diff --git a/src/main/java/com/github/f4b6a3/ulid/Ulid.java b/src/main/java/com/github/f4b6a3/ulid/Ulid.java
index a21c505..d811e93 100644
--- a/src/main/java/com/github/f4b6a3/ulid/Ulid.java
+++ b/src/main/java/com/github/f4b6a3/ulid/Ulid.java
@@ -28,8 +28,6 @@ import java.io.Serializable;
 import java.time.Instant;
 import java.util.UUID;
 
-import com.github.f4b6a3.ulid.util.UlidValidator;
-
 /**
  * This class represents a ULID.
  */
@@ -48,81 +46,89 @@ public final class Ulid implements Serializable, Comparable<Ulid> {
 	public static final int TIME_BYTES_LENGTH = 6;
 	public static final int RANDOM_BYTES_LENGTH = 10;
 
-	protected static final char[] ENCODING_CHARS = //
+	// 0xffffffffffffffffL + 1 = 0x0000000000000000L
+	private static final long INCREMENT_OVERFLOW = 0x0000000000000000L;
+
+	protected static final char[] ALPHABET_UPPERCASE = //
 			{ '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', //
 					'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'J', 'K', //
 					'M', 'N', 'P', 'Q', 'R', 'S', 'T', 'V', 'W', 'X', 'Y', 'Z' };
 
-	protected static final long[] ENCODING_VALUES = new long[128];
+	protected static final char[] ALPHABET_LOWERCASE = //
+			{ '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', //
+					'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'j', 'k', //
+					'm', 'n', 'p', 'q', 'r', 's', 't', 'v', 'w', 'x', 'y', 'z' };
+
+	protected static final long[] ALPHABET_VALUES = new long[128];
 	static {
-		for (int i = 0; i < ENCODING_VALUES.length; i++) {
-			ENCODING_VALUES[i] = -1;
+		for (int i = 0; i < ALPHABET_VALUES.length; i++) {
+			ALPHABET_VALUES[i] = -1;
 		}
 		// Numbers
-		ENCODING_VALUES['0'] = 0x00;
-		ENCODING_VALUES['1'] = 0x01;
-		ENCODING_VALUES['2'] = 0x02;
-		ENCODING_VALUES['3'] = 0x03;
-		ENCODING_VALUES['4'] = 0x04;
-		ENCODING_VALUES['5'] = 0x05;
-		ENCODING_VALUES['6'] = 0x06;
-		ENCODING_VALUES['7'] = 0x07;
-		ENCODING_VALUES['8'] = 0x08;
-		ENCODING_VALUES['9'] = 0x09;
+		ALPHABET_VALUES['0'] = 0x00;
+		ALPHABET_VALUES['1'] = 0x01;
+		ALPHABET_VALUES['2'] = 0x02;
+		ALPHABET_VALUES['3'] = 0x03;
+		ALPHABET_VALUES['4'] = 0x04;
+		ALPHABET_VALUES['5'] = 0x05;
+		ALPHABET_VALUES['6'] = 0x06;
+		ALPHABET_VALUES['7'] = 0x07;
+		ALPHABET_VALUES['8'] = 0x08;
+		ALPHABET_VALUES['9'] = 0x09;
 		// Lower case
-		ENCODING_VALUES['a'] = 0x0a;
-		ENCODING_VALUES['b'] = 0x0b;
-		ENCODING_VALUES['c'] = 0x0c;
-		ENCODING_VALUES['d'] = 0x0d;
-		ENCODING_VALUES['e'] = 0x0e;
-		ENCODING_VALUES['f'] = 0x0f;
-		ENCODING_VALUES['g'] = 0x10;
-		ENCODING_VALUES['h'] = 0x11;
-		ENCODING_VALUES['j'] = 0x12;
-		ENCODING_VALUES['k'] = 0x13;
-		ENCODING_VALUES['m'] = 0x14;
-		ENCODING_VALUES['n'] = 0x15;
-		ENCODING_VALUES['p'] = 0x16;
-		ENCODING_VALUES['q'] = 0x17;
-		ENCODING_VALUES['r'] = 0x18;
-		ENCODING_VALUES['s'] = 0x19;
-		ENCODING_VALUES['t'] = 0x1a;
-		ENCODING_VALUES['v'] = 0x1b;
-		ENCODING_VALUES['w'] = 0x1c;
-		ENCODING_VALUES['x'] = 0x1d;
-		ENCODING_VALUES['y'] = 0x1e;
-		ENCODING_VALUES['z'] = 0x1f;
+		ALPHABET_VALUES['a'] = 0x0a;
+		ALPHABET_VALUES['b'] = 0x0b;
+		ALPHABET_VALUES['c'] = 0x0c;
+		ALPHABET_VALUES['d'] = 0x0d;
+		ALPHABET_VALUES['e'] = 0x0e;
+		ALPHABET_VALUES['f'] = 0x0f;
+		ALPHABET_VALUES['g'] = 0x10;
+		ALPHABET_VALUES['h'] = 0x11;
+		ALPHABET_VALUES['j'] = 0x12;
+		ALPHABET_VALUES['k'] = 0x13;
+		ALPHABET_VALUES['m'] = 0x14;
+		ALPHABET_VALUES['n'] = 0x15;
+		ALPHABET_VALUES['p'] = 0x16;
+		ALPHABET_VALUES['q'] = 0x17;
+		ALPHABET_VALUES['r'] = 0x18;
+		ALPHABET_VALUES['s'] = 0x19;
+		ALPHABET_VALUES['t'] = 0x1a;
+		ALPHABET_VALUES['v'] = 0x1b;
+		ALPHABET_VALUES['w'] = 0x1c;
+		ALPHABET_VALUES['x'] = 0x1d;
+		ALPHABET_VALUES['y'] = 0x1e;
+		ALPHABET_VALUES['z'] = 0x1f;
 		// Lower case OIL
-		ENCODING_VALUES['o'] = 0x00;
-		ENCODING_VALUES['i'] = 0x01;
-		ENCODING_VALUES['l'] = 0x01;
+		ALPHABET_VALUES['o'] = 0x00;
+		ALPHABET_VALUES['i'] = 0x01;
+		ALPHABET_VALUES['l'] = 0x01;
 		// Upper case
-		ENCODING_VALUES['A'] = 0x0a;
-		ENCODING_VALUES['B'] = 0x0b;
-		ENCODING_VALUES['C'] = 0x0c;
-		ENCODING_VALUES['D'] = 0x0d;
-		ENCODING_VALUES['E'] = 0x0e;
-		ENCODING_VALUES['F'] = 0x0f;
-		ENCODING_VALUES['G'] = 0x10;
-		ENCODING_VALUES['H'] = 0x11;
-		ENCODING_VALUES['J'] = 0x12;
-		ENCODING_VALUES['K'] = 0x13;
-		ENCODING_VALUES['M'] = 0x14;
-		ENCODING_VALUES['N'] = 0x15;
-		ENCODING_VALUES['P'] = 0x16;
-		ENCODING_VALUES['Q'] = 0x17;
-		ENCODING_VALUES['R'] = 0x18;
-		ENCODING_VALUES['S'] = 0x19;
-		ENCODING_VALUES['T'] = 0x1a;
-		ENCODING_VALUES['V'] = 0x1b;
-		ENCODING_VALUES['W'] = 0x1c;
-		ENCODING_VALUES['X'] = 0x1d;
-		ENCODING_VALUES['Y'] = 0x1e;
-		ENCODING_VALUES['Z'] = 0x1f;
+		ALPHABET_VALUES['A'] = 0x0a;
+		ALPHABET_VALUES['B'] = 0x0b;
+		ALPHABET_VALUES['C'] = 0x0c;
+		ALPHABET_VALUES['D'] = 0x0d;
+		ALPHABET_VALUES['E'] = 0x0e;
+		ALPHABET_VALUES['F'] = 0x0f;
+		ALPHABET_VALUES['G'] = 0x10;
+		ALPHABET_VALUES['H'] = 0x11;
+		ALPHABET_VALUES['J'] = 0x12;
+		ALPHABET_VALUES['K'] = 0x13;
+		ALPHABET_VALUES['M'] = 0x14;
+		ALPHABET_VALUES['N'] = 0x15;
+		ALPHABET_VALUES['P'] = 0x16;
+		ALPHABET_VALUES['Q'] = 0x17;
+		ALPHABET_VALUES['R'] = 0x18;
+		ALPHABET_VALUES['S'] = 0x19;
+		ALPHABET_VALUES['T'] = 0x1a;
+		ALPHABET_VALUES['V'] = 0x1b;
+		ALPHABET_VALUES['W'] = 0x1c;
+		ALPHABET_VALUES['X'] = 0x1d;
+		ALPHABET_VALUES['Y'] = 0x1e;
+		ALPHABET_VALUES['Z'] = 0x1f;
 		// Upper case OIL
-		ENCODING_VALUES['O'] = 0x00;
-		ENCODING_VALUES['I'] = 0x01;
-		ENCODING_VALUES['L'] = 0x01;
+		ALPHABET_VALUES['O'] = 0x00;
+		ALPHABET_VALUES['I'] = 0x01;
+		ALPHABET_VALUES['L'] = 0x01;
 
 	}
 
@@ -133,6 +139,14 @@ public final class Ulid implements Serializable, Comparable<Ulid> {
 		this.lsb = leastSignificantBits;
 	}
 
+	public static Ulid of(Ulid ulid) {
+		return new Ulid(ulid.getMostSignificantBits(), ulid.getLeastSignificantBits());
+	}
+
+	public static Ulid of(UUID uuid) {
+		return new Ulid(uuid.getMostSignificantBits(), uuid.getLeastSignificantBits());
+	}
+
 	// TODO: test
 	public static Ulid of(byte[] bytes) {
 
@@ -165,43 +179,42 @@ public final class Ulid implements Serializable, Comparable<Ulid> {
 	}
 
 	// TODO: optimize
-	public static Ulid of(String ulid) {
+	public static Ulid of(String string) {
 
-		final char[] chars = ulid == null ? new char[0] : ulid.toCharArray();
-		UlidValidator.validate(chars);
+		final char[] chars = toCharArray(string);
 
 		long tm = 0;
 		long r1 = 0;
 		long r2 = 0;
 
-		tm |= ENCODING_VALUES[chars[0x00]] << 45;
-		tm |= ENCODING_VALUES[chars[0x01]] << 40;
-		tm |= ENCODING_VALUES[chars[0x02]] << 35;
-		tm |= ENCODING_VALUES[chars[0x03]] << 30;
-		tm |= ENCODING_VALUES[chars[0x04]] << 25;
-		tm |= ENCODING_VALUES[chars[0x05]] << 20;
-		tm |= ENCODING_VALUES[chars[0x06]] << 15;
-		tm |= ENCODING_VALUES[chars[0x07]] << 10;
-		tm |= ENCODING_VALUES[chars[0x08]] << 5;
-		tm |= ENCODING_VALUES[chars[0x09]];
+		tm |= ALPHABET_VALUES[chars[0x00]] << 45;
+		tm |= ALPHABET_VALUES[chars[0x01]] << 40;
+		tm |= ALPHABET_VALUES[chars[0x02]] << 35;
+		tm |= ALPHABET_VALUES[chars[0x03]] << 30;
+		tm |= ALPHABET_VALUES[chars[0x04]] << 25;
+		tm |= ALPHABET_VALUES[chars[0x05]] << 20;
+		tm |= ALPHABET_VALUES[chars[0x06]] << 15;
+		tm |= ALPHABET_VALUES[chars[0x07]] << 10;
+		tm |= ALPHABET_VALUES[chars[0x08]] << 5;
+		tm |= ALPHABET_VALUES[chars[0x09]];
 
-		r1 |= ENCODING_VALUES[chars[0x0a]] << 35;
-		r1 |= ENCODING_VALUES[chars[0x0b]] << 30;
-		r1 |= ENCODING_VALUES[chars[0x0c]] << 25;
-		r1 |= ENCODING_VALUES[chars[0x0d]] << 20;
-		r1 |= ENCODING_VALUES[chars[0x0e]] << 15;
-		r1 |= ENCODING_VALUES[chars[0x0f]] << 10;
-		r1 |= ENCODING_VALUES[chars[0x10]] << 5;
-		r1 |= ENCODING_VALUES[chars[0x11]];
+		r1 |= ALPHABET_VALUES[chars[0x0a]] << 35;
+		r1 |= ALPHABET_VALUES[chars[0x0b]] << 30;
+		r1 |= ALPHABET_VALUES[chars[0x0c]] << 25;
+		r1 |= ALPHABET_VALUES[chars[0x0d]] << 20;
+		r1 |= ALPHABET_VALUES[chars[0x0e]] << 15;
+		r1 |= ALPHABET_VALUES[chars[0x0f]] << 10;
+		r1 |= ALPHABET_VALUES[chars[0x10]] << 5;
+		r1 |= ALPHABET_VALUES[chars[0x11]];
 
-		r2 |= ENCODING_VALUES[chars[0x12]] << 35;
-		r2 |= ENCODING_VALUES[chars[0x13]] << 30;
-		r2 |= ENCODING_VALUES[chars[0x14]] << 25;
-		r2 |= ENCODING_VALUES[chars[0x15]] << 20;
-		r2 |= ENCODING_VALUES[chars[0x16]] << 15;
-		r2 |= ENCODING_VALUES[chars[0x17]] << 10;
-		r2 |= ENCODING_VALUES[chars[0x18]] << 5;
-		r2 |= ENCODING_VALUES[chars[0x19]];
+		r2 |= ALPHABET_VALUES[chars[0x12]] << 35;
+		r2 |= ALPHABET_VALUES[chars[0x13]] << 30;
+		r2 |= ALPHABET_VALUES[chars[0x14]] << 25;
+		r2 |= ALPHABET_VALUES[chars[0x15]] << 20;
+		r2 |= ALPHABET_VALUES[chars[0x16]] << 15;
+		r2 |= ALPHABET_VALUES[chars[0x17]] << 10;
+		r2 |= ALPHABET_VALUES[chars[0x18]] << 5;
+		r2 |= ALPHABET_VALUES[chars[0x19]];
 
 		final long msb = (tm << 16) | (r1 >>> 24);
 		final long lsb = (r1 << 40) | (r2 & 0xffffffffffL);
@@ -209,10 +222,6 @@ public final class Ulid implements Serializable, Comparable<Ulid> {
 		return new Ulid(msb, lsb);
 	}
 
-	public static Ulid of(UUID uuid) {
-		return new Ulid(uuid.getMostSignificantBits(), uuid.getLeastSignificantBits());
-	}
-
 	public static Ulid of(long time, byte[] random) {
 
 		if ((time & 0xffff000000000000L) != 0) {
@@ -241,6 +250,17 @@ public final class Ulid implements Serializable, Comparable<Ulid> {
 		return new Ulid(msb, lsb);
 	}
 
+	public UUID toUuid() {
+		return new UUID(this.msb, this.lsb);
+	}
+
+	// TODO: test
+	public UUID toUuid4() {
+		final long msb4 = (this.msb & 0xffffffffffff0fffL) | 0x0000000000004000L; // apply version 4
+		final long lsb4 = (this.lsb & 0x3fffffffffffffffL) | 0x8000000000000000L; // apply variant RFC-4122
+		return new UUID(msb4, lsb4);
+	}
+
 	// TODO: test
 	public byte[] toBytes() {
 
@@ -272,64 +292,29 @@ public final class Ulid implements Serializable, Comparable<Ulid> {
 		return Ulid.of(this.toUuid4()).toBytes();
 	}
 
-	// TODO: optimize
 	@Override
 	public String toString() {
-
-		final char[] chars = new char[ULID_LENGTH];
-		long long0 = this.msb;
-		long long1 = this.lsb;
-
-		long time = long0 >>> 16;
-		long random1 = ((long0 & 0xffffL) << 24) | (long1 >>> 40);
-		long random2 = (long1 & 0xffffffffffL);
-
-		chars[0x00] = ENCODING_CHARS[(int) (time >>> 45 & 0b11111)];
-		chars[0x01] = ENCODING_CHARS[(int) (time >>> 40 & 0b11111)];
-		chars[0x02] = ENCODING_CHARS[(int) (time >>> 35 & 0b11111)];
-		chars[0x03] = ENCODING_CHARS[(int) (time >>> 30 & 0b11111)];
-		chars[0x04] = ENCODING_CHARS[(int) (time >>> 25 & 0b11111)];
-		chars[0x05] = ENCODING_CHARS[(int) (time >>> 20 & 0b11111)];
-		chars[0x06] = ENCODING_CHARS[(int) (time >>> 15 & 0b11111)];
-		chars[0x07] = ENCODING_CHARS[(int) (time >>> 10 & 0b11111)];
-		chars[0x08] = ENCODING_CHARS[(int) (time >>> 5 & 0b11111)];
-		chars[0x09] = ENCODING_CHARS[(int) (time & 0b11111)];
-
-		chars[0x0a] = ENCODING_CHARS[(int) (random1 >>> 35 & 0b11111)];
-		chars[0x0b] = ENCODING_CHARS[(int) (random1 >>> 30 & 0b11111)];
-		chars[0x0c] = ENCODING_CHARS[(int) (random1 >>> 25 & 0b11111)];
-		chars[0x0d] = ENCODING_CHARS[(int) (random1 >>> 20 & 0b11111)];
-		chars[0x0e] = ENCODING_CHARS[(int) (random1 >>> 15 & 0b11111)];
-		chars[0x0f] = ENCODING_CHARS[(int) (random1 >>> 10 & 0b11111)];
-		chars[0x10] = ENCODING_CHARS[(int) (random1 >>> 5 & 0b11111)];
-		chars[0x11] = ENCODING_CHARS[(int) (random1 & 0b11111)];
-
-		chars[0x12] = ENCODING_CHARS[(int) (random2 >>> 35 & 0b11111)];
-		chars[0x13] = ENCODING_CHARS[(int) (random2 >>> 30 & 0b11111)];
-		chars[0x14] = ENCODING_CHARS[(int) (random2 >>> 25 & 0b11111)];
-		chars[0x15] = ENCODING_CHARS[(int) (random2 >>> 20 & 0b11111)];
-		chars[0x16] = ENCODING_CHARS[(int) (random2 >>> 15 & 0b11111)];
-		chars[0x17] = ENCODING_CHARS[(int) (random2 >>> 10 & 0b11111)];
-		chars[0x18] = ENCODING_CHARS[(int) (random2 >>> 5 & 0b11111)];
-		chars[0x19] = ENCODING_CHARS[(int) (random2 & 0b11111)];
-
-		return new String(chars);
+		return this.toUpperCase();
 	}
 
 	// TODO: test
-	public String toString4() {
-		return Ulid.of(this.toUuid4()).toString();
-	}
-
-	public UUID toUuid() {
-		return new UUID(this.msb, this.lsb);
+	public String toUpperCase() {
+		return toString(ALPHABET_UPPERCASE);
 	}
 
 	// TODO: test
-	public UUID toUuid4() {
-		final long msb4 = (this.msb & 0xffffffffffff0fffL) | 0x0000000000004000L; // apply version 4
-		final long lsb4 = (this.lsb & 0x3fffffffffffffffL) | 0x8000000000000000L; // apply variant RFC-4122
-		return new UUID(msb4, lsb4);
+	public String toUpperCase4() {
+		return Ulid.of(this.toUuid4()).toUpperCase();
+	}
+
+	// TODO: test
+	public String toLowerCase() {
+		return toString(ALPHABET_LOWERCASE);
+	}
+
+	// TODO: test
+	public String toLowerCase4() {
+		return Ulid.of(this.toUuid4()).toLowerCase();
 	}
 
 	public long getTime() {
@@ -348,6 +333,20 @@ public final class Ulid implements Serializable, Comparable<Ulid> {
 		return this.lsb;
 	}
 
+	// TODO: test
+	public Ulid increment() {
+
+		long msb1 = this.msb;
+		long lsb1 = this.lsb + 1; // Increment the LSB
+
+		if (lsb1 == INCREMENT_OVERFLOW) {
+			// Increment the random bits of the MSB
+			msb1 = (msb1 & 0xffffffffffff0000L) | ((msb1 + 1) & 0x000000000000ffffL);
+		}
+
+		return new Ulid(msb1, lsb1);
+	}
+
 	@Override
 	public int hashCode() {
 		final int prime = 31;
@@ -385,4 +384,86 @@ public final class Ulid implements Serializable, Comparable<Ulid> {
 			return 1;
 		return 0;
 	}
+
+	// TODO: optimize
+	protected String toString(char[] alphabet) {
+
+		final char[] chars = new char[ULID_LENGTH];
+
+		long time = this.msb >>> 16;
+		long random1 = ((this.msb & 0xffffL) << 24) | (this.lsb >>> 40);
+		long random2 = (this.lsb & 0xffffffffffL);
+
+		chars[0x00] = alphabet[(int) (time >>> 45 & 0b11111)];
+		chars[0x01] = alphabet[(int) (time >>> 40 & 0b11111)];
+		chars[0x02] = alphabet[(int) (time >>> 35 & 0b11111)];
+		chars[0x03] = alphabet[(int) (time >>> 30 & 0b11111)];
+		chars[0x04] = alphabet[(int) (time >>> 25 & 0b11111)];
+		chars[0x05] = alphabet[(int) (time >>> 20 & 0b11111)];
+		chars[0x06] = alphabet[(int) (time >>> 15 & 0b11111)];
+		chars[0x07] = alphabet[(int) (time >>> 10 & 0b11111)];
+		chars[0x08] = alphabet[(int) (time >>> 5 & 0b11111)];
+		chars[0x09] = alphabet[(int) (time & 0b11111)];
+
+		chars[0x0a] = alphabet[(int) (random1 >>> 35 & 0b11111)];
+		chars[0x0b] = alphabet[(int) (random1 >>> 30 & 0b11111)];
+		chars[0x0c] = alphabet[(int) (random1 >>> 25 & 0b11111)];
+		chars[0x0d] = alphabet[(int) (random1 >>> 20 & 0b11111)];
+		chars[0x0e] = alphabet[(int) (random1 >>> 15 & 0b11111)];
+		chars[0x0f] = alphabet[(int) (random1 >>> 10 & 0b11111)];
+		chars[0x10] = alphabet[(int) (random1 >>> 5 & 0b11111)];
+		chars[0x11] = alphabet[(int) (random1 & 0b11111)];
+
+		chars[0x12] = alphabet[(int) (random2 >>> 35 & 0b11111)];
+		chars[0x13] = alphabet[(int) (random2 >>> 30 & 0b11111)];
+		chars[0x14] = alphabet[(int) (random2 >>> 25 & 0b11111)];
+		chars[0x15] = alphabet[(int) (random2 >>> 20 & 0b11111)];
+		chars[0x16] = alphabet[(int) (random2 >>> 15 & 0b11111)];
+		chars[0x17] = alphabet[(int) (random2 >>> 10 & 0b11111)];
+		chars[0x18] = alphabet[(int) (random2 >>> 5 & 0b11111)];
+		chars[0x19] = alphabet[(int) (random2 & 0b11111)];
+
+		return new String(chars);
+	}
+
+	public static boolean isValidString(String string) {
+		return isValidArray(string == null ? null : string.toCharArray());
+	}
+
+	/**
+	 * Checks if the string is a valid ULID.
+	 * 
+	 * A valid ULID string is a sequence of 26 characters from Crockford's base 32
+	 * alphabet.
+	 * 
+	 * @param chars a char array
+	 * @return boolean true if valid
+	 */
+	protected static boolean isValidArray(final char[] chars) {
+
+		if (chars == null || chars.length != ULID_LENGTH) {
+			return false; // null or wrong size!
+		}
+
+		// the two extra bits added by base-32 encoding must be zero
+		if ((ALPHABET_VALUES[chars[0]] & 0b11000) != 0) {
+			return false; // overflow!
+		}
+
+		for (int i = 0; i < chars.length; i++) {
+			if (ALPHABET_VALUES[chars[i]] == -1) {
+				return false; // invalid character!
+			}
+		}
+
+		return true; // It seems to be OK.
+	}
+
+	protected static char[] toCharArray(String string) {
+		char[] chars = string == null ? new char[0] : string.toCharArray();
+		if (!isValidArray(chars)) {
+			throw new IllegalArgumentException(String.format("Invalid ULID: \"%s\"", string));
+		}
+		return chars;
+	}
 }
diff --git a/src/main/java/com/github/f4b6a3/ulid/UlidCreator.java b/src/main/java/com/github/f4b6a3/ulid/UlidCreator.java
index 2dacd47..08ca495 100644
--- a/src/main/java/com/github/f4b6a3/ulid/UlidCreator.java
+++ b/src/main/java/com/github/f4b6a3/ulid/UlidCreator.java
@@ -24,9 +24,8 @@
 
 package com.github.f4b6a3.ulid;
 
+import com.github.f4b6a3.ulid.creator.MonotonicUlidSpecCreator;
 import com.github.f4b6a3.ulid.creator.UlidSpecCreator;
-import com.github.f4b6a3.ulid.creator.impl.DefaultUlidSpecCreator;
-import com.github.f4b6a3.ulid.creator.impl.MonotonicUlidSpecCreator;
 
 public final class UlidCreator {
 
@@ -49,19 +48,19 @@ public final class UlidCreator {
 		return MonotonicCreatorHolder.INSTANCE.create(time);
 	}
 
-	public static DefaultUlidSpecCreator getDefaultCreator() {
-		return new DefaultUlidSpecCreator();
+	public static UlidSpecCreator getUlidSpecCreator() {
+		return new UlidSpecCreator();
 	}
 
-	public static MonotonicUlidSpecCreator getMonotonicCreator() {
+	public static UlidSpecCreator getMonotonicUlidSpecCreator() {
 		return new MonotonicUlidSpecCreator();
 	}
 
 	private static class DefaultCreatorHolder {
-		static final UlidSpecCreator INSTANCE = getDefaultCreator();
+		static final UlidSpecCreator INSTANCE = getUlidSpecCreator();
 	}
 
 	private static class MonotonicCreatorHolder {
-		static final UlidSpecCreator INSTANCE = getMonotonicCreator();
+		static final UlidSpecCreator INSTANCE = getMonotonicUlidSpecCreator();
 	}
 }
diff --git a/src/main/java/com/github/f4b6a3/ulid/exception/InvalidUlidException.java b/src/main/java/com/github/f4b6a3/ulid/creator/MonotonicUlidSpecCreator.java
similarity index 68%
rename from src/main/java/com/github/f4b6a3/ulid/exception/InvalidUlidException.java
rename to src/main/java/com/github/f4b6a3/ulid/creator/MonotonicUlidSpecCreator.java
index 516dc87..0d21d19 100644
--- a/src/main/java/com/github/f4b6a3/ulid/exception/InvalidUlidException.java
+++ b/src/main/java/com/github/f4b6a3/ulid/creator/MonotonicUlidSpecCreator.java
@@ -22,13 +22,27 @@
  * SOFTWARE.
  */
 
-package com.github.f4b6a3.ulid.exception;
+package com.github.f4b6a3.ulid.creator;
 
-public final class InvalidUlidException extends RuntimeException {
+import com.github.f4b6a3.ulid.Ulid;
 
-	private static final long serialVersionUID = 1L;
+public final class MonotonicUlidSpecCreator extends UlidSpecCreator {
 
-	public InvalidUlidException(String message) {
-		super(message);
+	private long lastTime;
+	private Ulid lastUlid;
+
+	@Override
+	public synchronized Ulid create(final long time) {
+		
+		if (time == this.lastTime) {
+			this.lastUlid = lastUlid.increment();
+		} else {
+			final byte[] random = new byte[10];
+			this.randomStrategy.nextBytes(random);
+			this.lastUlid = Ulid.of(time, random);
+		}
+
+		this.lastTime = time;
+		return this.lastUlid;
 	}
 }
diff --git a/src/main/java/com/github/f4b6a3/ulid/creator/UlidSpecCreator.java b/src/main/java/com/github/f4b6a3/ulid/creator/UlidSpecCreator.java
index b487a88..8dc532c 100644
--- a/src/main/java/com/github/f4b6a3/ulid/creator/UlidSpecCreator.java
+++ b/src/main/java/com/github/f4b6a3/ulid/creator/UlidSpecCreator.java
@@ -27,11 +27,10 @@ package com.github.f4b6a3.ulid.creator;
 import java.util.Random;
 
 import com.github.f4b6a3.ulid.Ulid;
+import com.github.f4b6a3.ulid.strategy.DefaultRandomStrategy;
 import com.github.f4b6a3.ulid.strategy.RandomStrategy;
-import com.github.f4b6a3.ulid.strategy.random.DefaultRandomStrategy;
-import com.github.f4b6a3.ulid.strategy.random.OtherRandomStrategy;
 
-public abstract class UlidSpecCreator {
+public class UlidSpecCreator {
 
 	protected RandomStrategy randomStrategy;
 
@@ -39,11 +38,15 @@ public abstract class UlidSpecCreator {
 		this.randomStrategy = new DefaultRandomStrategy();
 	}
 
-	public synchronized Ulid create() {
+	public Ulid create() {
 		return create(System.currentTimeMillis());
 	}
 
-	public abstract Ulid create(final long time);
+	public Ulid create(final long time) {
+		final byte[] random = new byte[10];
+		this.randomStrategy.nextBytes(random);
+		return Ulid.of(time, random);
+	}
 
 	/**
 	 * Replaces the default random strategy with another.
@@ -61,20 +64,4 @@ public abstract class UlidSpecCreator {
 		this.randomStrategy = randomStrategy;
 		return (T) this;
 	}
-
-	/**
-	 * Replaces the default random strategy with another that uses the input
-	 * {@link Random} instance.
-	 * 
-	 * It replaces the internal {@link DefaultRandomStrategy} with
-	 * {@link OtherRandomStrategy}.
-	 * 
-	 * @param random a random generator
-	 * @return {@link UlidSpecCreator}
-	 */
-	@SuppressWarnings("unchecked")
-	public synchronized <T extends UlidSpecCreator> T withRandomGenerator(Random random) {
-		this.randomStrategy = new OtherRandomStrategy(random);
-		return (T) this;
-	}
 }
diff --git a/src/main/java/com/github/f4b6a3/ulid/creator/impl/DefaultUlidSpecCreator.java b/src/main/java/com/github/f4b6a3/ulid/creator/impl/DefaultUlidSpecCreator.java
deleted file mode 100644
index 1c5803d..0000000
--- a/src/main/java/com/github/f4b6a3/ulid/creator/impl/DefaultUlidSpecCreator.java
+++ /dev/null
@@ -1,56 +0,0 @@
-/*
- * MIT License
- * 
- * Copyright (c) 2020 Fabio Lima
- * 
- * Permission is hereby granted, free of charge, to any person obtaining a copy
- * of this software and associated documentation files (the "Software"), to deal
- * in the Software without restriction, including without limitation the rights
- * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
- * copies of the Software, and to permit persons to whom the Software is
- * furnished to do so, subject to the following conditions:
- * 
- * The above copyright notice and this permission notice shall be included in all
- * copies or substantial portions of the Software.
- * 
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
- * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
- * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
- * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
- * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
- * SOFTWARE.
- */
-
-package com.github.f4b6a3.ulid.creator.impl;
-
-import com.github.f4b6a3.ulid.Ulid;
-import com.github.f4b6a3.ulid.creator.UlidSpecCreator;
-
-public final class DefaultUlidSpecCreator extends UlidSpecCreator {
-
-	@Override
-	public synchronized Ulid create(final long time) {
-
-		long msb = 0;
-		long lsb = 0;
-
-		final byte[] bytes = new byte[10];
-		this.randomStrategy.nextBytes(bytes);
-
-		msb |= time << 16;
-		msb |= (long) (bytes[0x0] & 0xff) << 8;
-		msb |= (long) (bytes[0x1] & 0xff);
-
-		lsb |= (long) (bytes[0x2] & 0xff) << 56;
-		lsb |= (long) (bytes[0x3] & 0xff) << 48;
-		lsb |= (long) (bytes[0x4] & 0xff) << 40;
-		lsb |= (long) (bytes[0x5] & 0xff) << 32;
-		lsb |= (long) (bytes[0x6] & 0xff) << 24;
-		lsb |= (long) (bytes[0x7] & 0xff) << 16;
-		lsb |= (long) (bytes[0x8] & 0xff) << 8;
-		lsb |= (long) (bytes[0x9] & 0xff);
-
-		return new Ulid(msb, lsb);
-	}
-}
diff --git a/src/main/java/com/github/f4b6a3/ulid/creator/impl/MonotonicUlidSpecCreator.java b/src/main/java/com/github/f4b6a3/ulid/creator/impl/MonotonicUlidSpecCreator.java
deleted file mode 100644
index 2ec38ce..0000000
--- a/src/main/java/com/github/f4b6a3/ulid/creator/impl/MonotonicUlidSpecCreator.java
+++ /dev/null
@@ -1,74 +0,0 @@
-/*
- * MIT License
- * 
- * Copyright (c) 2020 Fabio Lima
- * 
- * Permission is hereby granted, free of charge, to any person obtaining a copy
- * of this software and associated documentation files (the "Software"), to deal
- * in the Software without restriction, including without limitation the rights
- * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
- * copies of the Software, and to permit persons to whom the Software is
- * furnished to do so, subject to the following conditions:
- * 
- * The above copyright notice and this permission notice shall be included in all
- * copies or substantial portions of the Software.
- * 
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
- * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
- * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
- * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
- * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
- * SOFTWARE.
- */
-
-package com.github.f4b6a3.ulid.creator.impl;
-
-import com.github.f4b6a3.ulid.Ulid;
-import com.github.f4b6a3.ulid.creator.UlidSpecCreator;
-
-public final class MonotonicUlidSpecCreator extends UlidSpecCreator {
-
-	private long msb = 0;
-	private long lsb = 0;
-
-	private long lastTime;
-
-	// 0xffffffffffffffffL + 1 = 0x0000000000000000L
-	private static final long UNSIGNED_OVERFLOW = 0x0000000000000000L;
-
-	@Override
-	public synchronized Ulid create(final long time) {
-
-		// TODO: test
-		if (time == this.lastTime) {
-			if (++this.lsb == UNSIGNED_OVERFLOW) {
-				// Increment the random bits of the MSB
-				this.msb = (this.msb & 0xffffffffffff0000L) | ((this.msb + 1) & 0x000000000000ffffL);
-			}
-		} else {
-
-			this.msb = 0;
-			this.lsb = 0;
-
-			final byte[] bytes = new byte[10];
-			this.randomStrategy.nextBytes(bytes);
-
-			this.msb |= time << 16;
-			this.msb |= (long) (bytes[0x0] & 0xff) << 8;
-			this.msb |= (long) (bytes[0x1] & 0xff);
-
-			this.lsb |= (long) (bytes[0x2] & 0xff) << 56;
-			this.lsb |= (long) (bytes[0x3] & 0xff) << 48;
-			this.lsb |= (long) (bytes[0x4] & 0xff) << 40;
-			this.lsb |= (long) (bytes[0x5] & 0xff) << 32;
-			this.lsb |= (long) (bytes[0x6] & 0xff) << 24;
-			this.lsb |= (long) (bytes[0x7] & 0xff) << 16;
-			this.lsb |= (long) (bytes[0x8] & 0xff) << 8;
-			this.lsb |= (long) (bytes[0x9] & 0xff);
-		}
-
-		this.lastTime = time;
-		return new Ulid(msb, lsb);
-	}
-}
diff --git a/src/main/java/com/github/f4b6a3/ulid/strategy/random/DefaultRandomStrategy.java b/src/main/java/com/github/f4b6a3/ulid/strategy/DefaultRandomStrategy.java
similarity index 93%
rename from src/main/java/com/github/f4b6a3/ulid/strategy/random/DefaultRandomStrategy.java
rename to src/main/java/com/github/f4b6a3/ulid/strategy/DefaultRandomStrategy.java
index 5f54dc2..eec3935 100644
--- a/src/main/java/com/github/f4b6a3/ulid/strategy/random/DefaultRandomStrategy.java
+++ b/src/main/java/com/github/f4b6a3/ulid/strategy/DefaultRandomStrategy.java
@@ -22,13 +22,11 @@
  * SOFTWARE.
  */
 
-package com.github.f4b6a3.ulid.strategy.random;
+package com.github.f4b6a3.ulid.strategy;
 
 import java.security.SecureRandom;
 import java.util.Random;
 
-import com.github.f4b6a3.ulid.strategy.RandomStrategy;
-
 /**
  * It uses an instance of {@link java.security.SecureRandom}.
  */
diff --git a/src/main/java/com/github/f4b6a3/ulid/strategy/random/OtherRandomStrategy.java b/src/main/java/com/github/f4b6a3/ulid/strategy/random/OtherRandomStrategy.java
deleted file mode 100644
index be90846..0000000
--- a/src/main/java/com/github/f4b6a3/ulid/strategy/random/OtherRandomStrategy.java
+++ /dev/null
@@ -1,46 +0,0 @@
-/*
- * MIT License
- * 
- * Copyright (c) 2018-2020 Fabio Lima
- * 
- * Permission is hereby granted, free of charge, to any person obtaining a copy
- * of this software and associated documentation files (the "Software"), to deal
- * in the Software without restriction, including without limitation the rights
- * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
- * copies of the Software, and to permit persons to whom the Software is
- * furnished to do so, subject to the following conditions:
- * 
- * The above copyright notice and this permission notice shall be included in all
- * copies or substantial portions of the Software.
- * 
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
- * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
- * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
- * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
- * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
- * SOFTWARE.
- */
-
-package com.github.f4b6a3.ulid.strategy.random;
-
-import java.util.Random;
-
-import com.github.f4b6a3.ulid.strategy.RandomStrategy;
-
-/**
- * It uses an instance of {@link java.util.Random} injected by constructor.
- */
-public final class OtherRandomStrategy implements RandomStrategy {
-
-	private final Random random;
-
-	public OtherRandomStrategy(Random random) {
-		this.random = random;
-	}
-
-	@Override
-	public void nextBytes(byte[] bytes) {
-		this.random.nextBytes(bytes);
-	}
-}
diff --git a/src/main/java/com/github/f4b6a3/ulid/util/UlidValidator.java b/src/main/java/com/github/f4b6a3/ulid/util/UlidValidator.java
deleted file mode 100644
index ef2822d..0000000
--- a/src/main/java/com/github/f4b6a3/ulid/util/UlidValidator.java
+++ /dev/null
@@ -1,223 +0,0 @@
-/*
- * MIT License
- * 
- * Copyright (c) 2020 Fabio Lima
- * 
- * Permission is hereby granted, free of charge, to any person obtaining a copy
- * of this software and associated documentation files (the "Software"), to deal
- * in the Software without restriction, including without limitation the rights
- * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
- * copies of the Software, and to permit persons to whom the Software is
- * furnished to do so, subject to the following conditions:
- * 
- * The above copyright notice and this permission notice shall be included in all
- * copies or substantial portions of the Software.
- * 
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
- * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
- * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
- * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
- * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
- * SOFTWARE.
- */
-
-package com.github.f4b6a3.ulid.util;
-
-import com.github.f4b6a3.ulid.exception.InvalidUlidException;
-
-public final class UlidValidator {
-
-	protected static final int STRING_LENGTH = 26;
-
-	protected static final long[] BASE32_VALUES = new long[128];
-	static {
-		for (int i = 0; i < BASE32_VALUES.length; i++) {
-			BASE32_VALUES[i] = -1;
-		}
-		// Numbers
-		BASE32_VALUES['0'] = 0x00;
-		BASE32_VALUES['1'] = 0x01;
-		BASE32_VALUES['2'] = 0x02;
-		BASE32_VALUES['3'] = 0x03;
-		BASE32_VALUES['4'] = 0x04;
-		BASE32_VALUES['5'] = 0x05;
-		BASE32_VALUES['6'] = 0x06;
-		BASE32_VALUES['7'] = 0x07;
-		BASE32_VALUES['8'] = 0x08;
-		BASE32_VALUES['9'] = 0x09;
-		// Lower case
-		BASE32_VALUES['a'] = 0x0a;
-		BASE32_VALUES['b'] = 0x0b;
-		BASE32_VALUES['c'] = 0x0c;
-		BASE32_VALUES['d'] = 0x0d;
-		BASE32_VALUES['e'] = 0x0e;
-		BASE32_VALUES['f'] = 0x0f;
-		BASE32_VALUES['g'] = 0x10;
-		BASE32_VALUES['h'] = 0x11;
-		BASE32_VALUES['j'] = 0x12;
-		BASE32_VALUES['k'] = 0x13;
-		BASE32_VALUES['m'] = 0x14;
-		BASE32_VALUES['n'] = 0x15;
-		BASE32_VALUES['p'] = 0x16;
-		BASE32_VALUES['q'] = 0x17;
-		BASE32_VALUES['r'] = 0x18;
-		BASE32_VALUES['s'] = 0x19;
-		BASE32_VALUES['t'] = 0x1a;
-		BASE32_VALUES['v'] = 0x1b;
-		BASE32_VALUES['w'] = 0x1c;
-		BASE32_VALUES['x'] = 0x1d;
-		BASE32_VALUES['y'] = 0x1e;
-		BASE32_VALUES['z'] = 0x1f;
-		// Lower case OIL
-		BASE32_VALUES['o'] = 0x00;
-		BASE32_VALUES['i'] = 0x01;
-		BASE32_VALUES['l'] = 0x01;
-		// Upper case
-		BASE32_VALUES['A'] = 0x0a;
-		BASE32_VALUES['B'] = 0x0b;
-		BASE32_VALUES['C'] = 0x0c;
-		BASE32_VALUES['D'] = 0x0d;
-		BASE32_VALUES['E'] = 0x0e;
-		BASE32_VALUES['F'] = 0x0f;
-		BASE32_VALUES['G'] = 0x10;
-		BASE32_VALUES['H'] = 0x11;
-		BASE32_VALUES['J'] = 0x12;
-		BASE32_VALUES['K'] = 0x13;
-		BASE32_VALUES['M'] = 0x14;
-		BASE32_VALUES['N'] = 0x15;
-		BASE32_VALUES['P'] = 0x16;
-		BASE32_VALUES['Q'] = 0x17;
-		BASE32_VALUES['R'] = 0x18;
-		BASE32_VALUES['S'] = 0x19;
-		BASE32_VALUES['T'] = 0x1a;
-		BASE32_VALUES['V'] = 0x1b;
-		BASE32_VALUES['W'] = 0x1c;
-		BASE32_VALUES['X'] = 0x1d;
-		BASE32_VALUES['Y'] = 0x1e;
-		BASE32_VALUES['Z'] = 0x1f;
-		// Upper case OIL
-		BASE32_VALUES['O'] = 0x00;
-		BASE32_VALUES['I'] = 0x01;
-		BASE32_VALUES['L'] = 0x01;
-
-	}
-
-	private UlidValidator() {
-	}
-
-	/**
-	 * Checks if the string is a valid ULID.
-	 * 
-	 * A valid ULID string is a sequence of 26 characters from Crockford's base 32
-	 * alphabet.
-	 * 
-	 * It also checks if the timestamp is between 0 and 2^48-1.
-	 * 
-	 * <pre>
-	 * Examples of valid ULID strings:
-	 * - 0123456789ABCDEFGHJKMNPKRS (26 alphanumeric, case insensitive, except U)
-	 * - 0123456789ABCDEFGHIJKLMNOP (26 alphanumeric, case insensitive, including OIL, except U)
-	 * - 0123456789-ABCDEFGHJK-MNPKRS (26 alphanumeric, case insensitive, except U, with hyphens)
-	 * - 0123456789-ABCDEFGHIJ-KLMNOP (26 alphanumeric, case insensitive, including OIL, except U, with hyphens)
-	 * </pre>
-	 * 
-	 * @param ulid a ULID
-	 * @return boolean true if valid
-	 */
-	public static boolean isValid(String ulid) {
-		return (ulid != null && ulid.length() != 0 && isValidString(ulid.toCharArray()));
-	}
-
-	/**
-	 * Checks if the char array is a valid ULID.
-	 * 
-	 * A valid ULID string is a sequence of 26 characters from Crockford's base 32
-	 * alphabet.
-	 * 
-	 * It also checks if the timestamp is between 0 and 2^48-1.
-	 * 
-	 * <pre>
-	 * Examples of valid ULID strings:
-	 * - 0123456789ABCDEFGHJKMNPKRS (26 alphanumeric, case insensitive, except U)
-	 * - 0123456789ABCDEFGHIJKLMNOP (26 alphanumeric, case insensitive, including OIL, except U)
-	 * - 0123456789-ABCDEFGHJK-MNPKRS (26 alphanumeric, case insensitive, except U, with hyphens)
-	 * - 0123456789-ABCDEFGHIJ-KLMNOP (26 alphanumeric, case insensitive, including OIL, except U, with hyphens)
-	 * </pre>
-	 * 
-	 * @param ulid a ULID char array
-	 * @return boolean true if valid
-	 */
-	public static boolean isValid(char[] ulid) {
-		return (ulid != null && ulid.length != 0 && isValidString(ulid));
-	}
-
-	/**
-	 * Checks if the ULID string is valid.
-	 * 
-	 * See {@link UlidValidator#isValid(String)}.
-	 * 
-	 * @param ulid a ULID string
-	 * @throws InvalidUlidException if invalid
-	 */
-	public static void validate(String ulid) {
-		if (ulid == null || ulid.length() == 0 || !isValidString(ulid.toCharArray())) {
-			throw new InvalidUlidException("Invalid ULID: \"" + ulid + "\"");
-		}
-	}
-
-	/**
-	 * Checks if the ULID char array is valid.
-	 * 
-	 * See {@link UlidValidator#isValid(String)}.
-	 * 
-	 * @param ulid a ULID char array
-	 * @throws InvalidUlidException if invalid
-	 */
-	public static void validate(char[] ulid) {
-		if (ulid == null || ulid.length == 0 || !isValidString(ulid)) {
-			throw new InvalidUlidException("Invalid ULID: \"" + (ulid == null ? null : new String(ulid)) + "\"");
-		}
-	}
-
-	/**
-	 * Checks if the string is a valid ULID.
-	 * 
-	 * A valid ULID string is a sequence of 26 characters from Crockford's base 32
-	 * alphabet.
-	 * 
-	 * <pre>
-	 * Examples of valid ULID strings:
-	 * - 0123456789ABCDEFGHJKMNPKRS (26 alphanumeric, case insensitive, except U)
-	 * - 0123456789ABCDEFGHIJKLMNOP (26 alphanumeric, case insensitive, including OIL, except U)
-	 * - 0123456789-ABCDEFGHJK-MNPKRS (26 alphanumeric, case insensitive, except U, with hyphens)
-	 * - 0123456789-ABCDEFGHIJ-KLMNOP (26 alphanumeric, case insensitive, including OIL, except U, with hyphens)
-	 * </pre>
-	 * 
-	 * @param c a char array
-	 * @return boolean true if valid
-	 */
-	protected static boolean isValidString(final char[] c) {
-
-		// the two extra bits added by base-32 encoding must be zero
-		if ((BASE32_VALUES[c[0]] & 0b11000) != 0) {
-			return false; // overflow
-		}
-
-		int hyphen = 0;
-		for (int i = 0; i < c.length; i++) {
-			if (c[i] == '-') {
-				hyphen++;
-				continue;
-			}
-			if (c[i] == 'U' || c[i] == 'u') {
-				return false;
-			}
-			// ASCII codes: A-Z, 0-9, a-z
-			if (!((c[i] >= 0x41 && c[i] <= 0x5a) || (c[i] >= 0x30 && c[i] <= 0x39) || (c[i] >= 0x61 && c[i] <= 0x7a))) {
-				return false;
-			}
-		}
-		return (c.length - hyphen) == STRING_LENGTH;
-	}
-}
diff --git a/src/test/java/com/github/f4b6a3/ulid/TestSuite.java b/src/test/java/com/github/f4b6a3/ulid/TestSuite.java
index 3a4dd5f..fbbae66 100644
--- a/src/test/java/com/github/f4b6a3/ulid/TestSuite.java
+++ b/src/test/java/com/github/f4b6a3/ulid/TestSuite.java
@@ -3,22 +3,13 @@ package com.github.f4b6a3.ulid;
 import org.junit.runner.RunWith;
 import org.junit.runners.Suite;
 
+import com.github.f4b6a3.ulid.creator.MonotonicUlidSpecCreatorTest;
 import com.github.f4b6a3.ulid.creator.UlidSpecCreatorTest;
-import com.github.f4b6a3.ulid.ulid.UlidCreatorDefaultTest;
-import com.github.f4b6a3.ulid.ulid.UlidCreatorDefaultStringTest;
-import com.github.f4b6a3.ulid.ulid.UlidCreatorMonotonicTest;
-import com.github.f4b6a3.ulid.ulid.UlidCreatorMonotonicStringTest;
-import com.github.f4b6a3.ulid.util.UlidValidatorTest;
-import com.github.f4b6a3.ulid.util.internal.UlidTest;
 
 @RunWith(Suite.class)
 @Suite.SuiteClasses({
-	UlidCreatorDefaultTest.class,
-	UlidCreatorDefaultStringTest.class,
-	UlidCreatorMonotonicTest.class,
-	UlidCreatorMonotonicStringTest.class,
+	MonotonicUlidSpecCreatorTest.class,
 	UlidSpecCreatorTest.class,
-	UlidValidatorTest.class,
 	UlidTest.class,
 })
 
diff --git a/src/test/java/com/github/f4b6a3/ulid/util/internal/UlidTest.java b/src/test/java/com/github/f4b6a3/ulid/UlidTest.java
similarity index 78%
rename from src/test/java/com/github/f4b6a3/ulid/util/internal/UlidTest.java
rename to src/test/java/com/github/f4b6a3/ulid/UlidTest.java
index 85dd723..ed2a9b4 100644
--- a/src/test/java/com/github/f4b6a3/ulid/util/internal/UlidTest.java
+++ b/src/test/java/com/github/f4b6a3/ulid/UlidTest.java
@@ -1,6 +1,8 @@
-package com.github.f4b6a3.ulid.util.internal;
+package com.github.f4b6a3.ulid;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
 
 import java.util.Random;
 import java.util.UUID;
@@ -11,7 +13,7 @@ import com.github.f4b6a3.ulid.Ulid;
 
 public class UlidTest {
 
-	private static final int DEFAULT_LOOP_MAX = 100_000;
+	private static final int DEFAULT_LOOP_MAX = 10_000;
 
 	protected static final char[] ALPHABET_CROCKFORD = "0123456789ABCDEFGHJKMNPQRSTVWXYZ".toCharArray();
 	protected static final char[] ALPHABET_JAVA = "0123456789abcdefghijklmnopqrstuv".toCharArray(); // Long.parseUnsignedLong()
@@ -191,6 +193,40 @@ public class UlidTest {
 
 		return r1 + r2;
 	}
+	
+	@Test
+	public void isValidString() {
+
+		String ulid = null; // Null
+		assertFalse("Null ULID should be invalid.", Ulid.isValidString(ulid));
+
+		ulid = ""; // length: 0
+		assertFalse("ULID with empty string should be invalid .", Ulid.isValidString(ulid));
+
+		ulid = "0123456789ABCDEFGHJKMNPQRS"; // All upper case
+		assertTrue("ULID in upper case should valid.", Ulid.isValidString(ulid));
+
+		ulid = "0123456789abcdefghjklmnpqr"; // All lower case
+		assertTrue("ULID in lower case should be valid.", Ulid.isValidString(ulid));
+
+		ulid = "0123456789AbCdEfGhJkMnPqRs"; // Mixed case
+		assertTrue("Ulid in upper and lower case should valid.", Ulid.isValidString(ulid));
+
+		ulid = "0123456789ABCDEFGHJKLMNPQ"; // length: 25
+		assertFalse("ULID length lower than 26 should be invalid.", Ulid.isValidString(ulid));
+
+		ulid = "0123456789ABCDEFGHJKMNPQZZZ"; // length: 27
+		assertFalse("ULID length greater than 26 should be invalid.", Ulid.isValidString(ulid));
+
+		ulid = "u123456789ABCDEFGHJKMNPQRS"; // Letter u
+		assertFalse("ULID with 'u' or 'U' should be invalid.", Ulid.isValidString(ulid));
+
+		ulid = "#123456789ABCDEFGHJKMNPQRS"; // Special char
+		assertFalse("ULID with special chars should be invalid.", Ulid.isValidString(ulid));
+
+		ulid = "8ZZZZZZZZZABCDEFGHJKMNPQRS"; // timestamp > (2^48)-1
+		assertFalse("ULID with timestamp greater than (2^48)-1 should be invalid.", Ulid.isValidString(ulid));
+	}
 
 	private static String transliterate(String string, char[] alphabet1, char[] alphabet2) {
 		char[] output = string.toCharArray();
diff --git a/src/test/java/com/github/f4b6a3/ulid/UniquenessTest.java b/src/test/java/com/github/f4b6a3/ulid/UniquenessTest.java
index 9516985..9899d33 100644
--- a/src/test/java/com/github/f4b6a3/ulid/UniquenessTest.java
+++ b/src/test/java/com/github/f4b6a3/ulid/UniquenessTest.java
@@ -117,7 +117,7 @@ public class UniquenessTest {
 	}
 
 	public static void execute(boolean verbose, int threadCount, int requestCount) {
-		UlidSpecCreator creator = UlidCreator.getMonotonicCreator();
+		UlidSpecCreator creator = UlidCreator.getMonotonicUlidSpecCreator();
 
 		UniquenessTest test = new UniquenessTest(threadCount, requestCount, creator, verbose);
 		test.start();
diff --git a/src/test/java/com/github/f4b6a3/ulid/creator/AbstractUlidSpecCreatorTest.java b/src/test/java/com/github/f4b6a3/ulid/creator/AbstractUlidSpecCreatorTest.java
new file mode 100644
index 0000000..09b70d8
--- /dev/null
+++ b/src/test/java/com/github/f4b6a3/ulid/creator/AbstractUlidSpecCreatorTest.java
@@ -0,0 +1,53 @@
+package com.github.f4b6a3.ulid.creator;
+
+import java.util.HashSet;
+import java.util.Random;
+import java.util.Set;
+import java.util.UUID;
+
+public abstract class AbstractUlidSpecCreatorTest {
+
+	protected static final int DEFAULT_LOOP_MAX = 10_000;
+
+	protected static final String DUPLICATE_UUID_MSG = "A duplicate ULID was created.";
+
+	protected static final int THREAD_TOTAL = availableProcessors();
+
+	protected static final Random RANDOM = new Random();
+
+	protected static final long TIME_MASK = 0x0000ffffffffffffL;
+
+	private static int availableProcessors() {
+		int processors = Runtime.getRuntime().availableProcessors();
+		if (processors < 4) {
+			processors = 4;
+		}
+		return processors;
+	}
+
+	protected static class TestThread extends Thread {
+
+		public static Set<UUID> hashSet = new HashSet<>();
+		private UlidSpecCreator creator;
+		private int loopLimit;
+
+		public TestThread(UlidSpecCreator creator, int loopLimit) {
+			this.creator = creator;
+			this.loopLimit = loopLimit;
+		}
+
+		public static void clearHashSet() {
+			hashSet = new HashSet<>();
+		}
+
+		@Override
+		public void run() {
+			long timestamp = System.currentTimeMillis();
+			for (int i = 0; i < loopLimit; i++) {
+				synchronized (hashSet) {
+					hashSet.add(creator.create(timestamp).toUuid());
+				}
+			}
+		}
+	}
+}
diff --git a/src/test/java/com/github/f4b6a3/ulid/ulid/UlidCreatorMonotonicTest.java b/src/test/java/com/github/f4b6a3/ulid/creator/MonotonicUlidSpecCreatorTest.java
similarity index 63%
rename from src/test/java/com/github/f4b6a3/ulid/ulid/UlidCreatorMonotonicTest.java
rename to src/test/java/com/github/f4b6a3/ulid/creator/MonotonicUlidSpecCreatorTest.java
index 80efe8d..c8e1f2b 100644
--- a/src/test/java/com/github/f4b6a3/ulid/ulid/UlidCreatorMonotonicTest.java
+++ b/src/test/java/com/github/f4b6a3/ulid/creator/MonotonicUlidSpecCreatorTest.java
@@ -1,4 +1,4 @@
-package com.github.f4b6a3.ulid.ulid;
+package com.github.f4b6a3.ulid.creator;
 
 import org.junit.Test;
 
@@ -6,12 +6,11 @@ import com.github.f4b6a3.ulid.Ulid;
 import com.github.f4b6a3.ulid.UlidCreator;
 
 import static org.junit.Assert.*;
+
 import java.util.Arrays;
 import java.util.HashSet;
 
-public class UlidCreatorMonotonicTest {
-
-	private static final int DEFAULT_LOOP_MAX = 100_000;
+public class MonotonicUlidSpecCreatorTest extends AbstractUlidSpecCreatorTest {
 
 	@Test
 	public void testGetUlid() {
@@ -68,4 +67,34 @@ public class UlidCreatorMonotonicTest {
 			assertEquals("The ULID list is not ordered", list[i], other[i]);
 		}
 	}
+
+	@Test
+	public void testGetMonotonicUlidInParallel() throws InterruptedException {
+
+		Thread[] threads = new Thread[THREAD_TOTAL];
+		TestThread.clearHashSet();
+
+		// Instantiate and start many threads
+		for (int i = 0; i < THREAD_TOTAL; i++) {
+			threads[i] = new TestThread(UlidCreator.getMonotonicUlidSpecCreator(), DEFAULT_LOOP_MAX);
+			threads[i].start();
+		}
+
+		// Wait all the threads to finish
+		for (Thread thread : threads) {
+			thread.join();
+		}
+
+		// Check if the quantity of unique UUIDs is correct
+		assertEquals(DUPLICATE_UUID_MSG, TestThread.hashSet.size(), (DEFAULT_LOOP_MAX * THREAD_TOTAL));
+	}
+
+	@Test
+	public void testGetMonotonicUlidTime() {
+		for (int i = 0; i < 100; i++) {
+			long time = RANDOM.nextLong() & TIME_MASK;
+			Ulid ulid = UlidCreator.getMonotonicUlid(time);
+			assertEquals(time, ulid.getTime());
+		}
+	}
 }
diff --git a/src/test/java/com/github/f4b6a3/ulid/creator/UlidSpecCreatorTest.java b/src/test/java/com/github/f4b6a3/ulid/creator/UlidSpecCreatorTest.java
index 154df73..4cf38c4 100644
--- a/src/test/java/com/github/f4b6a3/ulid/creator/UlidSpecCreatorTest.java
+++ b/src/test/java/com/github/f4b6a3/ulid/creator/UlidSpecCreatorTest.java
@@ -1,10 +1,5 @@
 package com.github.f4b6a3.ulid.creator;
 
-import java.util.HashSet;
-import java.util.Random;
-import java.util.Set;
-import java.util.UUID;
-
 import org.junit.Test;
 
 import com.github.f4b6a3.ulid.Ulid;
@@ -12,24 +7,75 @@ import com.github.f4b6a3.ulid.UlidCreator;
 
 import static org.junit.Assert.*;
 
-public class UlidSpecCreatorTest {
+import java.util.HashSet;
 
-	private static final int DEFAULT_LOOP_MAX = 100_000;
+public class UlidSpecCreatorTest extends AbstractUlidSpecCreatorTest {
 
-	protected static final String DUPLICATE_UUID_MSG = "A duplicate ULID was created.";
+	@Test
+	public void testGetUlid() {
+		Ulid[] list = new Ulid[DEFAULT_LOOP_MAX];
 
-	protected static final int THREAD_TOTAL = availableProcessors();
+		long startTime = System.currentTimeMillis();
 
-	private static final Random RANDOM = new Random();
-
-	private static final long TIME_MASK = 0x0000ffffffffffffL;
-
-	private static int availableProcessors() {
-		int processors = Runtime.getRuntime().availableProcessors();
-		if (processors < 4) {
-			processors = 4;
+		for (int i = 0; i < DEFAULT_LOOP_MAX; i++) {
+			list[i] = UlidCreator.getUlid();
 		}
-		return processors;
+
+		long endTime = System.currentTimeMillis();
+
+		checkNullOrInvalid(list);
+		checkUniqueness(list);
+		checkCreationTime(list, startTime, endTime);
+	}
+
+	private void checkNullOrInvalid(Ulid[] list) {
+		for (Ulid ulid : list) {
+			assertNotNull("ULID is null", ulid);
+		}
+	}
+
+	private void checkUniqueness(Ulid[] list) {
+
+		HashSet<Ulid> set = new HashSet<>();
+
+		for (Ulid ulid : list) {
+			assertTrue(String.format("ULID is duplicated %s", ulid), set.add(ulid));
+		}
+
+		assertEquals("There are duplicated ULIDs", set.size(), list.length);
+	}
+
+	private void checkCreationTime(Ulid[] list, long startTime, long endTime) {
+
+		assertTrue("Start time was after end time", startTime <= endTime);
+
+		for (Ulid ulid : list) {
+			long creationTime = ulid.getTime();
+			assertTrue("Creation time was before start time " + creationTime + " " + startTime,
+					creationTime >= startTime);
+			assertTrue("Creation time was after end time", creationTime <= endTime);
+		}
+	}
+
+	@Test
+	public void testGetUlidInParallel() throws InterruptedException {
+
+		Thread[] threads = new Thread[THREAD_TOTAL];
+		TestThread.clearHashSet();
+
+		// Instantiate and start many threads
+		for (int i = 0; i < THREAD_TOTAL; i++) {
+			threads[i] = new TestThread(UlidCreator.getUlidSpecCreator(), DEFAULT_LOOP_MAX);
+			threads[i].start();
+		}
+
+		// Wait all the threads to finish
+		for (Thread thread : threads) {
+			thread.join();
+		}
+
+		// Check if the quantity of unique UUIDs is correct
+		assertEquals(DUPLICATE_UUID_MSG, TestThread.hashSet.size(), (DEFAULT_LOOP_MAX * THREAD_TOTAL));
 	}
 
 	@Test
@@ -40,81 +86,4 @@ public class UlidSpecCreatorTest {
 			assertEquals(time, ulid.getTime());
 		}
 	}
-
-	@Test
-	public void testGetMonotonicUlidTime() {
-		for (int i = 0; i < 100; i++) {
-			long time = RANDOM.nextLong() & TIME_MASK;
-			Ulid ulid = UlidCreator.getMonotonicUlid(time);
-			assertEquals(time, ulid.getTime());
-		}
-	}
-
-	@Test
-	public void testGetDefaultUlidInParallel() throws InterruptedException {
-
-		Thread[] threads = new Thread[THREAD_TOTAL];
-		TestThread.clearHashSet();
-
-		// Instantiate and start many threads
-		for (int i = 0; i < THREAD_TOTAL; i++) {
-			threads[i] = new TestThread(UlidCreator.getDefaultCreator(), DEFAULT_LOOP_MAX);
-			threads[i].start();
-		}
-
-		// Wait all the threads to finish
-		for (Thread thread : threads) {
-			thread.join();
-		}
-
-		// Check if the quantity of unique UUIDs is correct
-		assertEquals(DUPLICATE_UUID_MSG, TestThread.hashSet.size(), (DEFAULT_LOOP_MAX * THREAD_TOTAL));
-	}
-
-	@Test
-	public void testGetMonotonicUlidInParallel() throws InterruptedException {
-
-		Thread[] threads = new Thread[THREAD_TOTAL];
-		TestThread.clearHashSet();
-
-		// Instantiate and start many threads
-		for (int i = 0; i < THREAD_TOTAL; i++) {
-			threads[i] = new TestThread(UlidCreator.getMonotonicCreator(), DEFAULT_LOOP_MAX);
-			threads[i].start();
-		}
-
-		// Wait all the threads to finish
-		for (Thread thread : threads) {
-			thread.join();
-		}
-
-		// Check if the quantity of unique UUIDs is correct
-		assertEquals(DUPLICATE_UUID_MSG, TestThread.hashSet.size(), (DEFAULT_LOOP_MAX * THREAD_TOTAL));
-	}
-
-	public static class TestThread extends Thread {
-
-		public static Set<UUID> hashSet = new HashSet<>();
-		private UlidSpecCreator creator;
-		private int loopLimit;
-
-		public TestThread(UlidSpecCreator creator, int loopLimit) {
-			this.creator = creator;
-			this.loopLimit = loopLimit;
-		}
-
-		public static void clearHashSet() {
-			hashSet = new HashSet<>();
-		}
-
-		@Override
-		public void run() {
-			long timestamp = System.currentTimeMillis();
-			for (int i = 0; i < loopLimit; i++) {
-				synchronized (hashSet) {
-					hashSet.add(creator.create(timestamp).toUuid());
-				}
-			}
-		}
-	}
 }
diff --git a/src/test/java/com/github/f4b6a3/ulid/ulid/UlidCreatorDefaultStringTest.java b/src/test/java/com/github/f4b6a3/ulid/ulid/UlidCreatorDefaultStringTest.java
deleted file mode 100644
index ea7d23e..0000000
--- a/src/test/java/com/github/f4b6a3/ulid/ulid/UlidCreatorDefaultStringTest.java
+++ /dev/null
@@ -1,65 +0,0 @@
-package com.github.f4b6a3.ulid.ulid;
-
-import org.junit.Test;
-
-import com.github.f4b6a3.ulid.Ulid;
-import com.github.f4b6a3.ulid.UlidCreator;
-import com.github.f4b6a3.ulid.util.UlidValidator;
-
-import static org.junit.Assert.*;
-import java.util.HashSet;
-
-public class UlidCreatorDefaultStringTest {
-
-	private static final int ULID_LENGTH = 26;
-	private static final int DEFAULT_LOOP_MAX = 100_000;
-
-	@Test
-	public void testGetUlid() {
-		String[] list = new String[DEFAULT_LOOP_MAX];
-
-		long startTime = System.currentTimeMillis();
-
-		for (int i = 0; i < DEFAULT_LOOP_MAX; i++) {
-			list[i] = UlidCreator.getUlid().toString();
-		}
-
-		long endTime = System.currentTimeMillis();
-
-		checkNullOrInvalid(list);
-		checkUniqueness(list);
-		checkCreationTime(list, startTime, endTime);
-	}
-
-	private void checkNullOrInvalid(String[] list) {
-		for (String ulid : list) {
-			assertNotNull("ULID is null", ulid);
-			assertTrue("ULID is empty", !ulid.isEmpty());
-			assertEquals("ULID length is wrong", ULID_LENGTH, ulid.length());
-			assertTrue("ULID is not valid", UlidValidator.isValid(ulid));
-		}
-	}
-
-	private void checkUniqueness(String[] list) {
-
-		HashSet<String> set = new HashSet<>();
-
-		for (String ulid : list) {
-			assertTrue(String.format("ULID is duplicated %s", ulid), set.add(ulid));
-		}
-
-		assertEquals("There are duplicated ULIDs", set.size(), list.length);
-	}
-
-	private void checkCreationTime(String[] list, long startTime, long endTime) {
-
-		assertTrue("Start time was after end time", startTime <= endTime);
-
-		for (String ulid : list) {
-			long creationTime = Ulid.of(ulid).getTime();
-			assertTrue("Creation time was before start time " + creationTime + " " + startTime,
-					creationTime >= startTime);
-			assertTrue("Creation time was after end time", creationTime <= endTime);
-		}
-	}
-}
diff --git a/src/test/java/com/github/f4b6a3/ulid/ulid/UlidCreatorDefaultTest.java b/src/test/java/com/github/f4b6a3/ulid/ulid/UlidCreatorDefaultTest.java
deleted file mode 100644
index 565d991..0000000
--- a/src/test/java/com/github/f4b6a3/ulid/ulid/UlidCreatorDefaultTest.java
+++ /dev/null
@@ -1,60 +0,0 @@
-package com.github.f4b6a3.ulid.ulid;
-
-import org.junit.Test;
-
-import com.github.f4b6a3.ulid.Ulid;
-import com.github.f4b6a3.ulid.UlidCreator;
-
-import static org.junit.Assert.*;
-import java.util.HashSet;
-
-public class UlidCreatorDefaultTest {
-
-	private static final int DEFAULT_LOOP_MAX = 100_000;
-
-	@Test
-	public void testGetUlid() {
-		Ulid[] list = new Ulid[DEFAULT_LOOP_MAX];
-
-		long startTime = System.currentTimeMillis();
-
-		for (int i = 0; i < DEFAULT_LOOP_MAX; i++) {
-			list[i] = UlidCreator.getUlid();
-		}
-
-		long endTime = System.currentTimeMillis();
-
-		checkNullOrInvalid(list);
-		checkUniqueness(list);
-		checkCreationTime(list, startTime, endTime);
-	}
-
-	private void checkNullOrInvalid(Ulid[] list) {
-		for (Ulid ulid : list) {
-			assertNotNull("ULID is null", ulid);
-		}
-	}
-
-	private void checkUniqueness(Ulid[] list) {
-
-		HashSet<Ulid> set = new HashSet<>();
-
-		for (Ulid ulid : list) {
-			assertTrue(String.format("ULID is duplicated %s", ulid), set.add(ulid));
-		}
-
-		assertEquals("There are duplicated ULIDs", set.size(), list.length);
-	}
-
-	private void checkCreationTime(Ulid[] list, long startTime, long endTime) {
-
-		assertTrue("Start time was after end time", startTime <= endTime);
-
-		for (Ulid ulid : list) {
-			long creationTime = ulid.getTime();
-			assertTrue("Creation time was before start time " + creationTime + " " + startTime,
-					creationTime >= startTime);
-			assertTrue("Creation time was after end time", creationTime <= endTime);
-		}
-	}
-}
diff --git a/src/test/java/com/github/f4b6a3/ulid/ulid/UlidCreatorMonotonicStringTest.java b/src/test/java/com/github/f4b6a3/ulid/ulid/UlidCreatorMonotonicStringTest.java
deleted file mode 100644
index 57ecdda..0000000
--- a/src/test/java/com/github/f4b6a3/ulid/ulid/UlidCreatorMonotonicStringTest.java
+++ /dev/null
@@ -1,76 +0,0 @@
-package com.github.f4b6a3.ulid.ulid;
-
-import org.junit.Test;
-
-import com.github.f4b6a3.ulid.Ulid;
-import com.github.f4b6a3.ulid.UlidCreator;
-import com.github.f4b6a3.ulid.util.UlidValidator;
-
-import static org.junit.Assert.*;
-import java.util.Arrays;
-import java.util.HashSet;
-
-public class UlidCreatorMonotonicStringTest {
-
-	private static final int ULID_LENGTH = 26;
-	private static final int DEFAULT_LOOP_MAX = 100_000;
-
-	@Test
-	public void testGetUlid() {
-		String[] list = new String[DEFAULT_LOOP_MAX];
-
-		long startTime = System.currentTimeMillis();
-
-		for (int i = 0; i < DEFAULT_LOOP_MAX; i++) {
-			list[i] = UlidCreator.getMonotonicUlid().toString();
-		}
-
-		long endTime = System.currentTimeMillis();
-
-		checkNullOrInvalid(list);
-		checkUniqueness(list);
-		checkOrdering(list);
-		checkCreationTime(list, startTime, endTime);
-	}
-
-	private void checkNullOrInvalid(String[] list) {
-		for (String ulid : list) {
-			assertNotNull("ULID is null", ulid);
-			assertTrue("ULID is empty", !ulid.isEmpty());
-			assertEquals("ULID length is wrong", ULID_LENGTH, ulid.length());
-			assertTrue("ULID is not valid", UlidValidator.isValid(ulid));
-		}
-	}
-
-	private void checkUniqueness(String[] list) {
-
-		HashSet<String> set = new HashSet<>();
-
-		for (String ulid : list) {
-			assertTrue(String.format("ULID is duplicated %s", ulid), set.add(ulid));
-		}
-
-		assertEquals("There are duplicated ULIDs", set.size(), list.length);
-	}
-
-	private void checkCreationTime(String[] list, long startTime, long endTime) {
-
-		assertTrue("Start time was after end time", startTime <= endTime);
-
-		for (String ulid : list) {
-			long creationTime = Ulid.of(ulid).getTime();
-			assertTrue("Creation time was before start time " + creationTime + " " + startTime,
-					creationTime >= startTime);
-			assertTrue("Creation time was after end time", creationTime <= endTime);
-		}
-	}
-
-	private void checkOrdering(String[] list) {
-		String[] other = Arrays.copyOf(list, list.length);
-		Arrays.sort(other);
-
-		for (int i = 0; i < list.length; i++) {
-			assertEquals("The ULID list is not ordered", list[i], other[i]);
-		}
-	}
-}
diff --git a/src/test/java/com/github/f4b6a3/ulid/util/UlidValidatorTest.java b/src/test/java/com/github/f4b6a3/ulid/util/UlidValidatorTest.java
deleted file mode 100644
index 05e917c..0000000
--- a/src/test/java/com/github/f4b6a3/ulid/util/UlidValidatorTest.java
+++ /dev/null
@@ -1,47 +0,0 @@
-package com.github.f4b6a3.ulid.util;
-
-import static org.junit.Assert.*;
-
-import org.junit.Test;
-
-import com.github.f4b6a3.ulid.util.UlidValidator;
-
-public class UlidValidatorTest {
-
-	@Test
-	public void testIsValid() {
-
-		String ulid = null; // Null
-		assertFalse("Null ULID should be invalid.", UlidValidator.isValid(ulid));
-
-		ulid = ""; // length: 0
-		assertFalse("ULID with empty string should be invalid .", UlidValidator.isValid(ulid));
-
-		ulid = "0123456789ABCDEFGHJKMNPQRS"; // All upper case
-		assertTrue("ULID in upper case should valid.", UlidValidator.isValid(ulid));
-
-		ulid = "0123456789abcdefghjklmnpqr"; // All lower case
-		assertTrue("ULID in lower case should be valid.", UlidValidator.isValid(ulid));
-
-		ulid = "0123456789AbCdEfGhJkMnPqRs"; // Mixed case
-		assertTrue("Ulid in upper and lower case should valid.", UlidValidator.isValid(ulid));
-
-		ulid = "0123456789ABCDEFGHJKLMNPQ"; // length: 25
-		assertFalse("ULID length lower than 26 should be invalid.", UlidValidator.isValid(ulid));
-
-		ulid = "0123456789ABCDEFGHJKMNPQZZZ"; // length: 27
-		assertFalse("ULID length greater than 26 should be invalid.", UlidValidator.isValid(ulid));
-
-		ulid = "u123456789ABCDEFGHJKMNPQRS"; // Letter u
-		assertFalse("ULID with 'u' or 'U' should be invalid.", UlidValidator.isValid(ulid));
-
-		ulid = "#123456789ABCDEFGHJKMNPQRS"; // Special char
-		assertFalse("ULID with special chars should be invalid.", UlidValidator.isValid(ulid));
-
-		ulid = "01234-56789-ABCDEFGHJKM---NPQRS"; // Hyphens
-		assertTrue("ULID with hiphens should be valid.", UlidValidator.isValid(ulid));
-
-		ulid = "8ZZZZZZZZZABCDEFGHJKMNPQRS"; // timestamp > (2^48)-1
-		assertFalse("ULID with timestamp greater than (2^48)-1 should be invalid.", UlidValidator.isValid(ulid));
-	}
-}