Add some comments.

skywalking-commons/skywalking-trace/src/main/java/com/a/eye/skywalking/trace/TraceId/DistributedTraceId.java

@@ -24,6 +24,13 @@ public abstract class DistributedTraceId {
         return id;
     }
 
+    /**
+     * Compare the two <code>DistributedTraceId</code> by its {@link #id},
+     * even these two <code>DistributedTraceId</code>s are not the same instances.
+     *
+     * @param o target <code>DistributedTraceId</code>
+     * @return return if they have the same {@link #id}
+     */
     @Override
     public boolean equals(Object o) {
         if (this == o)

skywalking-commons/skywalking-trace/src/main/java/com/a/eye/skywalking/trace/TraceSegment.java

@@ -100,8 +100,7 @@ public class TraceSegment {
     private boolean sampled;
 
     /**
-     * Create a trace segment, by given segmentId.
-     * This segmentId is generated by TraceSegmentRef, AKA, from tracer/agent module.
+     * Create a trace segment, by the given applicationCode.
      */
     public TraceSegment(String applicationCode) {
         this();
@@ -109,7 +108,9 @@ public class TraceSegment {
     }
 
     /**
-     * Create a default/empty trace segment
+     * Create a default/empty trace segment,
+     * with current time as start time,
+     * and generate a new segment id.
      */
     public TraceSegment() {
         this.startTime = System.currentTimeMillis();
@@ -134,6 +135,11 @@ public class TraceSegment {
         }
     }
 
+    /**
+     * Establish the line between this segment and all relative global trace ids.
+     *
+     * @param distributedTraceIds multi global trace ids. @see {@link DistributedTraceId}
+     */
     public void relatedGlobalTraces(List<DistributedTraceId> distributedTraceIds) {
         if (distributedTraceIds == null || distributedTraceIds.size() == 0) {
             return;