Java interview

  • OOP
  • Inheritance
  • Encapsulation
  • Polymorphism
  • Abstraction
  • Constructor
  • Object
  • Interface и class
  • Access modifiers such as private, default, protected and public
  • Abstract class and method
  • Exception and Error


OOPs in Java organizes a program around the various objects and well-defined interfaces. The OOPs Concepts in Java are abstraction, encapsulation, inheritance, and polymorphism


- it is possible to inherit attributes and methods from one class to another
- is a mechanism in which one object acquires all the properties and behaviors of a parent object
- Java does not support Multiple inheritance


- By encapsulating a class's variables, other classes cannot access them, and only the methods of the class can access them
- hiding the implementation of the class and separating its internal representation from the external class or interface


- is the ability to apply the same methods with the same or different sets of parameters in the same class or in a group of classes connected by an inheritance relation


- is the process of hiding implementation details from the user, giving the user only the functionality
- the user will own the information about what the object does, not how it does it


- All classes have constructors, whether you define them or not, because Java automatically provides a default constructor
- The constructor is a method that creates a kind of "framework" for the class. Every new object of the class must conform to it


- is an individual member of a class that has a particular state and behavior that is completely determined by the class
- Variables are used to store the state of the object
- The object is an instance of the class
- Any object can have two main characteristics: state - some data that the object stores, and behavior - actions that the object can perform


- A class is a template structure that allows you to describe an object, its properties (class attributes or fields) and behavior (class methods) in a program
- Class can consist methods, objects and constructors. Class is a template of an object, he has a body with fields and methods 


- An interface is a reference type, it is similar to a class. It is a collection of abstract methods. 
- A class implements an interface, thus inherits the abstract methods of the interface
- There is no implementation inside the interface
The interface only describes the behavior of some object
- Only immutable final fields can be in the interface

Abstract class and method

- Abstract class describes some general state and behavior that future classes - descendants of the abstract class - will have.
- Similar to interfaces, abstract classes can have abstract methods, an abstract method is a method without a body (without implementation), but unlike interfaces, abstract methods in abstract classes must be explicitly declared as abstract.

Access modifiers

- public, protected, default, and private.
- Protected - declarations are visible within the package or all subclasses
- Default - declarations are visible only within the package (package private)


- Keyword this is a link to the current class instance, we can use this word to call constructors, variables and object method’s 


- Static methods and variables are related to the class
- Static methods belong to a class, not an object. You can call them without creating an instance of the class
- From a static method you can access only static variables or methods