Core Java OOPS Interview Questions and Answers

1.What are the principle concepts of OOPS?

There are four principle concepts upon which object oriented design and programming rest. They are:

• Abstraction
• Polymorphism
• Inheritance
• Encapsulation
(i.e. easily remembered as A-PIE).

2.What is Abstraction?

Abstraction refers to the act of representing essential features without including the background details or explanations.

3.What is Encapsulation?

Encapsulation is a technique used for hiding the properties and behaviors of an object and allowing outside access only as appropriate. It prevents other objects from directly altering or accessing the properties or methods of the encapsulated object.

4.What is the difference between abstraction and encapsulation?

• Abstraction focuses on the outside view of an object (i.e. the interface) Encapsulation (information hiding) prevents clients from seeing it’s inside view, where the behavior of the abstraction is implemented.
• Abstraction solves the problem in the design side while Encapsulation is the Implementation.
• Encapsulation is the deliverables of Abstraction. Encapsulation barely talks about grouping up your abstraction to suit the developer needs.

5.What is Inheritance?

• Inheritance is the process by which objects of one class acquire the properties of objects of another class.
• A class that is inherited is called a superclass.
• The class that does the inheriting is called a subclass.
• Inheritance is done by using the keyword extends.
• The two most common reasons to use inheritance are:
  • To promote code reuse
  • To use polymorphism

6.What is Polymorphism?

Polymorphism is briefly described as "one interface, many implementations." Polymorphism is a characteristic of being able to assign a different meaning or usage to something in different contexts - specifically, to allow an entity such as a variable, a function, or an object to have more than one form.

7.How does Java implement polymorphism?

(Inheritance, Overloading and Overriding are used to achieve Polymorphism in java).
Polymorphism manifests itself in Java in the form of multiple methods having the same name.

• In some cases, multiple methods have the same name, but different formal argument lists (overloaded methods).
• In other cases, multiple methods have the same name, same return type, and same formal argument list (overridden methods).

8.Explain the different forms of Polymorphism.

There are two types of polymorphism one is Compile time polymorphism and the other is run time polymorphism. Compile time polymorphism is method overloading. Runtime time polymorphism is done using inheritance and interface.
Note: From a practical programming viewpoint, polymorphism manifests itself in three distinct forms in Java:

• Method overloading
• Method overriding through inheritance
• Method overriding through the Java interface

9.What is runtime polymorphism or dynamic method dispatch?

In Java, runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass. The determination of the method to be called is based on the object being referred to by the reference variable.

10.What is Dynamic Binding?

Binding refers to the linking of a procedure call to the code to be executed in response to the call. Dynamic binding (also known as late binding) means that the code associated with a given procedure call is not known until the time of the call at run-time. It is associated with polymorphism and inheritance.

11.What is method overloading?

Method Overloading means to have two or more methods with same name in the same class with different arguments. The benefit of method overloading is that it allows you to implement methods that support the same semantic operation but differ by argument number or type.
Note:

• Overloaded methods MUST change the argument list
• Overloaded methods CAN change the return type
• Overloaded methods CAN change the access modifier
• Overloaded methods CAN declare new or broader checked exceptions
• A method can be overloaded in the same class or in a subclass

12.What is method overriding?

Method overriding occurs when sub class declares a method that has the same type arguments as a method declared by one of its superclass. The key benefit of overriding is the ability to define behavior that’s specific to a particular subclass type.
Note:

• The overriding method cannot have a more restrictive access modifier than the method being overridden (Ex: You can’t override a method marked public and make it protected).
• You cannot override a method marked final
• You cannot override a method marked static

13.What are the differences between method overloading and method overriding?

	Overloaded Method	Overridden Method
Arguments	Must change	Must not change
Return type	Can change	Can’t change except for covariant returns
Exceptions	Can change	Can reduce or eliminate. Must not throw new or broader checked exceptions
Access	Can change	Must not make more restrictive (can be less restrictive)
Invocation	Reference type determines which overloaded version is selected. Happens at compile time.	Object type determines which method is selected. Happens at runtime.

14.Can overloaded methods be override too?

Yes, derived classes still can override the overloaded methods. Polymorphism can still happen. Compiler will not binding the method calls since it is overloaded, because it might be overridden now or in the future.

15.Is it possible to override the main method?

NO, because main is a static method. A static method can't be overridden in Java.

16.How to invoke a superclass version of an Overridden method?

To invoke a superclass method that has been overridden in a subclass, you must either call the method directly through a superclass instance, or use the super prefix in the subclass itself. From the point of the view of the subclass, the super prefix provides an explicit reference to the superclass' implementation of the method.

  // From subclass
  super.overriddenMethod();

17.What is super?

super is a keyword which is used to access the method or member variables from the superclass. If a method hides one of the member variables in its superclass, the method can refer to the hidden variable through the use of the super keyword. In the same way, if a method overrides one of the methods in its superclass, the method can invoke the overridden method through the use of the super keyword.
Note:

• You can only go back one level.
• In the constructor, if you use super(), it must be the very first code, and you cannot access any this.xxx variables or methods to compute its parameters.

18.How do you prevent a method from being overridden?

To prevent a specific method from being overridden in a subclass, use the final modifier on the method declaration, which means "this is the final implementation of this method", the end of its inheritance hierarchy.

                        public final void exampleMethod() {
                          //  Method statements
                          }

19.What is an Interface?

An interface is a description of a set of methods that conforming implementing classes must have.
Note:

• You can’t mark an interface as final.
• Interface variables must be static.
• An Interface cannot extend anything but another interfaces.

20.Can we instantiate an interface?

You can’t instantiate an interface directly, but you can instantiate a class that implements an interface.

21.Can we create an object for an interface?

Yes, it is always necessary to create an object implementation for an interface. Interfaces cannot be instantiated in their own right, so you must write a class that implements the interface and fulfill all the methods defined in it.

22.Do interfaces have member variables?

Interfaces may have member variables, but these are implicitly public, static, and final- in other words, interfaces can declare only constants, not instance variables that are available to all implementations and may be used as key references for method arguments for example.

23.What modifiers are allowed for methods in an Interface?

Only public and abstract modifiers are allowed for methods in interfaces.

24.What is a marker interface?

Marker interfaces are those which do not declare any required methods, but signify their compatibility with certain operations. The java.io.Serializable interface and Cloneable are typical marker interfaces. These do not contain any methods, but classes must implement this interface in order to be serialized and de-serialized.

25.What is an abstract class?

Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation.
Note:

• If even a single method is abstract, the whole class must be declared abstract.
• Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.
• You can’t mark a class as both abstract and final.

26.Can we instantiate an abstract class?

An abstract class can never be instantiated. Its sole purpose is to be extended (subclassed).

27.What are the differences between Interface and Abstract class?

Abstract Class	Interfaces
An abstract class can provide complete, default code and/or just the details that have to be overridden.	An interface cannot provide any code at all,just the signature.
In case of abstract class, a class may extend only one abstract class.	A Class may implement several interfaces.
An abstract class can have non-abstract methods.	All methods of an Interface are abstract.
An abstract class can have instance variables.	An Interface cannot have instance variables.
An abstract class can have any visibility: public, private, protected.	An Interface visibility must be public (or) none.
If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly.	If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method.
An abstract class can contain constructors .	An Interface cannot contain constructors .
Abstract classes are fast.	Interfaces are slow as it requires extra indirection to find corresponding method in the actual class.

28.When should I use abstract classes and when should I use interfaces?

Use Interfaces when…

• You see that something in your design will change frequently.
• If various implementations only share method signatures then it is better to use Interfaces.
  
• you need some classes to use some methods which you don't want to be included in the class, then you go for the interface, which makes it easy to just implement and make use of the methods defined in the interface.

Use Abstract Class when…

• If various implementations are of the same kind and use common behavior or status then abstract class is better to use.
• When you want to provide a generalized form of abstraction and leave the implementation task with the inheriting subclass.
  
• Abstract classes are an excellent way to create planned inheritance hierarchies. They're also a good choice for nonleaf classes in class hierarchies.

29.When you declare a method as abstract, can other nonabstract methods access it?

Yes, other nonabstract methods can access a method that you declare as abstract.

30.Can there be an abstract class with no abstract methods in it?

Yes, there can be an abstract class without abstract methods.

  31.What is Constructor?

• A constructor is a special method whose task is to initialize the object of its class.
• It is special because its name is the same as the class name.
• They do not have return types, not even void and therefore they cannot return values.
• They cannot be inherited, though a derived class can call the base class constructor.
• Constructor is invoked whenever an object of its associated class is created.

32.How does the Java default constructor be provided?

If a class defined by the code does not have any constructor, compiler will automatically provide one no-parameter-constructor (default-constructor) for the class in the byte code. The access modifier (public/private/etc.) of the default constructor is the same as the class itself.

33.Can constructor be inherited?

No, constructor cannot be inherited, though a derived class can call the base class constructor.

34.What are the differences between Contructors and Methods?

	Constructors	Methods
Purpose	Create an instance of a class	Group Java statements
Modifiers	Cannot be abstract, final, native, static, or synchronized	Can be abstract, final, native, static, or synchronized
Return Type	No return type, not even void	void or a valid return type
Name	Same name as the class (first letter is capitalized by convention) -- usually a noun	Any name except the class. Method names begin with a lowercase letter by convention -- usually the name of an action
this	Refers to another constructor in the same class. If used, it must be the first line of the constructor	Refers to an instance of the owning class. Cannot be used by static methods.
super	Calls the constructor of the parent class. If used, must be the first line of the constructor	Calls an overridden method in the parent class
Inheritance	Constructors are not inherited	Methods are inherited

35.How are this() and super() used with constructors?

• Constructors use this to refer to another constructor in the same class with a different parameter list.
• Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.

36.What are the differences between Class Methods and Instance Methods?

Class Methods	Instance Methods
Class methods are methods which are declared as static. The method can be called without creating an instance of the class	Instance methods on the other hand require an instance of the class to exist before they can be called, so an instance of a class needs to be created by using the new keyword. Instance methods operate on specific instances of classes.
Class methods can only operate on class members and not on instance members as class methods are unaware of instance members.	Instance methods of the class can also not be called from within a class method unless they are being called on an instance of that class.
Class methods are methods which are declared as static. The method can be called without creating an  instance of the class.	Instance methods are not declared as static.

37.How are this() and super() used with constructors?

• Constructors use this to refer to another constructor in the same class with a different parameter list.
• Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.

38.What are Access Specifiers?

One of the techniques in object-oriented programming is encapsulation. It concerns the hiding of data in a class and making this class available only through methods. Java allows you to control access to classes, methods, and fields via so-called access specifiers..

39.What are Access Specifiers available in Java?

Java offers four access specifiers, listed below in decreasing accessibility:

• Public- public classes, methods, and fields can be accessed from everywhere.
• Protected- protected methods and fields can only be accessed within the same class to which the methods and fields belong, within its subclasses, and within classes of the same package.
• Default(no specifier)- If you do not set access to specific level, then such a class, method, or field will be accessible from inside the same package to which the class, method, or field belongs, but not from outside this package.
• Private- private methods and fields can only be accessed within the same class to which the methods and fields belong. private methods and fields are not visible within subclasses and are not inherited by subclasses.

Situation	public	protected	default	private
Accessible to class  from same package?	yes	yes	yes	no
Accessible to class  from different package?	yes	no, unless it is a subclass	no	no

40.What is final modifier?

The final modifier keyword makes that the programmer cannot change the value anymore. The actual meaning depends on whether it is applied to a class, a variable, or a method.

• final Classes- A final class cannot have subclasses.
• final Variables- A final variable cannot be changed once it is initialized.
• final Methods- A final method cannot be overridden by subclasses.

41.What are the uses of final method?

There are two reasons for marking a method as final:

• Disallowing subclasses to change the meaning of the method.
• Increasing efficiency by allowing the compiler to turn calls to the method into inline Java code.

42.What is static block?

Static block which exactly executed exactly once when the class is first loaded into JVM. Before going to the main method the static block will execute.

43.What are static variables?

Variables that have only one copy per class are known as static variables. They are not attached to a particular instance of a class but rather belong to a class as a whole. They are declared by using the static keyword as a modifier.

   static type  varIdentifier;

where, the name of the variable is varIdentifier and its data type is specified by type.
Note: Static variables that are not explicitly initialized in the code are automatically initialized with a default value. The default value depends on the data type of the variables.

44.What is the difference between static and non-static variables?

A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

45.What are static methods?

Methods declared with the keyword static as modifier are called static methods or class methods. They are so called because they affect a class as a whole, not a particular instance of the class. Static methods are always invoked without reference to a particular instance of a class.
Note:The use of a static method suffers from the following restrictions:

• A static method can only call other static methods.
• A static method must only access static data.
• A static method cannot reference to the current object using keywords super or this.                           46.What is an Iterator ?
• The Iterator interface is used to step through the elements of a Collection.
• Iterators let you process each element of a Collection.
• Iterators are a generic way to go through all the elements of a Collection no matter how it is organized.
• Iterator is an Interface implemented a different way for every Collection.

47.How do you traverse through a collection using its Iterator?

To use an iterator to traverse through the contents of a collection, follow these steps:

• Obtain an iterator to the start of the collection by calling the collection’s iterator() method.
• Set up a loop that makes a call to hasNext(). Have the loop iterate as long as hasNext() returns true.
• Within the loop, obtain each element by calling next().

48.How do you remove elements during Iteration?

Iterator also has a method remove() when remove is called, the current element in the iteration is deleted.

49.What is the difference between Enumeration and Iterator?

Enumeration	Iterator
Enumeration doesn't have a remove() method	Iterator has a remove() method
Enumeration acts as Read-only interface, because it has the methods only to traverse and fetch the objects	Can be abstract, final, native, static, or synchronized

Note: So Enumeration is used whenever we want to make Collection objects as Read-only.

50.How is ListIterator?

ListIterator is just like Iterator, except it allows us to access the collection in either the forward or backward direction and lets us modify an element

51.What is the List interface?

• The List interface provides support for ordered collections of objects.
• Lists may contain duplicate elements.

52.What are the main implementations of the List interface ?

The main implementations of the List interface are as follows :

• ArrayList : Resizable-array implementation of the List interface. The best all-around implementation of the List interface.
• Vector : Synchronized resizable-array implementation of the List interface with additional "legacy methods."
• LinkedList : Doubly-linked list implementation of the List interface. May provide better performance than the ArrayList implementation if elements are frequently inserted or deleted within the list. Useful for queues and double-ended queues (deques).

53.What are the advantages of ArrayList over arrays ?

Some of the advantages ArrayList has over arrays are:

• It can grow dynamically
• It provides more powerful insertion and search mechanisms than arrays.

54.Difference between ArrayList and Vector ?

ArrayList	Vector
ArrayList is NOT synchronized by default.	Vector List is synchronized by default.
ArrayList can use only Iterator to access the elements.	Vector list can use Iterator and Enumeration Interface to access the elements.
The ArrayList increases its array size by 50 percent if it runs out of room.	A Vector defaults to doubling the size of its array if it runs out of room
ArrayList has no default size.	While vector has a default size of 10.

55.How to obtain Array from an ArrayList ?

Array can be obtained from an ArrayList using toArray() method on ArrayList.

 List arrayList = new ArrayList();
   arrayList.add(…

 Object  a[] = arrayList.toArray();

56.Why insertion and deletion in ArrayList is slow compared to LinkedList ?

• ArrayList internally uses and array to store the elements, when that array gets filled by inserting elements a new array of roughly 1.5 times the size of the original array is created and all the data of old array is copied to new array.
• During deletion, all elements present in the array after the deleted elements have to be moved one step back to fill the space created by deletion. In linked list data is stored in nodes that have reference to the previous node and the next node so adding element is simple as creating the node an updating the next pointer on the last node and the previous pointer on the new node. Deletion in linked list is fast because it involves only updating the next pointer in the node before the deleted node and updating the previous pointer in the node after the deleted node.

57.Why are Iterators returned by ArrayList called Fail Fast ?

Because, if list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

58.How do you decide when to use ArrayList and When to use LinkedList?

If you need to support random access, without inserting or removing elements from any place other than the end, then ArrayList offers the optimal collection. If, however, you need to frequently add and remove elements from the middle of the list and only access the list elements sequentially, then LinkedList offers the better implementation.

59.What is the Set interface ?

    The Set interface provides methods for accessing the elements of a finite mathematical set
    Sets do not allow duplicate elements
    Contains no methods other than those inherited from Collection
    It adds the restriction that duplicate elements are prohibited
    Two Set objects are equal if they contain the same elements

60.What are the main Implementations of the Set interface ?

The main implementations of the List interface are as follows:

    HashSet
    TreeSet
    LinkedHashSet
    EnumSet

61.What is a HashSet ?

    A HashSet is an unsorted, unordered Set.
    It uses the hashcode of the object being inserted (so the more efficient your hashcode() implementation the better access performance you’ll get).
    Use this class when you want a collection with no duplicates and you don’t care about order when you iterate through it.

62.What is a TreeSet ?

TreeSet is a Set implementation that keeps the elements in sorted order. The elements are sorted according to the natural order of elements or by the comparator provided at creation time.

63.What is an EnumSet ?

An EnumSet is a specialized set for use with enum types, all of the elements in the EnumSet type that is specified, explicitly or implicitly, when the set is created.

64.Difference between HashSet and TreeSet ?

HashSet     TreeSet
HashSet is under set interface i.e. it  does not guarantee for either sorted order or sequence order.     TreeSet is under set i.e. it provides elements in a sorted  order (acceding order).
We can add any type of elements to hash set.     We can add only similar types
of elements to tree set.


65.What is a Map ?

    A map is an object that stores associations between keys and values (key/value pairs).
    Given a key, you can find its value. Both keys  and  values are objects.
    The keys must be unique, but the values may be duplicated.
    Some maps can accept a null key and null values, others cannot.


66.What are the main Implementations of the Map interface ?

The main implementations of the List interface are as follows:

    HashMap
    HashTable
    TreeMap
    EnumMap


67.What is a TreeMap ?

TreeMap actually implements the SortedMap interface which extends the Map interface. In a TreeMap the data will be sorted in ascending order of keys according to the natural order for the key's class, or by the comparator provided at creation time. TreeMap is based on the Red-Black tree data structure.

68.How do you decide when to use HashMap and when to use TreeMap ?

For inserting, deleting, and locating elements in a Map, the HashMap offers the best alternative. If, however, you need to traverse the keys in a sorted order, then TreeMap is your better alternative. Depending upon the size of your collection, it may be faster to add elements to a HashMap, then convert the map to a TreeMap for sorted key traversal.

69.Difference between HashMap and Hashtable ?

HashMap     Hashtable
HashMap lets you have null values as well as one null key.     HashTable  does not allows null values as key and value.
The iterator in the HashMap is fail-safe (If you change the map while iterating, you’ll know).     The enumerator for the Hashtable is not fail-safe.
HashMap is unsynchronized.     Hashtable is synchronized.

Note: Only one NULL is allowed as a key in HashMap. HashMap does not allow multiple keys to be NULL. Nevertheless, it can have multiple NULL values.


70.How does a Hashtable internally maintain the key-value pairs?

TreeMap actually implements the SortedMap interface which extends the Map interface. In a TreeMap the data will be sorted in ascending order of keys according to the natural order for the key's class, or by the comparator provided at creation time. TreeMap is based on the Red-Black tree data structure.

71.What Are the different Collection Views That Maps Provide?

Maps Provide Three Collection Views.

    Key Set - allow a map's contents to be viewed as a set of keys.
    Values Collection - allow a map's contents to be viewed as a set of values.
    Entry Set - allow a map's contents to be viewed as a set of key-value mappings.


72.What is a KeySet View ?

KeySet is a set returned by the keySet() method of the Map Interface, It is a set that contains all the keys present in the Map.

73.What is a Values Collection View ?

Values Collection View is a collection returned by the values() method of the Map Interface, It contains all the objects present as values in the map.

74.What is an EntrySet View ?

Entry Set view is a set that is returned by the entrySet() method in the map and contains Objects of type Map. Entry each of which has both Key and Value.

75.How do you sort an ArrayList (or any list) of user-defined objects ?

Create an implementation of the java.lang.Comparable interface that knows how to order your objects and pass it to java.util.Collections.sort(List, Comparator).

76.What is the Comparable interface ?

The Comparable interface is used to sort collections and arrays of objects using the Collections.sort() and java.utils.Arrays.sort() methods respectively. The objects of the class implementing the Comparable interface can be ordered.

The Comparable interface in the generic form is written as follows:

    interface Comparable<T>

where T is the name of the type parameter.

All classes implementing the Comparable interface must implement the compareTo() method that has the return type as an integer. The signature of the compareTo() method is as follows:

      int i = object1.compareTo(object2)

    If object1 < object2: The value of i returned will be negative.
    If object1 > object2: The value of i returned will be positive.
    If object1 = object2: The value of i returned will be zero.


77.What are the differences between the Comparable and Comparator interfaces ?

Comparable     Comparato

It uses the compareTo() method.

int objectOne.compareTo(objectTwo).
    t uses the compare() method.

int compare(ObjOne, ObjTwo)
It is necessary to modify the class whose instance is going to be sorted.     A separate class can be created in order to sort the instances.
Only one sort sequence can be created.     Many sort sequences can be created.
It is frequently used by the API classes.     It used by third-party classes to sort instances.