Java Interfaces from Scratch: Core Concepts and Practical Use

In large software projects, many developers work on separate parts and need a common contract to collaborate; this contract is called an interface.

Just like the on/off buttons on a TV or water dispenser provide a uniform way for users to operate different devices, interfaces hide internal details and present a simple, consistent way to interact with components.

In Java, an interface is a reference type similar to a class. It cannot be instantiated because it has no constructors; it may contain nested types, abstract method signatures, and constants.

Defining an interface uses the interface keyword followed by the name and method signatures ending with semicolons, for example:

interface Animal {
    // constants can be declared here
    // method declarations
    public void eat();
    public void travel();
}

To use an interface, a class must implement it and provide concrete implementations for every method declared in the interface.

Interfaces also serve as APIs (Application Programming Interfaces). An API is a set of predefined functions that allow developers to use functionality without accessing source code or understanding internal mechanisms, similar to how a client uses a software package through its documented interface.

Java does not allow multiple class inheritance, but a class can implement multiple interfaces, offering a form of multiple inheritance. Consequently, a variable of an interface type can reference any instance of any class that implements that interface.