Interfaces in Java

Interface in Java is a concept to separate contract (what) from implementation (how).

The keyword interface creates an entity which defines how a class is going to look like with method names, their list & type of arguments and their return type. Interface does not define any method body.

One or more classes can implement the interface to provide the actual method body for all the methods declared in the interface . Since interfaces do not contain any method body, Java does not allow creation of objects of interface. Objects can be created for class that implements the interface.


Similar to inheritance, when defining class for interface we specify name of the interface it is implementing. Here we use the keyword implements to define the class.

A class can implement multiple interfaces by separating Interface names using comma after the implements keyword. An object of such class can be upcasted to any of the interfaces. This is Java’s way of multiple inheritance which is directly not allowed with extends keyword.


Abstract Class

We have discussed about classes and interfaces, there is another entity in Java called Abstract Class which falls into same category but is midway between class and interface. Mid-way because we can provide definition of some methods (like classes) and leave some as abstract or as declaration  (like interfaces). Java does not allow instantiation of abstract class but allows it to be extended like any other class using extends keyword.


In this example, we have created a class TennisPlayer by inheriting from abstract class Person and implementing interface Player. TennisPlayer gets eat() capability from Person but is forced to define work() capability as mandated in same Person abstract class. Similarly, TennisPlayer is forced to implement play() capability of Player interface. Since playing tennis is the work for a tennis player, we reused play as work.

Benefits of using Interfaces & Abstract classes-

  • Re-usability: Using inheritance we can design in a way such that code is reused to its maximum. In above example – common activity of a Human like eat is defined in the abstract class.
  • Extensibility – Using interfaces we can design applications that are easily extensible in future. In above example – SoccerPlayer can be added easily by implementing Player and extending Person.

Word of caution: Using Interfaces is somewhat tricky because it makes sense to use them only when we have multiple implementations either currently or in future. If there is no scope of multiple implementation in future then its wiser to avoid interface and use a class directly.


OOP Fundamentals (3 of 3)

Polymorphism is the third essential feature of OOP.

Polymorphism means many forms – an object can behave in different forms and the correct behavior will be evaluated during run-time only.


Taking the concept of inheritance further,  an object of derived class can be used as its own type or as a type of its base class. Although this statement might appear as incorrect – why would Java allow one type to behave like another type, but if we think in a object oriented manner it will make sense.

Inheritance is making a base class (generic) into a more specific derived class. So, derived class object still posses all the generic behavior of base class. By doing upcasting we are loosing specifics of a derived class but retaining generics of base class.

Let’s take this example


Output :


Although this is the desired output, but looking at the method park() it doesn’t make sense. In the park() method we are invoking move() of Vehicle but actually move() of Car is invoked.

Dynamic Binding

Connecting a method call to method body is called binding. When this binding happens during compile time, its called static binding. When this binding happens during run time (based on type of object), its called dynamic binding or runtime binding.

In the above example, even when Car object is upcasted to Vehicle in park(), it still knows its type information and behave in polymorphic manner to produce desired output.

Downcasting is opposite of upcasting. Here we cast a generic object into more specific one. While Upcasting is always safe and happens implicitly, Downcasting requires explicit type casting operator and throws a runtime ClassCastException if the instance to be downcasted does not belong to the correct subclass.




OOP Fundamentals (2 of 3)

Object Oriented Programming Fundamentals

An important aspect of OOP is reuse of classes using composition and inheritance.

Composition is using objects of existing classes into a new class. It represents ‘has-a‘ relationship. e.g. car has-a engine.


Inheritance is creating new classes as type of existing classes. It represents ‘is-a‘ relationship. In Java code, after specifying the name of derived (new) class use the keyword extends followed by name of base (existing) class .

With inheritance derived class gets all the public & protected attributes and methods of base class. There are two more access specifier (other than private and public) –

  • Protected – Like private, protected  entity is accessible to only the Class which defines it. Unlike private, protected entity of base class is available in the derived class.
  • Package – This is the default access where entity is accessible within the package but is private out of package.


The derived class may add more attribute & methods to the base class. Internally derived class has a object of base class but externally it exposes an extended interface of base class. So,  derived class is a wrapper over base class with some more specific functionality.


Since base class is wrapped inside derived class, it becomes responsibility of derived class to do initialization of base class. Java helps here by calling constructor of base class from constructor of derived class, but this automatic help is available only in case of default constructors. When dealing with constructors having arguments, we need to explicitly invoke the base-class constructor using super keyword as first thing in the derived-class constructor. For example,

Class Furniture {

Furniture(int i) {

print(“Creating Furniture”);



Class Table extends Furniture {

Table(int i) {

super (i);

print(“Creating Furniture”);



OOP Fundamentals (1 of 3)

Object Oriented Programming Fundamentals

OOP is a paradigm to solve real-world business problems by dividing the problem into a set of objects communicating with each other.

An Object consists of attributes and actions to be performed on the attributes. In programming jargon, attributes are called data members and actions are called member functions. Member functions are also referred as methods in Java.

In Java, Class is a type of objects which have same attributes & methods. In other words, a class is definition of attributes and methods. Once we have a class definition or blueprint ready, we can realize the class to create Objects. This realization is called instantiation in programming jargon.

Although Classes are blueprint and Objects are the instances of class, but the term ‘Object‘ is generally used to represent either of them.

Here’s an example of Account class and two of its instances having id 1010 and 1011.

Account class

Encapsulation and Abstraction

A class encapsulates data members and member functions into a single reusable entity. Re-usability of classes a benefit of using OOP; a class once written & tested can be re-used in different scenarios to solve similar business problems.

Abstraction is hiding internal details while exposing only relevant & required information to outside world. In java, abstraction is achieved by use of access specifiers. Here we are introducing only two of four access specifiers in Java –

Public – The entity (class, data member, method) is accessible to everyone.

Private – The entity is accessible to only the Class which defines it.

Let’s take real world example of a Car – which is made-up of a lot of attributes like engine, steering, brake pedal, gear-box, tyres, brake shoe, axle, etc. Car exposes interface to the user (methods) like changing gear, applying brake, etc. without telling the user that there is a brake shoe. Someone driving a car does not need to know that it has brake shoe but they need to know how to apply brake.

The benefit to using abstraction is that Car manufacturer can actually switch a mechanical brake with a hydraulic brake without any impact on the way people use or apply brake.

A software world example can be a AlarmLogger class – whose job is to write alarms in a file using a specific format. This class exposes a public method logAlarm and hides internal file-handing and formatting from clients calling this class by keeping them as private. This same class can be reused in multiple Programs like ecommerce app, banking app.