[JAVA-4543]: Improve Guide to UUID in Java (#12962)

Co-authored-by: Harpal Singh <harpal.singh@kaleyra.com>

core-java-modules/core-java-uuid/src/main/java/com/baeldung/uuid/UUIDGenerator.java

@@ -3,8 +3,6 @@ package com.baeldung.uuid;
 import java.nio.charset.StandardCharsets;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
-import java.time.Duration;
-import java.time.LocalDateTime;
 import java.util.Arrays;
 import java.util.Random;
 import java.util.UUID;
@@ -13,7 +11,8 @@ public final class UUIDGenerator {
 
     private static final char[] hexArray = "0123456789ABCDEF".toCharArray();
 
-    private UUIDGenerator() {}
+    private UUIDGenerator() {
+    }
 
     /**
      * Type 1 UUID Generation
@@ -33,14 +32,12 @@ public final class UUIDGenerator {
     }
 
     private static long get64MostSignificantBitsForVersion1() {
-        final LocalDateTime start = LocalDateTime.of(1582, 10, 15, 0, 0, 0);
+        final long timeForUuidIn100Nanos = System.currentTimeMillis();
-        final Duration duration = Duration.between(start, LocalDateTime.now());
+        final long time_low = (timeForUuidIn100Nanos & 0x0000_0000_FFFF_FFFFL) << 32;
-        final long seconds = duration.getSeconds();
+        final long time_mid = ((timeForUuidIn100Nanos >> 32) & 0xFFFF) << 16;
-        final long nanos = duration.getNano();
-        final long timeForUuidIn100Nanos = seconds * 10000000 + nanos * 100;
-        final long least12SignificantBitOfTime = (timeForUuidIn100Nanos & 0x000000000000FFFFL) >> 4;
         final long version = 1 << 12;
-        return (timeForUuidIn100Nanos & 0xFFFFFFFFFFFF0000L) + version + least12SignificantBitOfTime;
+        final long time_hi = ((timeForUuidIn100Nanos >> 48) & 0x0FFF);
+        return time_low + time_mid + version + time_hi;
     }
 
     /**
@@ -94,34 +91,16 @@ public final class UUIDGenerator {
         long lsb = 0;
         assert data.length == 16 : "data must be 16 bytes in length";
 
-        for (int i = 0; i < 8; i++) {msb = (msb << 8) | (data[i] & 0xff);}
+        for (int i = 0; i < 8; i++) {
+            msb = (msb << 8) | (data[i] & 0xff);
+        }
 
-        for (int i = 8; i < 16; i++) {lsb = (lsb << 8) | (data[i] & 0xff);}
+        for (int i = 8; i < 16; i++) {
+            lsb = (lsb << 8) | (data[i] & 0xff);
+        }
         return new UUID(msb, lsb);
     }
 
-    /**
-     * Unique Keys Generation Using Message Digest and Type 4 UUID
-     * @throws NoSuchAlgorithmException
-     */
-    public static String generateUniqueKeysWithUUIDAndMessageDigest() throws NoSuchAlgorithmException {
-        final MessageDigest salt = MessageDigest.getInstance("SHA-256");
-        salt.update(UUID.randomUUID()
-                        .toString()
-                        .getBytes(StandardCharsets.UTF_8));
-        return bytesToHex(salt.digest());
-    }
-
-    private static String bytesToHex(byte[] bytes) {
-        final char[] hexChars = new char[bytes.length * 2];
-        for (int j = 0; j < bytes.length; j++) {
-            final int v = bytes[j] & 0xFF;
-            hexChars[j * 2] = hexArray[v >>> 4];
-            hexChars[j * 2 + 1] = hexArray[v & 0x0F];
-        }
-        return new String(hexChars);
-    }
-
     private static byte[] bytesFromUUID(String uuidHexString) {
         final String normalizedUUIDHexString = uuidHexString.replace("-", "");
 

core-java-modules/core-java-uuid/src/test/java/com/baeldung/uuid/UUIDGeneratorUnitTest.java

@@ -3,6 +3,10 @@ package com.baeldung.uuid;
 import org.junit.jupiter.api.Test;
 
 import java.io.UnsupportedEncodingException;
+import java.time.Instant;
+import java.time.LocalDate;
+import java.time.LocalDateTime;
+import java.time.ZoneId;
 import java.util.UUID;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
@@ -13,17 +17,23 @@ class UUIDGeneratorUnitTest {
     private static final String NAMESPACE_DNS = "6ba7b810-9dad-11d1-80b4-00c04fd430c8";
 
     @Test
-    public void version_1_UUID_is_generated_with_correct_length_version_and_variant() {
+    void shouldGenerateType1UUIDWithCorrectVersionAndVariant() {
-
         UUID uuid = UUIDGenerator.generateType1UUID();
-
         assertEquals(36, uuid.toString().length());
         assertEquals(1, uuid.version());
         assertEquals(2, uuid.variant());
     }
 
     @Test
-    public void version_3_UUID_is_correctly_generated_for_domain_baeldung_com() throws UnsupportedEncodingException {
+    void shouldGenerateType1UUIDWithTheCurrentDate() {
+        UUID uuid = UUIDGenerator.generateType1UUID();
+        long time = uuid.timestamp();
+        LocalDateTime dateTime = LocalDateTime.ofInstant(Instant.ofEpochMilli(time), ZoneId.systemDefault());
+        assertEquals(LocalDate.now(), dateTime.toLocalDate());
+    }
+
+    @Test
+    void version_3_UUID_is_correctly_generated_for_domain_baeldung_com() {
 
         UUID uuid = UUIDGenerator.generateType3UUID(NAMESPACE_DNS, "baeldung.com");
 
@@ -33,7 +43,7 @@ class UUIDGeneratorUnitTest {
     }
 
     @Test
-    public void version_3_UUID_is_correctly_generated_for_domain_d() throws UnsupportedEncodingException {
+    void version_3_UUID_is_correctly_generated_for_domain_d() {
 
         UUID uuid = UUIDGenerator.generateType3UUID(NAMESPACE_DNS, "d");