How to prepare to Java Interview


  • 4 principles OOP


    Encapsulation means that the internal representation of an object is generally hidden from view outside of the object’s definition. Typically, only the object’s own methods can directly inspect or manipulate its fields.


    Abstraction denotes a model, a view, or some other focused representation for an actual item. In short, data abstraction is nothing more than the implementation of an object that contains the same essential properties and actions we can find in the original object we are representing.


    Inheritance is a way to reuse code of existing objects, or to establish a subtype from an existing object, or both, depending upon programming language support. In classical inheritance where objects are defined by classes, classes can inherit attributes and behavior from pre-existing classes called base classes, superclasses, parent classes or ancestor classes. The resulting classes are known as derived classes, subclasses or child classes. The relationships of classes through inheritance gives rise to a hierarchy.

    Subclasses and Superclasses: A subclass is a modular, derivative class that inherits one or more properties from another class (called the superclass). The properties commonly include class data variables, properties, and methods or functions. The superclass establishes a common interface and foundational functionality, which specialized subclasses can inherit, modify, and supplement. The software inherited by a subclass is considered reused in the subclass. In some cases, a subclass may customize or redefine a method inherited from the superclass. A superclass method which can be redefined in this way is called a virtual method.


    Polymorphism means one name, many forms. Polymorphism manifests itself by having multiple methods all with the same name, but slightly different functionality.

    There are 2 basic types of polymorphism:

    Overridding, also called run-time polymorphism. For method overloading, the compiler determines which method will be executed, and this decision is made when the code gets compiled. Overloading, which is referred to as compile-time polymorphism. Method will be used for method overriding is determined at runtime based on the dynamic type of an object.

  • Object Oriented Programming vs. Functional Programming

    Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which are data structures that contain data, in the form of fields, often known as attributes; and code, in the form of procedures, often known as methods

    Functional programming is a programming paradigm, a style of building the structure and elements of computer programs, that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data

    Both OOP and FP have the shared goal of creating understandable, flexible programs that are free of bugs. But they have two different approaches for how to best create those programs.

    In all programs, there are two primary components: the data (the stuff a program knows) and the behaviors (the stuff a program can do to/with that data). OOP says that bringing together data and its associated behavior in a single location (called an “object”) makes it easier to understand how a program works. FP says that data and behavior are distinctively different things and should be kept separate for clarity.

  • What is the difference between association, aggregation and composition?

Java core

String & StringBuffer

equals and ==

checked and unchecked exceptions

interface and abstract class

    • Thread class and runnable interface, which one is better