feat(Java12): add demo showcasing switch expressions and mention Shenandoah GC

What
- Added JAVA12 class in package Java12.
- Demonstrates new Java 12 features:
  - Switch expressions (preview feature at the time).
    - Example uses integer day to categorize as "Weekday", "Weekend", or "Invalid day".
    - Uses new arrow (->) syntax and yield in block form.
  - Mention of Shenandoah GC introduction as experimental low-pause garbage collector.
    - Prints note about enabling it with JVM flag -XX:+UseShenandoahGC.

Why
- Illustrates concise switch expressions replacing verbose switch-case blocks.
- Demonstrates yield keyword in multi-statement branch.
- Educates about Shenandoah GC, a low-latency garbage collector added in Java 12.

How
- Declared int day = 3.
- Used switch expression:
  - 1–5 → "Weekday".
  - 6–7 → "Weekend".
  - default → prints warning and yields "Invalid day".
- Printed result to stdout.
- Printed informational notes about Shenandoah GC usage.

Logic
- Inputs: day = 3.
- Outputs:
  - "Day type: Weekday".
  - Notes about Shenandoah GC.
- Flow:
  1. switch expression matches day = 3.
  2. Returns "Weekday".
  3. Print result.
  4. Print informational note about Shenandoah GC.
- Edge cases:
  - day outside 1–7 triggers default branch with yield "Invalid day".
- Complexity / performance: O(1) branching.
- Concurrency / thread-safety: Purely local computation, no concurrency issues.
- Error handling: None required.

Real-life applications
- Switch expressions improve readability for mapping values, classification, or simple rules.
- Useful in parsers, state machines, or business logic.
- Shenandoah GC suited for applications needing predictable low-latency GC pauses (financial systems, gaming servers, etc.).

Notes
- switch expressions return values directly, making assignment concise.
- yield keyword is mandatory when block case is used with multiple statements.
- Shenandoah GC was experimental in Java 12 but later matured in subsequent releases.

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

Author: Someshdiwan

Java-Versions/Java 12/src/JAVA12.java

@@ -0,0 +1,21 @@
+package Java12;
+
+public class JAVA12 {
+    public static void main(String[] args) {
+        // Switch Expressions (Preview Feature)
+        int day = 3;
+        String dayType = switch (day) {
+            case 1, 2, 3, 4, 5 -> "Weekday";
+            case 6, 7 -> "Weekend";
+            default -> {
+                System.out.println("Unknown day, assigning default...");
+                yield "Invalid day";
+            }
+        };
+
+        System.out.println("Day type: " + dayType);
+
+        System.out.println("\n[Note] Shenandoah GC was introduced as an experimental GC in Java 12.");
+        System.out.println("Use JVM flags like: -XX:+UseShenandoahGC to test its low-pause capabilities.");
+    }
+}