Allow arbitrary strings as entity type

This change is based on the discussion in Berlin last week.
It should be possible to add new entity types in the future,
maybe even in some local configuration, and hence the types
need to be extendable. The strings used to identify the known
types are the IRIs of the corresponding classes in the RDF
export.

wdtk-datamodel/src/main/java/org/wikidata/wdtk/datamodel/implementation/ItemIdImpl.java

@@ -20,6 +20,7 @@
  * #L%
  */
 
+import org.wikidata.wdtk.datamodel.interfaces.EntityId;
 import org.wikidata.wdtk.datamodel.interfaces.ItemId;
 
 /**
@@ -46,8 +47,8 @@ public class ItemIdImpl extends EntityIdImpl implements ItemId {
 	}
 
 	@Override
-	public EntityType getEntityType() {
-		return EntityType.ITEM;
+	public String getEntityType() {
+		return EntityId.ET_ITEM;
 	}
 
 }

wdtk-datamodel/src/main/java/org/wikidata/wdtk/datamodel/implementation/PropertyIdImpl.java

@@ -20,6 +20,7 @@
  * #L%
  */
 
+import org.wikidata.wdtk.datamodel.interfaces.EntityId;
 import org.wikidata.wdtk.datamodel.interfaces.PropertyId;
 
 /**
@@ -46,8 +47,8 @@ public class PropertyIdImpl extends EntityIdImpl implements PropertyId {
 	}
 
 	@Override
-	public EntityType getEntityType() {
-		return EntityType.PROPERTY;
+	public String getEntityType() {
+		return EntityId.ET_PROPERTY;
 	}
 
 }

wdtk-datamodel/src/main/java/org/wikidata/wdtk/datamodel/interfaces/EntityId.java

@@ -35,20 +35,22 @@
 public interface EntityId extends IriValue {
 
 	/**
-	 * Enum for the possible entity types, currently Item and Property.
-	 * 
-	 * @author Markus Kroetzsch
+	 * IRI of the type of an entity that is an item.
+	 */
+	static final String ET_ITEM = "http://www.wikidata.org/ontology#Item";
+	/**
+	 * IRI of the type of an entity that is a property.
 	 */
-	enum EntityType {
-		ITEM, PROPERTY
-	}
+	static final String ET_PROPERTY = "http://www.wikidata.org/ontology#Property";
 
 	/**
-	 * Get the type of this entity.
+	 * Get the type of this entity. This should be an IRI that identifies an
+	 * entity type, such as {@link EntityId#ET_ITEM} or
+	 * {@link EntityId#ET_PROPERTY}.
 	 * 
-	 * @return EntityType
+	 * @return String
 	 */
-	EntityType getEntityType();
+	String getEntityType();
 
 	/**
 	 * Get the id of this entity.

wdtk-datamodel/src/test/java/org/wikidata/wdtk/datamodel/implementation/ItemIdImplTest.java

@@ -19,15 +19,15 @@
  * limitations under the License.
  * #L%
  */
-import org.wikidata.wdtk.datamodel.implementation.ItemIdImpl;
-import org.wikidata.wdtk.datamodel.interfaces.EntityId;
-
 import static org.hamcrest.CoreMatchers.equalTo;
 import static org.hamcrest.CoreMatchers.not;
-import static org.junit.Assert.*;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertThat;
 
-import org.junit.Test;
 import org.junit.Before;
+import org.junit.Test;
+import org.wikidata.wdtk.datamodel.interfaces.EntityId;
 
 public class ItemIdImplTest {
 
@@ -46,7 +46,7 @@ public void setUp() {
 
 	@Test
 	public void entityTypeIsItem() {
-		assertEquals(item1.getEntityType(), EntityId.EntityType.ITEM);
+		assertEquals(item1.getEntityType(), EntityId.ET_ITEM);
 	}
 
 	@Test

wdtk-datamodel/src/test/java/org/wikidata/wdtk/datamodel/implementation/PropertyIdImplTest.java

@@ -20,14 +20,15 @@
  * #L%
  */
 
-import org.wikidata.wdtk.datamodel.interfaces.EntityId;
-
 import static org.hamcrest.CoreMatchers.equalTo;
 import static org.hamcrest.CoreMatchers.not;
-import static org.junit.Assert.*;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertThat;
 
-import org.junit.Test;
 import org.junit.Before;
+import org.junit.Test;
+import org.wikidata.wdtk.datamodel.interfaces.EntityId;
 
 public class PropertyIdImplTest {
 
@@ -46,7 +47,7 @@ public void setUp() {
 
 	@Test
 	public void entityTypeIsProperty() {
-		assertEquals(prop1.getEntityType(), EntityId.EntityType.PROPERTY);
+		assertEquals(prop1.getEntityType(), EntityId.ET_PROPERTY);
 	}
 
 	@Test