feat(functional-interfaces): add demo using Function<T,R> to compute string length

What
- Introduced FunctionalInterfaceDemo class.
- Demonstrates usage of built-in functional interface `Function<T,R>` from `java.util.function`.
- Example logic:
  - Defines a Function<String, Integer> mapping a string to its length.
  - Applies it to the string "Java 8".
  - Prints the computed length.

Why
- Shows practical usage of a functional interface without writing custom ones.
- Highlights how lambdas implement single abstract methods cleanly.
- Reinforces role of functional interfaces in functional programming style introduced in Java 8.

Logic
1. Function<T,R> has one abstract method → `R apply(T t)`.
2. Lambda `str -> str.length()` implements this method, mapping a String input to an Integer output.
3. The function is applied with input `"Java 8"` returning `7`.
4. Demonstrates concise functional programming over traditional anonymous classes.

Real-life applications
- Transforming data in Streams (`map` operations).
- Converting domain objects (e.g., User → UserDTO).
- Processing strings (extracting substrings, normalizing cases).
- Mapping request data to processed formats in APIs.

Notes
- Functional interface `Function<T,R>` is parameterized: T = input type, R = return type.
- Commonly chained with `andThen` and `compose` for function composition.
- Part of core functional interfaces in `java.util.function` alongside Predicate, Consumer, Supplier.

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

Author: Someshdiwan

Java 8 Crash Course/Functional Interface/src/FunctionalInterfaceDemo.java

@@ -0,0 +1,12 @@
+import java.util.function.Function;
+
+public class FunctionalInterfaceDemo {
+    public static void main(String[] args) {
+        // Using Function<T, R> to convert a String to its length.
+        Function<String, Integer> stringLengthFunction = str -> str.length();
+
+        // Applying the function
+        int length = stringLengthFunction.apply("Java 8");
+        System.out.println("Length of 'Java 8': " + length);
+    }
+}