Collections in Java

 

HashMap:

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hashCode() method is used to get the hash code of the object.

In HashMap, hashCode() is used to calculate the bucket.

Bucket - used to store nodes. 2 or more nodes can have same bucket. Those nodes are linked via linked list.

index of bucket = hashCode(key) & (n-1)

n=size of bucket array

HashMap hm = new HashMap();

hm.put("a",1);

First, hashCode of key 'a' is calculated and then the index of bucket array.

Second, the details of element is stored as a linked list.

{

  int hash = 115

  Key key = {"saurabh"}

  Integer value = 30

  Node next = null

}

Third, in case of same index the new element is saved as an linked list with address saved on next variable of first element.

Collection:

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1. Iterable Interface: Root interface of all collection class.

it contains only one abstract method i.e., Iterator<T> iterator()

2. Collection Interface : implemented by all class in collection framework.

Declares the method that every collection will have. eg., Boolean add(Object obj), boolean addAll(Collection c), void clear()

3. List Interface : Ordered collection of object. duplicacy allowed

List <data-type> list1= new ArrayList();  

List <data-type> list2 = new LinkedList();  

List <data-type> list3 = new Vector();  

List <data-type> list4 = new Stack();  

list1.add("a")

Iterator i = list1.iterator();

while(i.hasNext()){

sop(i.next());

}

Stack is a subclass of Vector. It implements LIFO.

few methods: Boolean push(), Boolean peek() - it returns the first element ie., head of stack ---- In stack the head element is the last element.

4. Queue: its a interface which maintains the first in first out order. It is an ordered list. classes which implements Queue are PriorityQueue, Deque, Arraydeque.

5. Set: unordered set of elements with no duplicacy. can store 1 null. 

Set<data-type> s1 = new HashSet<data-type>();

-non synchronized means multiple thread cant use the same object efficiently

-doesnt maintain insertion order

-Best for searching operation

Set<data-type> s2 = new LinkedHashSet<data-type>();  

-non synchronized

-Maintains insertion order

Set<data-type> s3 = new TreeSet<data-type>();  

-Access and retrieval is fast

-not allow null

-non synchronized

-ascending order i.e., sorted order

6. Map: Interface

HashMap - it doesn't maintain any order.

LinkedHashMap - It maintains insertion order.

TreeMap - It maintains ascending order.

7. Collections.sort(al,Collections.reverseOrder());