diff --git a/webapp/src/edu/cornell/mannlib/vitro/webapp/auth/identifier/common/HasPermission.java b/webapp/src/edu/cornell/mannlib/vitro/webapp/auth/identifier/common/HasPermission.java
index 9621842b0..11de68c9b 100644
--- a/webapp/src/edu/cornell/mannlib/vitro/webapp/auth/identifier/common/HasPermission.java
+++ b/webapp/src/edu/cornell/mannlib/vitro/webapp/auth/identifier/common/HasPermission.java
@@ -14,7 +14,7 @@ import edu.cornell.mannlib.vitro.webapp.auth.permissions.Permission;
  * The current user has this Permission, through one or more PermissionSets.
  */
 public class HasPermission extends AbstractCommonIdentifier implements
-		Identifier {
+		Identifier, Comparable<HasPermission> {
 	public static Collection<HasPermission> getIdentifiers(IdentifierBundle ids) {
 		return getIdentifiersForClass(ids, HasPermission.class);
 	}
@@ -27,9 +27,12 @@ public class HasPermission extends AbstractCommonIdentifier implements
 		return set;
 	}
 
-	private final Permission permission;
+	private final Permission permission; // never null
 
 	public HasPermission(Permission permission) {
+		if (permission == null) {
+			throw new NullPointerException("permission may not be null.");
+		}
 		this.permission = permission;
 	}
 
@@ -41,4 +44,29 @@ public class HasPermission extends AbstractCommonIdentifier implements
 	public String toString() {
 		return "HasPermission[" + permission + "]";
 	}
+
+	@Override
+	public int hashCode() {
+		return permission.hashCode();
+	}
+
+	@Override
+	public boolean equals(Object obj) {
+		if (obj == this) {
+			return true;
+		}
+		if (obj == null) {
+			return false;
+		}
+		if (!(obj instanceof HasPermission)) {
+			return false;
+		}
+		HasPermission that = (HasPermission) obj;
+		return this.permission.equals(that.permission);
+	}
+
+	@Override
+	public int compareTo(HasPermission that) {
+		return this.permission.compareTo(that.permission);
+	}
 }
diff --git a/webapp/src/edu/cornell/mannlib/vitro/webapp/auth/identifier/common/IsUser.java b/webapp/src/edu/cornell/mannlib/vitro/webapp/auth/identifier/common/IsUser.java
index 635069a80..220e814af 100644
--- a/webapp/src/edu/cornell/mannlib/vitro/webapp/auth/identifier/common/IsUser.java
+++ b/webapp/src/edu/cornell/mannlib/vitro/webapp/auth/identifier/common/IsUser.java
@@ -25,9 +25,13 @@ public class IsUser extends AbstractCommonIdentifier implements Identifier {
 		return set;
 	}
 
-	private final String uri;
+	private final String uri; // never null
 
 	public IsUser(String uri) {
+		if (uri == null) {
+			throw new NullPointerException("uri may not be null.");
+		}
+
 		this.uri = uri;
 	}
 
@@ -35,6 +39,26 @@ public class IsUser extends AbstractCommonIdentifier implements Identifier {
 		return uri;
 	}
 
+	@Override
+	public int hashCode() {
+		return uri.hashCode();
+	}
+
+	@Override
+	public boolean equals(Object obj) {
+		if (obj == this) {
+			return true;
+		}
+		if (obj == null) {
+			return false;
+		}
+		if (!(obj instanceof IsUser)) {
+			return false;
+		}
+		IsUser that = (IsUser) obj;
+		return this.uri.equals(that.uri);
+	}
+
 	@Override
 	public String toString() {
 		return "IsUser[" + uri + "]";