Core Java


Question: What are the principle concepts of OOPS?

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

  1.  Abstraction
  2.  Polymorphism
  3.  Inheritance
  4. Encapsulation

i.e. easily remembered as A-PIE


Question:  What is Abstraction?

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


Question: What is Encapsulation?

Answer: 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.


Question: What is the difference between abstraction and encapsulation?


  1. 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.
  2. Abstraction solves the problem in the design side while Encapsulation is the Implementation.
  3. Encapsulation is the deliverables of Abstraction. Encapsulation barely talks about grouping up your abstraction to suit the developer needs.


Question: What is Inheritance?

Answer: 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:

    1. To promote code reuse
    2. To use polymorphism


Question: What is Polymorphism?

Answer: 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.


Question: How does Java implement polymorphism?

Answer: Inheritance, Overloading and Overriding are used to achieve Polymorphism in java.

Polymorphism establishes 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).


Question: Explain the different forms of Polymorphism?

Answer: 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.

According to practical programming viewpoint, polymorphism manifests itself in three distinct forms in Java:

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


Question: What is runtime Polymorphism or dynamic method dispatch?

Answer: 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.


Question: What is Dynamic Binding?

Answer: Binding states 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.


Question: What is Method Overloading?

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


  • 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.


Question: What is Method Overriding?


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


  • 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).
  • We cannot override a method marked Final
  • We cannot override a method marked static

Question: 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 The overloaded version is selected. Happens at compile time. Object type determines which method is selected. Happens at runtime


Question: Can overloaded methods be overridden too?

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


Question: Is it possible to override the main method?

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


Question: How to invoke a superclass version of an Overridden method?

Answer: To invoke a superclass method that has been overridden in a subclass, we 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



Question: What is super?

Answer:  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.


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


Question: How do you prevent a method from being overridden?

Answer: 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



Question: What is an Interface?

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


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


Question:  Can we instantiate an interface?

Answer: No, We can’t instantiate an interface directly, but we can instantiate a class that implements an interface.


Question: Can we create an object for an interface?

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


Question: Do interfaces have member variables?

Answer: 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.


Question: What modifiers are allowed for methods in an Interface?

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


Question:  What is a marker interface?

Answer: Marker interfaces are those which do not declare any required methods, but signify their compatibility with certain operations. The 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.


Question: What is an abstract class?

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


  • 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.
  • We can’t mark a class as both abstract and final.


Question: Can we instantiate an abstract class?

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


Question: 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 the class then we have the option of providing the 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 an implementation of 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 the corresponding method in the actual class.


Question: When should I use abstract classes and when should I use interfaces? Use Interfaces when…


  • When something in design will change frequently.
  • In various implementations only share method signatures then it is better to use Interfaces.
  • We need some classes to use some methods which we don’t want to be included in the class, then we 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…

  • In various implementations are of the same kind and use common behavior or status then abstract class is better to use.
  • When we 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 non-leaf classes in class hierarchies.


 Question: When we declare a method as abstract, can other non-abstract methods access it?

Answer: Yes, other non-abstract methods can access a method that we declare as abstract.


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

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


Question: What is Constructor?


  • A constructor is a special method whose task is to initialize the object of its class.
  • t 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.
  • The constructor is invoked whenever an object of its associated class is created.


Question: How does the Java default constructor be provided?

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


Question: Can a constructor be inherited?

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


Question: What are the differences between Contractors and Methods?


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


Question: 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.


Question: What are the differences between Class Methods and Instance Methods?


Class Methods Instance Methods
 Class methods are methods which are declared as static. 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.
The method can be called without creating an instance of the class
Class methods can only operate on class members 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.

Question: How are this() and super() used with construtors?


  • 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.


Question: What are Access, Specifiers?

Answer: 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.


Question: What are Access Specifiers available in Java?

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

  • Publicpublic classes, methods, and fields can be accessed from everywhere.
  • Protectedprotected 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 we 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.
  • PrivatePrivate 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 the same package? yes yes yes no
Accessible to class from different package? yes no, unless it is a subclass no no


Question: What is final modifier?

Answer: 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.


Question: What are the uses of final method?

Answer: 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.


Question: What is static block?

Answer: 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.


Question: What are static variables?

Answer: 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.

Question: What is the difference between static and non-static variables?

Answer: 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.

Question: What are static methods?

Answer: 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.

Question: 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.
  • An iterator is an Interface implemented a different way for every Collection.


Question: How do we traverse through a collection using its Iterator?

Answer: 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()
  • 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().


Question: How do we remove elements during Iteration?

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


Question: 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.

Question: How is ListIterator?

Answer: 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







Leave a Reply

Your email address will not be published. Required fields are marked *