Interfaces and Packages

1. Interfaces in Java

An interface is a blueprint for a class that defines a set of abstract methods. Interfaces are implemented by classes using the implements keyword.

Creating and Implementing an Interface

    
    // Defining an interface
    interface Vehicle {
        void start(); // Abstract method (no body)
        void stop();  // Abstract method
    }
    
    // Implementing the interface
    class Car implements Vehicle {
        public void start() {
            System.out.println("Car is starting");
        }
        public void stop() {
            System.out.println("Car has stopped");
        }
    }
    
    class Main {
        public static void main(String[] args) {
            Car myCar = new Car();
            myCar.start();
            myCar.stop();
        }
    }
    
    

Key Features of Interfaces

• All methods in an interface are implicitly public and abstract.
• Interfaces cannot have instance variables (only constants).
• A class can implement multiple interfaces (unlike classes that only support single inheritance).

Difference Between Abstract Class and Interface

• Abstract Class: Can have both abstract and concrete methods; supports constructors and fields.
• Interface: Contains only abstract methods (before Java 8); supports multiple inheritance.

2. Packages in Java

A package is a collection of related classes and interfaces. It helps in organizing the code and avoids class name conflicts.

Creating a Package

    
    // Creating a package
    package mypackage;
    
    public class Example {
        public void display() {
            System.out.println("This is a package example");
        }
    }
    
    

Importing a Package

To use a class from another package, use the import keyword.

    
    import mypackage.Example; // Importing specific class
    
    class Main {
        public static void main(String[] args) {
            Example obj = new Example();
            obj.display();
        }
    }
    
    

Types of Packages

• Built-in Packages: Predefined packages like java.util, java.io.
• User-defined Packages: Custom packages created by the programmer.

3. Viva/Interview Questions