An interface in Java acts similarly to an abstract class, which means that an object can never be produced an interface. As a result, although an interface lacks constructors, it does have methods.
In earlier versions of Java, like before Java 8, interface could specify the activities that its implementing class performs but not how the implementing class should perform them
Developers now have the option to choose from the traditionally present abstract method or the newer and concrete methods present within interfaces.
Before understanding interface, we need to take a look at polymorphism. Polymorphism is a key notion in object-oriented programming that allows a developer to use classes to generate generalised and specialised behaviour.
So, what is the relationship between polymorphism and a Java interface? An interface in Java allows classes that aren’t normally connected to have access to the same operations. A Java interface, in its most basic form, is a template that can be simply used by the various classes that implement it. These implementing classes can then convert an interface’s function from a generalist to a more specialised state in order to complete a specified purpose.
The most important benefit of using interfaces in Java 8 is the capability of making methods concrete. Another advantageous feature of Java 8 Interfaces is the ability to add new concrete methods to an already implemented interface without affecting the programme. Prior to Java 8, a class that implemented an interface but did not use all of its methods had to be tagged as abstract. The programme would crash if this were not the case.
When should be using interface?
The main function of interface is to make the process of writing the code easily. Same functionality can be achieved without the use of interface too, but their use will make the program organised better and also reduce the time of development.
If you are acquainted with inheritance, you will be knowing that it is too a concept of Object-Oriented Programming that facilitates generalisation. Inheritance groups comparable classes into parent-child relationships.
Classes that would otherwise not have access to the same methods (due to a lack of parent-child connections) may now be processed polymorphically thanks to polymorphism (which is the programming behaviour that an interface shows).
The accounting department of a software company is an excellent illustration of how to use an interface. When it comes to producing an employee’s payslip and a customer’s invoice, this department will most likely use the same techniques (or activities).