Add UUIDGeneratorDemo showing usage examples

Co-authored-by: simbo1905 <[email protected]>

Author: Copilot

json-java21/src/main/java/jdk/sandbox/demo/UUIDGeneratorDemo.java

@@ -0,0 +1,86 @@
+package jdk.sandbox.demo;
+
+import jdk.sandbox.java.util.json.UUIDGenerator;
+
+import java.util.UUID;
+
+/// Demonstrates usage of {@link UUIDGenerator} with both UUIDv7 and unique-then-time modes.
+///
+/// This demo shows:
+/// - Default UUIDv7 generation
+/// - Direct UUIDv7 creation with specific timestamps
+/// - Unique-then-time UUID generation
+/// - Configuration mode detection
+public final class UUIDGeneratorDemo {
+    
+    public static void main(final String[] args) {
+        System.out.println("=== UUID Generator Demo ===\n");
+        
+        // Show current configuration
+        final UUIDGenerator.Mode mode = UUIDGenerator.getConfiguredMode();
+        System.out.println("Configured mode: " + mode);
+        System.out.println("System property: " + System.getProperty(UUIDGenerator.MODE_PROPERTY, "(not set)"));
+        System.out.println();
+        
+        // Generate UUIDs using the configured mode
+        System.out.println("--- Generating UUIDs with configured mode ---");
+        for (int i = 0; i < 5; i++) {
+            final UUID uuid = UUIDGenerator.generateUUID();
+            System.out.println("UUID " + (i + 1) + ": " + uuid);
+            if (mode == UUIDGenerator.Mode.V7) {
+                System.out.println("  Version: " + uuid.version() + ", Variant: " + uuid.variant());
+            }
+        }
+        System.out.println();
+        
+        // Demonstrate UUIDv7 with specific timestamps
+        System.out.println("--- UUIDv7 with specific timestamps ---");
+        final long baseTime = System.currentTimeMillis();
+        for (int i = 0; i < 3; i++) {
+            final long timestamp = baseTime + (i * 1000); // 1 second apart
+            final UUID uuid = UUIDGenerator.ofEpochMillis(timestamp);
+            System.out.println("Timestamp: " + timestamp + " -> " + uuid);
+            System.out.println("  Version: " + uuid.version() + ", Variant: " + uuid.variant());
+        }
+        System.out.println();
+        
+        // Demonstrate unique-then-time mode
+        System.out.println("--- Unique-then-time mode ---");
+        for (int i = 0; i < 3; i++) {
+            final long uniqueMsb = 0x1000000000000000L + i;
+            final UUID uuid = UUIDGenerator.uniqueThenTime(uniqueMsb);
+            System.out.println("Unique MSB: " + Long.toHexString(uniqueMsb) + " -> " + uuid);
+        }
+        System.out.println();
+        
+        // Demonstrate monotonicity of UUIDv7
+        System.out.println("--- UUIDv7 Monotonicity (time-ordered) ---");
+        UUID previous = null;
+        for (int i = 0; i < 5; i++) {
+            final long timestamp = baseTime + (i * 100); // 100ms apart
+            final UUID current = UUIDGenerator.ofEpochMillis(timestamp);
+            if (previous != null) {
+                final int comparison = current.compareTo(previous);
+                System.out.println(current + " > " + previous + " ? " + (comparison > 0));
+            } else {
+                System.out.println(current + " (first)");
+            }
+            previous = current;
+        }
+        System.out.println();
+        
+        // Show configuration examples
+        System.out.println("=== Configuration Examples ===");
+        System.out.println("To use UUIDv7 (default):");
+        System.out.println("  java -jar app.jar");
+        System.out.println("  or");
+        System.out.println("  java -D" + UUIDGenerator.MODE_PROPERTY + "=v7 -jar app.jar");
+        System.out.println();
+        System.out.println("To use unique-then-time mode:");
+        System.out.println("  java -D" + UUIDGenerator.MODE_PROPERTY + "=unique-then-time -jar app.jar");
+        System.out.println();
+        System.out.println("On Android, set in Application.onCreate():");
+        System.out.println("  System.setProperty(\"" + UUIDGenerator.MODE_PROPERTY + "\", \"v7\");");
+        System.out.println("  Note: Must be set before first UUIDGenerator access");
+    }
+}