feat(HowUseLambda): add Employee functional interface and Main with lambda usage

What
- Added Employee interface in package HowUseLambda.
  - Defines a single abstract method getName().
  - Acts as a functional interface suitable for lambda expressions.
- Added Main class demonstrating lambda usage with Employee.
  - Creates Employee E = () -> "Software Engineer".
  - Prints result of E.getName().
  - Creates another Employee editor = () -> "Editor".
  - Prints result of editor.getName().

Why
- Demonstrates how functional interfaces can be implemented with lambda expressions in Java.
- Provides educational example of replacing verbose implementation classes with concise lambdas.
- Shows how interface references can hold lambda expressions, enabling cleaner and more expressive code.

How
- Declared Employee interface with single method getName().
- In Main, used lambda () -> "Software Engineer" to provide implementation at runtime.
- Called getName() on Employee reference to print "Software Engineer".
- Repeated with editor lambda returning "Editor".

Logic
- Inputs: none (hardcoded strings).
- Outputs:
  1. "Software Engineer"
  2. "Editor"
- Flow:
  1. Define Employee E as lambda returning "Software Engineer".
  2. Invoke E.getName() → print "Software Engineer".
  3. Define Employee editor as lambda returning "Editor".
  4. Invoke editor.getName() → print "Editor".
- Edge cases handled:
  - Functional interface constraint ensures only one abstract method is present.
  - Lambdas provide direct implementation without boilerplate.
- Complexity / performance: O(1) operations, negligible overhead.
- Concurrency / thread-safety:
  - Lambdas here are stateless, thus inherently thread-safe.
- Error handling:
  - No exceptions thrown in current design.

Real-life applications
- Using functional interfaces for callbacks, event handling, or business logic injection.
- Simplifying code when passing behavior as parameter (e.g., with Java Streams, Runnable, Comparator).
- Replacing anonymous classes with concise lambdas in GUI or concurrent programming.
- Teaching material for introducing functional programming in Java.

Notes
- Employee interface could be annotated with @FunctionalInterface to enforce contract at compile time.
- Example uses hardcoded strings, but lambdas can encapsulate more complex logic or captured variables.
- Lambdas improve readability, reduce boilerplate, and align Java with modern functional programming paradigms.

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan

Java 8 Crash Course/Lambda Expression/src/HowUseLambda/Employee.java

@@ -0,0 +1,6 @@
+package HowUseLambda;
+
+public interface Employee {
+    String getName();
+    //Only one method means its functional interface.
+}