and synchronized versions of
Collections
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Algorithms/Utility methods in
Collections class
• Common set of methods that work on all types
of Collections and Lists
• min, max – can work on any Collection
• sort, binarySearch, shuffe, reverse, copy, fill –
can work on any List
• unmodifiable and synchronized versions of
Collections
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Legacy Collections
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Vector
Hashtable
Enumeration
Properties
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TradeProcessor Examples
TradeProcessor1 through TradeProcessor11
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TradeProcessor3
Using collections of Custom classes – ArrayList<Trade>
Your custom class must implement “Comparable”
if you want to use Collections.sort method to sort
a collection of your class objects
Example: TradeProcessor4
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TradeProcessor5
Using custom Comparator to sort
Your custom class must implement “Comparable”
if you want to use Collections.sort method to sort
a collection of your class objects
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TradeProcessor6
Removing Duplicates using HashSet
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TradeProcessor7
Removing Duplicates using HashSet
Your custom class must implement hashCode and
equals methods in order to be added to HashSet
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HashSet uses hashCode and equals
TradeProcessor
HashSet
add(t)
Trade
t.hashCode()
Hash value x
If (bucket x not empty)
If any of the calls to
equals returns true, then
duplicate is not added
and “false” is returned
else item is added and
“true is returned
for each s in bucket x
s.equals(t)
True/false
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hashCode and equals
• If two objects are equal, they MUST have matching
hashcode
• If two objects are equal, calling equals() on either object
MUST return true (if a.equals(b), then b.equals(a) must be
true)
• If two objects have the same hashcode value, they are NOT
required to be equal. But if they are equal, they MUST
have the same hashcode value.
• So, if you override equals, you MUST override hashCode
• The default behavior of hashCode is to generate a unique
integer based on its memory address
• The default behavior of equals is to do an = = comparison.
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Generics
• Create type-safe Collections
• More problems may be caught at compile
time rather than at run time
• For example – without generics any
collection can keep anything of type
“Object” and when you get it back from the
collection you have to “cast” it to your type.
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Using Generics
new ArrayList<Trade>
List<Trade> tradeList = new
ArrayList<Trade>
void cancelTrades(List<Trade>)
tp.cancelTrades(tradeList);
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Writing Generics
pulic class ArrayList<E> extends
AbstractList<E> implements List<E> …
public boolean add(E o)
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Type Parameter
ArrayList<Trade> tradeList = new
ArrayList<Trade>
pulic class ArrayList<E> extends
AbstractList<E> implements List<E> … {
public boolean add(E o)
pulic class ArrayList<Trade> extends
AbstractList<Trade> … {
public boolean add(Trade o)
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Type Parameter
pulic class ArrayList<E> extends
AbstractList<E> implements List<E> … {
public boolean add(E o)
pulic class TradeProcessor {
public <T extends Trade>
cancelTrades(ArrayList<T> trades);
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Type Parameter
pulic class TradeProcessor {
public <T extends Trade>
cancelTrades(ArrayList<T> trades);
pulic class TradeProcessor {
public cancelTrades(ArrayList<Trade>
trades);
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Arrays vs Generics
TestArrays.java
TestArrays2.java
TestGenerics1.java
TestGenerics2.java
TestGenerics3.java
TestGenerics4.java
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Type Checking in Generics
• Array types are checked again at runtime but collection
type checks are done only when you compile.
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