Java Collection
Framework
Collection Framework provides
an architecture to store and manipulate the group of objects.
All the operations that you perform on a data such as searching,
sorting, insertion, deletion etc. can be performed by Java Collection Framework.
Collection simply means a single unit of objects.
What is Collection?
Collection represents a single unit of objects i.e. a group.
What is framework?
- provides
readymade architecture.
- represents
set of classes and interface.
- is
optional.
Hierarchy of Collection Framework
Let us see the hierarchy of collection framework.The java.util package
contains all the classes and interfaces for Collection framework.
Methods of Collection interface
There are many methods declared in the Collection interface. They
are as follows:
|
No.
|
Method
|
Description
|
|
1
|
public boolean add(Object element)
|
is used to insert an element in this collection.
|
|
2
|
public boolean addAll(collection c)
|
is used to insert the specified collection elements in the
invoking collection.
|
|
3
|
public boolean remove(Object element)
|
is used to delete an element from this collection.
|
|
4
|
public boolean removeAll(Collection c)
|
is used to delete all the elements of specified collection from
the invoking collection.
|
|
5
|
public boolean retainAll(Collection c)
|
is used to delete all the elements of invoking collection except
the specified collection.
|
|
6
|
public int size()
|
return the total number of elements in the collection.
|
|
7
|
public void clear()
|
removes the total no of element from the collection.
|
|
8
|
public boolean contains(object element)
|
is used to search an element.
|
|
9
|
public boolean containsAll(Collection c)
|
is used to search the specified collection in this collection.
|
|
10
|
public Iterator iterator()
|
returns an iterator.
|
|
11
|
public Object[] toArray()
|
converts collection into array.
|
|
12
|
public boolean isEmpty()
|
checks if collection is empty.
|
|
13
|
public boolean equals(Object element)
|
matches two collection.
|
|
14
|
public int hashCode()
|
returns the hashcode number for collection.
|
Iterator interface
|
Iterator interface provides the facility of iterating the
elements in forward direction only.
|
Methods of Iterator interface
|
There are only three methods in the Iterator interface. They
are:
|
|
1. public boolean hasNext() it
returns true if iterator has more elements.
2. public object next() it
returns the element and moves the cursor pointer to the next element.
3. public void remove() it
removes the last elements returned by the iterator. It is rarely used.
|
ArrayList class:
|
·
uses a dynamic array for
storing the elements.It extends AbstractList class and implements List
interface.
·
can contain duplicate
elements.
·
maintains insertion order.
·
random access because array
works at the index basis.
·
manipulation slow because a
lot of shifting needs to be occurred.
|
Example of
ArrayList:
import java.util.*;
class Simple{
public static void main(String args[]){
ArrayList al=new ArrayList();
al.add("Ravi");
al.add("Vijay");
al.add("Ravi");
al.add("Ajay");
Iterator itr=al.iterator();
while(itr.hasNext()){
System.out.println(itr.next());
}
}
}
Output:Ravi
Vijay
Ravi
Ajay
List Interface:
List Interface is the subinterface of Collection.It contains
methods to insert and delete elements in index basis.It is a factory of
ListIterator interface.
Difference between List and Set:
List can contain duplicate
elements whereas Set contains unique elements only.
HashSet
class:
- uses
hashtable to store the elements.It extends AbstractSet class and
implements Set interface.
- contains
unique elements only.
Example of
HashSet class:
import java.util.*;
class Simple{
public static void main(String args[]){
HashSet al=new HashSet();
al.add("Ravi");
al.add("Vijay");
al.add("Ravi");
al.add("Ajay");
Iterator itr=al.iterator();
while(itr.hasNext()){
System.out.println(itr.next());
}
}
}
Output:Ajay
Vijay
Ravi
Sorting
We can sort the elements of:
- String
objects
- Wrapper
class objects
|
Collections class provides
static methods for sorting the elements of collection.If collection elements
are of Set type, we can use TreeSet.But We cannot sort the elements of
List.Collections class provides methods for sorting the elements of List type
elements.
|
Method of Collections class for sorting List
elements
|
public void sort(List list): is
used to sort the elements of List.List elements must be of Comparable type.
|
Note: String class and Wrapper classes implements the
Comparable interface.So if you store the objects of string or wrapper classes,
it will be Comparable.
Example of Sorting the elements of List that
contains string objects
import java.util.*;
class Simple12{
public static void main(String args[]){
ArrayList al=new ArrayList();
al.add("Viru");
al.add("Saurav");
al.add("Mukesh");
al.add("Tahir");
Collections.sort(al);
Iterator itr=al.iterator();
while(itr.hasNext()){
System.out.println(itr.next());
}
}
}
Output:Mukesh
Saurav
Tahir
Viru
Example of Sorting the elements of List that
contains Wrapper class objects
1.
import java.util.*;
2.
class Simple12{
3.
public static void main(String args[]){
4.
5.
ArrayList al=new ArrayList();
6.
al.add(Integer.valueOf(201));
7.
al.add(Integer.valueOf(101));
8.
al.add(230);//internally will be converted into objects as Integer.valueOf(230)
9.
10.
Collections.sort(al);
11.
12.
Iterator itr=al.iterator();
13.
while(itr.hasNext()){
14.
System.out.println(itr.next());
15.
}
16.
}
17. }
Output:101
201
230
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