An Interface in Java is a reference type, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types. It provides a way to achieve 100% abstraction (before Java 8) and multiple inheritance of type.

Use interfaces to define a "contract" that classes must follow (e.g., Runnable, Comparable). Use them when you want to define what a class should do, but not how it should do it.

• implements Keyword: Used by a class to inherit from an interface.
• Default Methods (Java 8+): Methods with a body that provide a default implementation.
• Multiple Inheritance: A class can implement multiple interfaces.
• Loose Coupling: Interfaces help in reducing dependencies between classes.

java Standard
interface Printable {
    void print(); // Abstract method
    
    default void scan() {
        System.out.println("Scanning...");
    }
}

class LaserPrinter implements Printable {
    public void print() {
        System.out.println("Printing using Laser...");
    }
}

public class Main {
    public static void main(String[] args) {
        LaserPrinter lp = new LaserPrinter();
        lp.print();
        lp.scan(); // Uses default implementation
    }
}

• Instantiating Interfaces: Trying to do new MyInterface() (interfaces cannot be instantiated).
• Public access: Forgetting that all methods in an interface are public and abstract by default.
• Overusing Default Methods: Using default methods for core logic (they should be used for backward compatibility).