Sets

A Set is a Collection that cannot contain duplicate elements. It models the mathematical set abstraction. The Set interface contains only methods inherited from Collection and adds the restriction that duplicate elements are prohibited.

HashSets :

• Java HashSet class is used to create a collection that uses a hash table for storage.
• It inherits the AbstractSet class and implements Set interface.
• The important points about Java HashSet class are:
• HashSet stores the elements by using a mechanism called hashing.
• HashSet contains unique elements only.

Hierarchy of HashSet class : The HashSet class extends AbstractSet class which implements Set interface. The Set interface inherits Collection and Iterable interfaces in hierarchical order.

HashSet class declaration

public class HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable, Serializable

Constructors of Java HashSet class :

Constructor	Description
HashSet()	It is used to construct a default HashSet.
HashSet(Collection c)	It is used to initialize the hash set by using the elements of the collection c.
HashSet(int capacity)	It is used to initialize the capacity of the hash set to the given integer value capacity. The capacity grows automatically as elements are added to the HashSet.

Methods of Java HashSet class:

Method	Description
void clear()	It is used to remove all of the elements from this set.
boolean contains(Object o)	It is used to return true if this set contains the specified element.
boolean add(Object o)	It is used to adds the specified element to this set if it is not already present.
boolean isEmpty()	It is used to return true if this set contains no elements.
boolean remove(Object o)	It is used to remove the specified element from this set if it is present.
Object clone()	It is used to return a shallow copy of this HashSet instance: the elements themselves are not cloned.
Iterator iterator()	It is used to return an iterator over the elements in this set.
int size()	It is used to return the number of elements in this set.

Example :

import java.util.*;  
class TestCollection9{  
 public static void main(String args[]){  
  //Creating HashSet and adding elements  
  HashSet<String> set=new HashSet<String>();  
  set.add("Ravi");  
  set.add("Vijay");  
  set.add("Ravi");  
  set.add("Ajay");  
  //Traversing elements  
  Iterator<String> itr=set.iterator();  
  while(itr.hasNext()){  
   System.out.println(itr.next());  
  }  
 }  
}  

Output :

       Ajay
       Vijay
       Ravi

LinkedHashSet :

Java LinkedHashSet class is a Hash table and Linked list implementation of the set interface. It inherits HashSet class and implements Set interface.

The important points about Java LinkedHashSet class are:

• Contains unique elements only like HashSet.
• Provides all optional set operations, and permits null elements.
• Maintains insertion order.

Hierarchy of LinkedHashSet class

The LinkedHashSet class extends HashSet class which implements Set interface. The Set interface inherits Collection and Iterable interfaces in hierarchical order.

LinkedHashSet class declaration

public class LinkedHashSet<E> extends HashSet<E> implements Set<E>, Cloneable, Serializable

Constructors of Java LinkedHashSet class

Constructor	Description
HashSet()	It is used to construct a default HashSet.
HashSet(Collection c)	It is used to initialize the hash set by using the elements of the collection c.
LinkedHashSet(int capacity)	It is used initialize the capacity of the linkedhashset to the given integer value capacity.
LinkedHashSet(int capacity, float fillRatio)	It is used to initialize both the capacity and the fill ratio (also called load capacity) of the hash set from its argument.

Example :

import java.util.*;  
class TestCollection10{  
 public static void main(String args[]){  
  LinkedHashSet<String> al=new LinkedHashSet<String>();  
  al.add("Ravi");  
  al.add("Vijay");  
  al.add("Ravi");  
  al.add("Ajay");  
  Iterator<String> itr=al.iterator();  
  while(itr.hasNext()){  
   System.out.println(itr.next());  
  }  
 }  
}  

Output :

       Ravi
       Vijay
       Ajay

TreeSet :

Java TreeSet class implements the Set interface that uses a tree for storage. It inherits AbstractSet class and implements NavigableSet interface. The objects of TreeSet class are stored in ascending order.

The important points about Java TreeSet class are:

• Contains unique elements only like HashSet.
• Access and retrieval times are quiet fast.
• Maintains ascending order.
• Hierarchy of TreeSet class
• Java TreeSet class implements NavigableSet interface. The NavigableSet interface extends SortedSet, Set, Collection and Iterable interfaces in hierarchical order.

TreeSet class declaration :

public class TreeSet<E> extends AbstractSet<E> implements NavigableSet<E>, Cloneable, Serializable

Constructors of Java TreeSet class :

Constructor	Description
TreeSet()	It is used to construct an empty tree set that will be sorted in an ascending order according to the natural order of the tree set.
TreeSet(Collection c)	It is used to build a new tree set that contains the elements of the collection c.
TreeSet(Comparator comp)	It is used to construct an empty tree set that will be sorted according to given comparator.
TreeSet(SortedSet ss)	It is used to build a TreeSet that contains the elements of the given SortedSet.

Methods of Java TreeSet class :

Method	Description
boolean addAll(Collection c)	It is used to add all of the elements in the specified collection to this set.
boolean contains(Object o)	It is used to return true if this set contains the specified element.
boolean isEmpty()	It is used to return true if this set contains no elements.
boolean remove(Object o)	It is used to remove the specified element from this set if it is present.
void add(Object o)	It is used to add the specified element to this set if it is not already present.
void clear()	It is used to remove all of the elements from this set.
Object clone()	It is used to return a shallow copy of this TreeSet instance.
Object first()	It is used to return the first (lowest) element currently in this sorted set.
Object last()	It is used to return the last (highest) element currently in this sorted set.
int size()	It is used to return the number of elements in this set.

**Example : **

import java.util.*;  
class TestCollection11{  
 public static void main(String args[]){  
  //Creating and adding elements  
  TreeSet<String> al=new TreeSet<String>();  
  al.add("Ravi");  
  al.add("Vijay");  
  al.add("Ravi");  
  al.add("Ajay");  
  //Traversing elements  
  Iterator<String> itr=al.iterator();  
  while(itr.hasNext()){  
   System.out.println(itr.next());  
  }  
 }  
}  

Output :

Ajay
Ravi
Vijay