1) OOPs Concept.

   Abstraction

         Hiding internal details and showing functionality is known as abstraction. For example phone call, we don't know the          internal processing.

         In Java, we use abstract class and interface to achieve abstraction.

   Polymorphism

         If one task is performed by different ways, it is known as polymorphism. For example: to convince the customer          differently, to draw something, for example, shape, triangle, rectangle, etc.

         In Java, we use method overloading and method overriding to achieve polymorphism.

   Inheritence

         When one object acquires all the properties and behaviors of a parent object, it is known as inheritance. It provides          code reusability. It is used to achieve runtime polymorphism.

   Encapsulation

         Binding (or wrapping) code and data together into a single unit are known as encapsulation. For example capsule, it is          wrapped with different medicines.

         A java class is the example of encapsulation. Java bean is the fully encapsulated class because all the data members          are private here.

2) Difference between Abstraction and Encapsulation

   Encapsulation is wrapping, just hiding properties and methods. Encapsulation is used for hide the code and data in    a single unit to protect the data from the outside the world. Class is the best example    of encapsulation. Abstraction refers to showing only the necessary details to the intended user.

3) Difference between HashMap and ConcurrentHashMap

• HashMap is non-Synchronized in nature i.e. HashMap is not Thread-safe whereas ConcurrentHashMap is Thread-safe in nature.
• HashMap performance is relatively high because it is non-synchronized in nature and any number of threads can perform simultaneously. But ConcurrentHashMap performance is low sometimes because sometimes Threads are required to wait on ConcurrentHashMap.
• While one thread is Iterating the HashMap object, if other thread try to add/modify the contents of Object then we will get Run-time exception saying ConcurrentModificationException. Whereas In ConcurrentHashMap we wont get any exception while performing any modification at the time of Iteration.

4) Difference between HashSet and HashMap

   Differences:

      HASHSET

         HashSet class implements the Set interface

         In HashSet we store objects(elements or values) e.g. If we have a HashSet of string elements then it could depict a set          of HashSet elements: {“Hello”, “Hi”, “Bye”, “Run”}

         HashSet does not allow duplicate elements that means you can not store duplicate values in HashSet.

         HashSet permits to have a single null value.

      HASHMAP

         HashMap class implements the Map interface

         HashMap is used for storing key & value pairs. In short it maintains the mapping of key & value (The HashMap class is           roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.) This is how you could represent          HashMap elements if it has integer key and value of String type: e.g. {1->”Hello”, 2->”Hi”, 3->”Bye”, 4->”Run”}

         HashMap does not allow duplicate keys however it allows to have duplicate values.

         HashMap permits single null key and any number of null values.

5)