Collections

Collection Framework:

what are Collections.?

-It is a framework which provides the architecture to store and manipulate the group of objects.

- The collection is a container in which group of elements are combined together to form a single       unit.

Collection in Java classified into 3 categories:

1. Interface: It is an abstract data-type(ADT) which represents the collection.
2. Implementation: It is a concrete implementation of the interface.
3. Algorithms: It contains methods which perform useful computations like searching and sorting.

The following ways collections are classified:

1. Set: It is special kind of interface which does not allow duplicates.
2. List: It allows duplicates and maintains the order
3. Map: It is key, value pair.

SET in Detail : (java.util )

• It is special kind of interface which does not allow duplicates.
• Set is an interface which extends Collection. 
• It is an unordered collection of objects in which duplicate values cannot be stored.

Syntax: Set <Object> someName= new HashSet<Object>();

Example: Set <String> s= new HashSet<String>();

METHODS: 

boolean add(Object o)--->It is used to add the specified element to this set if it is not already present.

Eg: s.add("Nihal");

       s.add("Kurre"); 

boolean remove(Object o) ---> It is used to remove the specified element from this set if it is present.

Eg: s.remove("Nihal");

void clear()---> It is used to remove all of the elements from this set.

Eg: s.clear();

int size()---> It is used to return the number of elements in this set.

Eg: s.size();

boolean contains(Object o)---> It is used to return true if this set contains the specified element.

Eg: s.contains("Nihal");

boolean isEmpty()---> It is used to return true if this set contains no elements.

Eg: s.isEmpty();

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.

HashSet:  Does not sort.can contain null values.

LinkedHashSet:  Takes the insertion order. It can have Null values

TreeSet: It is sorted and null value depends

ConcurrentSkip List set: It is sorted and weakly consistent. It does not have null values.

List In Detail : 

• It is special kind of interface which does not allow duplicates.
• Set is an interface which extends Collection. 
• It is an unordered collection of objects in which duplicate values cannot be stored.

Syntax: List<Object> someName= new HashTable<Object>();

Example: List<String> l= new HashTable<String>();

METHODS: 

void add(int index,Object element)--->It is used to insert an element into the invoking list at the index passed in the index.

Eg: l.add("Nihal");

boolean addAll(int index,Collection c)--->It is used to insert all elements of c into the invoking list at the index passed in the index.

object get(int index)--->It is used to return the object stored at the specified index within the invoking collection.

object set(int index,Object element)---> It is used to assign an element to the location specified by index within the invoking list.

object remove(int index)---> It is used to remove the element at position index from the invoking list and return the deleted element.

ListIterator listIterator()---> It is used to return an iterator to the start of the invoking list.

ListIterator listIterator(int index)---> It is used to return an iterator to the invoking list that begins at the specified index

.

Map In Detail: 

• The java.util.Map interface represents a mapping between a key and a value.
• A Map cannot contain duplicate keys and each key can map to at most one value.
•  Some implementations allow null key and the null value (HashMap and LinkedHashMap) but some do not (TreeMap).

Why Map: 

A map of zip codes and cities.

A map of managers and employees. Each manager (key) is associated with a list of deatils of employees (value) he manages.

A map of classes and students. Each class (key) is associated with a list of students values.

Best: Map is best for searching.

It implements a serializable and clonable interface.

Syntax: Map<Object, Object>someName= new HashMap<Object, Object>();

Eg: Map<Integer, Integer>map= new HashMap<Integer, Integer>();

METHODS: 

Object put(Object key, Object value)--->It is used to insert an entry in this map.

void putAll(Map map)---> It is used to insert the specified map in this map.

Object remove(Object key)---> It is used to delete an entry for the specified key.

Object get(Object key)---> It is used to return the value for the specified key.

boolean containsKey(Object key)---> It is used to search the specified key from this map.

Set keySet()---> It is used to return the Set view containing all the keys.

Set entrySet()---> It is used to return the Set view containing all the keys and values.