feat(functional-interface): add Function demo for String length and student filtering by prefix

What
- Added `Main` class demonstrating multiple uses of `Function<T,R>`:
  1. `Function<String,Integer>` → calculates string length (`"Jackson"` → 7).
  2. `Function<String,String>` → extracts substring (first 3 characters).
  3. `Function<List<Student>, List<Student>>` → custom function to filter students whose names start with `"vip"` (case-insensitive).
- Introduced inner `Student` class with fields `id` and `name`, along with getters, setters, and overridden `toString()`.

Why
- Shows versatility of the `Function` interface:
  - Simple transformations (`String → Integer`).
  - Substring extraction (`String → String`).
  - Business logic functions (`List<Student> → List<Student>`).
- Highlights functional programming approach where behavior is encapsulated in reusable, composable functions.
- Demonstrates filtering logic without external loops in main method, improving readability.

Logic
1. **String Length Function**
   - `Function<String,Integer> function = x -> x.length();`
   - `function.apply("Jackson")` → returns `7`.

2. **Substring Function**
   - `Function<String,String> function2 = s -> s.substring(0,3);`
   - Applied to any string to get the first three characters.

3. **Student Filtering Function**
   - Iterates over input `List<Student>`.
   - Checks: `s.getName() != null && s.getName().toLowerCase().startsWith("vip")`.
   - If condition passes → adds to result list.
   - Applied on a list of students:
     - Students:
       `2 - VIP`
       `3 - Jackson`
       `4 - Stark`
     - Output: `[2 - VIP]` (only student with `"vip"` prefix).

Real-life applications
- String transformation pipelines (e.g., trimming, validation, encoding).
- Filtering domain objects (e.g., students, employees, products) based on dynamic business rules.
- Encapsulating data transformation logic into reusable `Function` objects for cleaner APIs.
- Can be combined with Java Streams (`stream().map(...).filter(...)`) for declarative data processing.

Notes
- `Function<T,R>` encourages separation of transformation logic from execution.
- Use `.andThen()` or `.compose()` to chain multiple `Function` objects for complex transformations.
- Inner `Student` class kept simple for demo purposes; in production, prefer separate class files with immutability or records (Java 14+).

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

Author: Someshdiwan

Java 8 Crash Course/Function/src/Main.java

@@ -0,0 +1,64 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.function.Function;
+
+public class Main {
+    public static void main(String[] args) {
+        Function<String, Integer> function = x -> x.length();
+        System.out.println(function.apply("Jackson"));
+
+        Function<String, String> function2 = s -> s.substring(0,3);
+        System.out.println();
+
+        Function<List<Student>, List<Student>> studentsWithVipAsPrefix = li -> {
+            List<Student> result = new ArrayList<>();
+
+            for(Student s : li) {
+                if (s.getName() != null && s.getName().toLowerCase().startsWith("vip")) {
+                    result.add(s);
+                }
+            }
+            return result;
+        };
+       Student s1 = new Student(2,"VIP");
+       Student s2 = new Student(3,"Jackson");
+       Student s3 = new Student(4,"Stark");
+
+       List<Student> students = Arrays.asList(s1, s2, s3);
+
+       List<Student> filteredStudents = studentsWithVipAsPrefix.apply(students);
+        System.out.println(filteredStudents);
+    }
+
+    private static class Student {
+        private  int id;
+        private String name;
+
+        public Student(int id, String name) {
+            this.id = id;
+            this.name = name;
+        }
+
+        public int getId() {
+            return id;
+        }
+
+        public void setId(int id) {
+            this.id = id;
+        }
+
+        public String getName() {
+            return name;
+        }
+
+        public void setName(String name) {
+            this.name = name;
+        }
+
+        @Override
+        public String toString() {
+            return id + " - " + name;
+        }
+    }
+}