diff --git a/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/Identifier.java b/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/Identifier.java
index bcc2a37..8c555d7 100644
--- a/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/Identifier.java
+++ b/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/Identifier.java
@@ -7,6 +7,10 @@
 
 public final class Identifier {
 
+	private static final int MAX_ID_LENGTH = 19;
+
+	private static final String ZERO_ID = "0";
+
 	private final Long id;
 
 	private final Anchor anchor;
@@ -48,12 +52,12 @@ public Optional<Anchor> getAnchor() {
 
 	public static boolean isId(String str) {
 
-		if (str.equals("0")) {
+		if (ZERO_ID.equals(str)) {
 			throw new IllegalArgumentException("id should be greater than 0");
 		}
 
 		int length = str.length();
-		if (length > 19) {
+		if (length > MAX_ID_LENGTH) {
 			return false;
 		}
 
diff --git a/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/MediaReference.java b/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/MediaReference.java
index fb2c746..fef465b 100644
--- a/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/MediaReference.java
+++ b/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/MediaReference.java
@@ -71,17 +71,13 @@ protected Builder(MediaReference media) {
 		}
 
 		public Builder mediaId(String mediaId) {
-			if (mediaId == null) {
-				throw new NullPointerException("mediaId is null");
-			}
+			Objects.requireNonNull(mediaId, "mediaId is null");
 			this.mediaId = mediaId;
 			return this;
 		}
 
 		public Builder usedBy(String usedBy) {
-			if (usedBy == null) {
-				throw new NullPointerException("usedBy is null");
-			}
+			Objects.requireNonNull(usedBy, "usedBy is null");
 			this.usedBy = usedBy;
 			return this;
 		}
diff --git a/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/query/CursorBasedQuery.java b/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/query/CursorBasedQuery.java
index c2c0003..41978aa 100644
--- a/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/query/CursorBasedQuery.java
+++ b/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/query/CursorBasedQuery.java
@@ -51,7 +51,9 @@ public final boolean equals(Object o) {
 	}
 
 	/**
-	 * @see <a href="https://www.artima.com/articles/how-to-write-an-equality-method-in-java">How to Write an Equality Method in Java</a>
+	 * @see <a href="https://www.artima.com/articles/how-to-write-an-equality-method-in-java">
+	 * 	How to Write an Equality Method in Java
+	 * </a>
 	 */
 	@Override
 	public boolean canEqual(Object other) {
diff --git a/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/query/Query.java b/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/query/Query.java
index 9c181a4..07e1677 100644
--- a/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/query/Query.java
+++ b/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/query/Query.java
@@ -60,7 +60,9 @@ public boolean equals(Object o) {
 	}
 
 	/**
-	 * @see <a href="https://www.artima.com/articles/how-to-write-an-equality-method-in-java">How to Write an Equality Method in Java</a>
+	 * @see <a href="https://www.artima.com/articles/how-to-write-an-equality-method-in-java">
+	 * 	How to Write an Equality Method in Java
+	 * </a>
 	 */
 	public boolean canEqual(Object other) {
 		return (other instanceof Query);
diff --git a/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/query/SubTreeQuery.java b/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/query/SubTreeQuery.java
index be6410f..f1b2c9d 100644
--- a/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/query/SubTreeQuery.java
+++ b/src/main/java/com/sitepark/ies/contentrepository/core/domain/entity/query/SubTreeQuery.java
@@ -46,7 +46,9 @@ public final boolean equals(Object o) {
 	}
 
 	/**
-	 * @see <a href="https://www.artima.com/articles/how-to-write-an-equality-method-in-java">How to Write an Equality Method in Java</a>
+	 * @see <a href="https://www.artima.com/articles/how-to-write-an-equality-method-in-java">
+	 * 	How to Write an Equality Method in Java
+	 * </a>
 	 */
 	@Override
 	public boolean canEqual(Object other) {
diff --git a/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/EntityBulkExecutionTest.java b/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/EntityBulkExecutionTest.java
index a0f669a..c049f07 100644
--- a/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/EntityBulkExecutionTest.java
+++ b/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/EntityBulkExecutionTest.java
@@ -16,6 +16,7 @@
 
 import nl.jqno.equalsverifier.EqualsVerifier;
 
+@SuppressWarnings("PMD.AvoidDuplicateLiterals")
 class EntityBulkExecutionTest {
 
 	@Test
diff --git a/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/EntityLockTest.java b/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/EntityLockTest.java
index e4d33ee..e2bb9a6 100644
--- a/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/EntityLockTest.java
+++ b/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/EntityLockTest.java
@@ -13,6 +13,7 @@
 
 import nl.jqno.equalsverifier.EqualsVerifier;
 
+@SuppressWarnings("PMD.AvoidDuplicateLiterals")
 class EntityLockTest {
 
 	@Test
diff --git a/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/HistoryEntryTest.java b/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/HistoryEntryTest.java
index 02886da..e58e717 100644
--- a/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/HistoryEntryTest.java
+++ b/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/HistoryEntryTest.java
@@ -11,6 +11,7 @@
 
 import nl.jqno.equalsverifier.EqualsVerifier;
 
+@SuppressWarnings("PMD.AvoidDuplicateLiterals")
 class HistoryEntryTest {
 
 	@Test
diff --git a/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/IdentifierTest.java b/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/IdentifierTest.java
index fccca5c..967ea24 100644
--- a/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/IdentifierTest.java
+++ b/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/IdentifierTest.java
@@ -9,6 +9,7 @@
 
 import nl.jqno.equalsverifier.EqualsVerifier;
 
+@SuppressWarnings("PMD.AvoidDuplicateLiterals")
 class IdentifierTest {
 
 	@Test
diff --git a/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/MediaReferenceTest.java b/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/MediaReferenceTest.java
index a0d644a..e943d09 100644
--- a/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/MediaReferenceTest.java
+++ b/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/MediaReferenceTest.java
@@ -7,6 +7,7 @@
 
 import nl.jqno.equalsverifier.EqualsVerifier;
 
+@SuppressWarnings("PMD.AvoidDuplicateLiterals")
 class MediaReferenceTest {
 
 	@Test
diff --git a/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/RecycleBinItemTest.java b/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/RecycleBinItemTest.java
index b6a3c2d..f2378f3 100644
--- a/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/RecycleBinItemTest.java
+++ b/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/RecycleBinItemTest.java
@@ -14,6 +14,7 @@
 
 import nl.jqno.equalsverifier.EqualsVerifier;
 
+@SuppressWarnings("PMD.AvoidDuplicateLiterals")
 class RecycleBinItemTest {
 
 	@Test
diff --git a/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/filter/FilterTest.java b/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/filter/FilterTest.java
index a3baa59..bc15a7e 100644
--- a/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/filter/FilterTest.java
+++ b/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/filter/FilterTest.java
@@ -19,6 +19,7 @@
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.sitepark.ies.contentrepository.core.domain.databind.DatabindModule;
 
+@SuppressWarnings("PMD.AvoidDuplicateLiterals")
 class FilterTest {
 
 	@Test
diff --git a/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/query/CursorBasedQueryTest.java b/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/query/CursorBasedQueryTest.java
index ca1f07b..ea3aa7e 100644
--- a/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/query/CursorBasedQueryTest.java
+++ b/src/test/java/com/sitepark/ies/contentrepository/core/domain/entity/query/CursorBasedQueryTest.java
@@ -6,6 +6,7 @@
 
 import nl.jqno.equalsverifier.EqualsVerifier;
 
+@SuppressWarnings("PMD.AvoidDuplicateLiterals")
 class CursorBasedQueryTest {
 
 	@Test
diff --git a/src/test/java/com/sitepark/ies/contentrepository/core/domain/exception/EntityNotFoundExceptionTest.java b/src/test/java/com/sitepark/ies/contentrepository/core/domain/exception/EntityNotFoundExceptionTest.java
index 529418a..ec34776 100644
--- a/src/test/java/com/sitepark/ies/contentrepository/core/domain/exception/EntityNotFoundExceptionTest.java
+++ b/src/test/java/com/sitepark/ies/contentrepository/core/domain/exception/EntityNotFoundExceptionTest.java
@@ -5,6 +5,7 @@
 
 import org.junit.jupiter.api.Test;
 
+@SuppressWarnings("PMD.AvoidDuplicateLiterals")
 class EntityNotFoundExceptionTest {
 
 	@Test
diff --git a/src/test/java/com/sitepark/ies/contentrepository/core/domain/exception/GroupNotEmptyExceptionTest.java b/src/test/java/com/sitepark/ies/contentrepository/core/domain/exception/GroupNotEmptyExceptionTest.java
index b506747..e39bda3 100644
--- a/src/test/java/com/sitepark/ies/contentrepository/core/domain/exception/GroupNotEmptyExceptionTest.java
+++ b/src/test/java/com/sitepark/ies/contentrepository/core/domain/exception/GroupNotEmptyExceptionTest.java
@@ -5,6 +5,7 @@
 
 import org.junit.jupiter.api.Test;
 
+@SuppressWarnings("PMD.AvoidDuplicateLiterals")
 class GroupNotEmptyExceptionTest {
 
 	@Test