Add javadoc documentations

src/main/java/seedu/address/model/util/DateTimeComparator.java

@@ -1,16 +1,29 @@
 package seedu.address.model.util;
 
+import java.util.Comparator;
+
 import seedu.address.model.activity.Activity;
 import seedu.address.model.activity.DateTime;
 
-import java.util.Comparator;
-
 //@@author karenfrilya97
+/**
+ * Comparator class that compares Activity based on dateTime attributes.
+ * For Event objects, this class only compares based on the startDateTime,
+ * and does not take into account the endDateTime.
+ */
 public class DateTimeComparator implements Comparator<Activity> {
 
     public DateTimeComparator () {
     }
 
+    /**
+     * Compares two activities
+     * @param o1 and
+     * @param o2,
+     * @return a negative integer, zero, or a positive integer
+     * if the first activity's dateTime is earlier than, equal to or later than
+     * the second activity's dateTime respectively.
+     */
     public int compare(Activity o1, Activity o2) {
         DateTime dt1 = o1.getDateTime();
         DateTime dt2 = o2.getDateTime();