diff --git a/graph/src/main/scala/spark/graph/EdgeTriplet.scala b/graph/src/main/scala/spark/graph/EdgeTriplet.scala
index 9a819481ba..3ed8052794 100644
--- a/graph/src/main/scala/spark/graph/EdgeTriplet.scala
+++ b/graph/src/main/scala/spark/graph/EdgeTriplet.scala
@@ -1,17 +1,53 @@
 package spark.graph
 
-
+/**
+ * An edge triplet represents two vertices and edge along with their attributes.
+ *
+ * @tparam VD the type of the vertex attribute.
+ * @tparam ED the type of the edge attribute
+ */
 class EdgeTriplet[@specialized(Char, Int, Boolean, Byte, Long, Float, Double) VD,
                   @specialized(Char, Int, Boolean, Byte, Long, Float, Double) ED] {
+  /**
+   * The vertex (id and attribute) corresponding to the source vertex.
+   */
   var src: Vertex[VD] = _
+
+  /**
+   * The vertex (id and attribute) corresponding to the target vertex.
+   */
   var dst: Vertex[VD] = _
+
+  /**
+   * The attribute associated with the edge.
+   */
   var data: ED = _
 
-  def otherVertex(vid: Vid): Vertex[VD] = if (src.id == vid) dst else src
+  /**
+   * Given one vertex in the edge return the other vertex.
+   *
+   * @param vid the id one of the two vertices on the edge.
+   * @return the other vertex on the edge.
+   */
+  def otherVertex(vid: Vid): Vertex[VD] =
+    if (src.id == vid) dst else { assert(dst.id == vid); src }
 
-  def vertex(vid: Vid): Vertex[VD] = if (src.id == vid) src else dst
+  /**
+   * Get the vertex object for the given vertex in the edge.
+   *
+   * @param vid the id of one of the two vertices on the edge
+   * @return the vertex object with that id.
+   */
+  def vertex(vid: Vid): Vertex[VD] =
+    if (src.id == vid) src else { assert(dst.id == vid); dst }
+
+  /**
+   * Return the relative direction of the edge to the corresponding vertex.
+   *
+   * @param vid the id of one of the two vertices in the edge.
+   * @return the relative direction of the edge to the corresponding vertex.
+   */
+  def relativeDirection(vid: Vid): EdgeDirection =
+    if (vid == src.id) EdgeDirection.Out else { assert(vid == dst.id); EdgeDirection.In }
 
-  def relativeDirection(vid: Vid): EdgeDirection = {
-    if (vid == src.id) EdgeDirection.Out else EdgeDirection.In
-  }
 }