Index: lams_common/src/java/org/lamsfoundation/lams/learningdesign/Activity.java
===================================================================
RCS file: /usr/local/cvsroot/lams_common/src/java/org/lamsfoundation/lams/learningdesign/Activity.java,v
diff -u -r1.5 -r1.6
--- lams_common/src/java/org/lamsfoundation/lams/learningdesign/Activity.java	16 Feb 2005 22:02:11 -0000	1.5
+++ lams_common/src/java/org/lamsfoundation/lams/learningdesign/Activity.java	17 Feb 2005 04:20:35 -0000	1.6
@@ -5,11 +5,12 @@
 import org.apache.commons.lang.builder.EqualsBuilder;
 import org.apache.commons.lang.builder.HashCodeBuilder;
 import org.apache.commons.lang.builder.ToStringBuilder;
+import org.lamsfoundation.lams.util.Nullable;
 
 /**
  * @hibernate.class table="lams_learning_activity"
  */
-public abstract class Activity implements Serializable {
+public abstract class Activity implements Serializable,Nullable {
 
 	public static final int TOOL_ACTIVITY_TYPE = 1;
 
Index: lams_common/src/java/org/lamsfoundation/lams/learningdesign/GroupingActivity.java
===================================================================
RCS file: /usr/local/cvsroot/lams_common/src/java/org/lamsfoundation/lams/learningdesign/GroupingActivity.java,v
diff -u -r1.3 -r1.4
--- lams_common/src/java/org/lamsfoundation/lams/learningdesign/GroupingActivity.java	16 Feb 2005 22:03:26 -0000	1.3
+++ lams_common/src/java/org/lamsfoundation/lams/learningdesign/GroupingActivity.java	17 Feb 2005 04:20:35 -0000	1.4
@@ -132,4 +132,12 @@
 	public void setCreateGroupingUIID(Integer create_grouping_ui_id) {
 		this.createGroupingUIID = create_grouping_ui_id;
 	}
+
+    /**
+     * @see org.lamsfoundation.lams.util.Nullable#isNull()
+     */
+    public boolean isNull()
+    {
+        return false;
+    }
 }
Index: lams_common/src/java/org/lamsfoundation/lams/learningdesign/NullActivity.java
===================================================================
RCS file: /usr/local/cvsroot/lams_common/src/java/org/lamsfoundation/lams/learningdesign/NullActivity.java,v
diff -u
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ lams_common/src/java/org/lamsfoundation/lams/learningdesign/NullActivity.java	17 Feb 2005 04:20:35 -0000	1.1
@@ -0,0 +1,19 @@
+
+package org.lamsfoundation.lams.learningdesign;
+
+
+/**
+ * Null activity object. 
+ * @author dgarth
+ */
+public class NullActivity extends Activity {
+
+    /**
+     * @see org.lamsfoundation.lams.util.Nullable#isNull()
+     */
+    public boolean isNull()
+    {
+        return true;
+    }
+
+}
Index: lams_common/src/java/org/lamsfoundation/lams/learningdesign/OptionsActivity.java
===================================================================
RCS file: /usr/local/cvsroot/lams_common/src/java/org/lamsfoundation/lams/learningdesign/OptionsActivity.java,v
diff -u -r1.5 -r1.6
--- lams_common/src/java/org/lamsfoundation/lams/learningdesign/OptionsActivity.java	16 Feb 2005 23:22:18 -0000	1.5
+++ lams_common/src/java/org/lamsfoundation/lams/learningdesign/OptionsActivity.java	17 Feb 2005 04:20:35 -0000	1.6
@@ -159,4 +159,12 @@
 	public void setOptionsInstructions(String options_instructions) {
 		this.optionsInstructions = options_instructions;
 	}
+
+    /**
+     * @see org.lamsfoundation.lams.util.Nullable#isNull()
+     */
+    public boolean isNull()
+    {
+        return false;
+    }
 }
Index: lams_common/src/java/org/lamsfoundation/lams/learningdesign/ParallelActivity.java
===================================================================
RCS file: /usr/local/cvsroot/lams_common/src/java/org/lamsfoundation/lams/learningdesign/ParallelActivity.java,v
diff -u -r1.6 -r1.7
--- lams_common/src/java/org/lamsfoundation/lams/learningdesign/ParallelActivity.java	17 Feb 2005 01:07:42 -0000	1.6
+++ lams_common/src/java/org/lamsfoundation/lams/learningdesign/ParallelActivity.java	17 Feb 2005 04:20:35 -0000	1.7
@@ -115,4 +115,12 @@
             .toString();
     }
 
+    /**
+     * @see org.lamsfoundation.lams.util.Nullable#isNull()
+     */
+    public boolean isNull()
+    {
+        return false;
+    }
+
 }
Index: lams_common/src/java/org/lamsfoundation/lams/learningdesign/PermissionGateActivity.java
===================================================================
RCS file: /usr/local/cvsroot/lams_common/src/java/org/lamsfoundation/lams/learningdesign/PermissionGateActivity.java,v
diff -u -r1.5 -r1.6
--- lams_common/src/java/org/lamsfoundation/lams/learningdesign/PermissionGateActivity.java	17 Feb 2005 01:24:57 -0000	1.5
+++ lams_common/src/java/org/lamsfoundation/lams/learningdesign/PermissionGateActivity.java	17 Feb 2005 04:20:35 -0000	1.6
@@ -121,5 +121,13 @@
         .append("activityId", getActivityId())
         .toString();
     }
+
+    /**
+     * @see org.lamsfoundation.lams.util.Nullable#isNull()
+     */
+    public boolean isNull()
+    {
+        return false;
+    }
     
 }
Index: lams_common/src/java/org/lamsfoundation/lams/learningdesign/ScheduleGateActivity.java
===================================================================
RCS file: /usr/local/cvsroot/lams_common/src/java/org/lamsfoundation/lams/learningdesign/ScheduleGateActivity.java,v
diff -u -r1.4 -r1.5
--- lams_common/src/java/org/lamsfoundation/lams/learningdesign/ScheduleGateActivity.java	17 Feb 2005 01:25:48 -0000	1.4
+++ lams_common/src/java/org/lamsfoundation/lams/learningdesign/ScheduleGateActivity.java	17 Feb 2005 04:20:35 -0000	1.5
@@ -174,5 +174,12 @@
             .append("activityId", getActivityId())
             .toString();
     }
+    /**
+     * @see org.lamsfoundation.lams.util.Nullable#isNull()
+     */
+    public boolean isNull()
+    {
+        return false;
+    }
 
 }
Index: lams_common/src/java/org/lamsfoundation/lams/learningdesign/SequenceActivity.java
===================================================================
RCS file: /usr/local/cvsroot/lams_common/src/java/org/lamsfoundation/lams/learningdesign/SequenceActivity.java,v
diff -u -r1.5 -r1.6
--- lams_common/src/java/org/lamsfoundation/lams/learningdesign/SequenceActivity.java	17 Feb 2005 01:07:57 -0000	1.5
+++ lams_common/src/java/org/lamsfoundation/lams/learningdesign/SequenceActivity.java	17 Feb 2005 04:20:35 -0000	1.6
@@ -117,4 +117,13 @@
             .toString();
     }
 
+
+    /**
+     * @see org.lamsfoundation.lams.util.Nullable#isNull()
+     */
+    public boolean isNull()
+    {
+        return false;
+    }
+
 }
Index: lams_common/src/java/org/lamsfoundation/lams/learningdesign/SynchGateActivity.java
===================================================================
RCS file: /usr/local/cvsroot/lams_common/src/java/org/lamsfoundation/lams/learningdesign/SynchGateActivity.java,v
diff -u -r1.4 -r1.5
--- lams_common/src/java/org/lamsfoundation/lams/learningdesign/SynchGateActivity.java	17 Feb 2005 03:20:17 -0000	1.4
+++ lams_common/src/java/org/lamsfoundation/lams/learningdesign/SynchGateActivity.java	17 Feb 2005 04:20:35 -0000	1.5
@@ -118,4 +118,12 @@
             .toString();
     }
 
+    /**
+     * @see org.lamsfoundation.lams.util.Nullable#isNull()
+     */
+    public boolean isNull()
+    {
+        return false;
+    }
+
 }
Index: lams_common/src/java/org/lamsfoundation/lams/learningdesign/ToolActivity.java
===================================================================
RCS file: /usr/local/cvsroot/lams_common/src/java/org/lamsfoundation/lams/learningdesign/ToolActivity.java,v
diff -u -r1.6 -r1.7
--- lams_common/src/java/org/lamsfoundation/lams/learningdesign/ToolActivity.java	17 Feb 2005 00:56:08 -0000	1.6
+++ lams_common/src/java/org/lamsfoundation/lams/learningdesign/ToolActivity.java	17 Feb 2005 04:20:35 -0000	1.7
@@ -192,5 +192,13 @@
 	 */
 	public void setToolSessions(Set toolSessions) {
 		this.toolSessions = toolSessions;
-	}	
+	}
+
+    /**
+     * @see org.lamsfoundation.lams.util.Nullable#isNull()
+     */
+    public boolean isNull()
+    {
+        return false;
+    }	
 }
Index: lams_common/src/java/org/lamsfoundation/lams/lesson/LessonCompleteActivity.java
===================================================================
RCS file: /usr/local/cvsroot/lams_common/src/java/org/lamsfoundation/lams/lesson/LessonCompleteActivity.java,v
diff -u -r1.1 -r1.2
--- lams_common/src/java/org/lamsfoundation/lams/lesson/LessonCompleteActivity.java	17 Feb 2005 02:57:31 -0000	1.1
+++ lams_common/src/java/org/lamsfoundation/lams/lesson/LessonCompleteActivity.java	17 Feb 2005 04:21:45 -0000	1.2
@@ -6,6 +6,8 @@
  */
 package org.lamsfoundation.lams.lesson;
 
+import org.lamsfoundation.lams.learningdesign.NullActivity;
+
 /**
  * @author dgarth
  *
Fisheye: Tag 1.2 refers to a dead (removed) revision in file `lams_common/src/java/org/lamsfoundation/lams/lesson/NullActivity.java'.
Fisheye: No comparison available.  Pass `N' to diff?
Index: lams_common/src/java/org/lamsfoundation/lams/lesson/ParallelWaitActivity.java
===================================================================
RCS file: /usr/local/cvsroot/lams_common/src/java/org/lamsfoundation/lams/lesson/ParallelWaitActivity.java,v
diff -u -r1.1 -r1.2
--- lams_common/src/java/org/lamsfoundation/lams/lesson/ParallelWaitActivity.java	17 Feb 2005 02:57:31 -0000	1.1
+++ lams_common/src/java/org/lamsfoundation/lams/lesson/ParallelWaitActivity.java	17 Feb 2005 04:21:45 -0000	1.2
@@ -6,6 +6,8 @@
  */
 package org.lamsfoundation.lams.lesson;
 
+import org.lamsfoundation.lams.learningdesign.NullActivity;
+
 /**
  * @author dgarth
  *
Index: lams_common/src/java/org/lamsfoundation/lams/util/Nullable.java
===================================================================
RCS file: /usr/local/cvsroot/lams_common/src/java/org/lamsfoundation/lams/util/Nullable.java,v
diff -u
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ lams_common/src/java/org/lamsfoundation/lams/util/Nullable.java	17 Feb 2005 04:21:45 -0000	1.1
@@ -0,0 +1,28 @@
+/* ********************************************************************************
+ *  Copyright Notice
+ *  =================
+ * This file contains propriety information of LAMS Foundation. 
+ * Copying or reproduction with prior written permission is prohibited.
+ * Copyright (c) 2004 
+ * Created on 2004-12-3
+ ******************************************************************************** */
+
+package org.lamsfoundation.lams.util;
+
+
+/**
+ * Null Object pattern. This interface is defined to avoid the use of NULL. 
+ * The domain object that allows NULL as return value should implement this
+ * interface.
+ * 
+ * @author Jacky Fang 2004-12-3
+ * 
+ */
+public interface Nullable
+{
+    /**
+     * contract to indicate whether current object is null.
+     * @return
+     */
+    public boolean isNull();
+}