diff --git a/src/org/ntlab/trace/Reference.java b/src/org/ntlab/trace/Reference.java
index 1accbac..a832d5c 100644
--- a/src/org/ntlab/trace/Reference.java
+++ b/src/org/ntlab/trace/Reference.java
@@ -110,7 +110,11 @@
 	}
 	
 	public int hashCode() {
-		return Integer.parseInt(srcObj.getId()) + Integer.parseInt(dstObj.getId());
+		if (srcObj.getId().matches("[0-9]{1,}") && dstObj.getId().matches("[0-9]{1,}")) {
+			return Integer.parseInt(srcObj.getId()) + Integer.parseInt(dstObj.getId());			
+		}
+		// If objectId contains string not only number.
+		return srcObj.getId().hashCode() + dstObj.getId().hashCode();			
 	}
 	
 	public String toString() {