SDE - 1

You are given a number 'N'. Your task is to find Nth prime number.
A prime number is a number greater than 1 that is not a product of two smaller natural numbers. Prime numbers have only two factors – 1 and the number itself.
Any number greater than 0 is a natural number i.e. natural numbers N = {1, 2, 3, 4,....}

The differences between Abstract Class and Interface are as follows : 

Abstract Class:
1) Abstract classes have a default constructor and it is called whenever the concrete subclass is instantiated.

2) It contains Abstract methods as well as Non-Abstract methods.

3) The class which extends the Abstract class shouldn’t require the implementation of all the methods, only Abstract
methods need to be implemented in the concrete sub-class.

4) Abstract class contains instance variables.



Interface:
1 )It doesn’t have any constructor and couldn’t be instantiated.

2) The abstract method alone should be declared.

3) Classes that implement the interface should provide the implementation for all the methods.

4) The interface contains only constants.

According to ConcurrentHashMap Oracle docs,

The constructor of ConcurrentHashMap looks like this :

public ConcurrentHashMap (int initialCapacity, float loadFactor, int concurrencyLevel)

So the above line creates a new, empty map with the specified initial capacity, load factor and concurrency level.
where,
Important Parameters to consider from ConcurrentHashMap Constructor :

initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate this many elements.

concurrencyLevel - the estimated number of concurrently updating threads. The implementation performs internal
sizing to try to accommodate this many threads.

In the ConcurrentHashMap Api , you will find the following constants.

static final int DEFAULT_INITIAL_CAPACITY = 16;
static final int DEFAULT_CONCURRENCY_LEVEL = 16;

initial capacity parameter and concurrency level parameters of ConcurrentHashMap constructor (or Object) are set to
16 by default.

1) Thus, instead of a map wide lock, ConcurrentHashMap maintains a list of 16 locks by default ( number of locks equal
to the initial capacity , which is by default 16) each of which is used to lock on a single bucket of the Map.

2) This indicates that 16 threads (number of threads equal to the concurrency level , which is by default 16) can modify the collection at the same time , given ,each thread works on different bucket. 

3) So unlike hashtable, we perform any sort of operation ( update ,delete ,read ,create) without locking on entire map in ConcurrentHashMap.


Retrieval operations (including get) generally do not block, so may overlap with update operations (including put and
remove). Retrievals reflect the results of the most recently completed update operations holding upon their onset.


1) The allowed concurrency among update operations is guided by the optional concurrencyLevel constructor argument
(default 16), which is used as a hint for internal sizing. 

2) The table is internally partitioned to try to permit the indicated number of concurrent updates without contention. 

3) Because placement in hash tables is essentially random, the actual concurrency will vary. 

4) Ideally, you should choose a value to accommodate as many threads as will ever concurrently modify the table. 

5) Using a significantly higher value than you need can waste space and time, and a significantly lower value can lead to thread contention.

In Java, the final keyword is used as defining something as constant /final and represents the non-access modifier.

1) final variable :
i) When a variable is declared as final in Java, the value can’t be modified once it has been assigned.

ii) If any value has not been assigned to that variable, then it can be assigned only by the constructor of the class.


2) final method :
i) A method declared as final cannot be overridden by its children's classes.

ii) A constructor cannot be marked as final because whenever a class is inherited, the constructors are not inherited.
Hence, marking it final doesn't make sense. Java throws compilation error saying - modifier final not allowed here


3) final class :
i) No classes can be inherited from the class declared as final. But that final class can extend other classes for its
usage.

Design and implement a data structure for Least Recently Used (LRU) cache to support the following operations: