Cache IntervalWindow hashCode

sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/windowing/IntervalWindow.java

@@ -38,12 +38,18 @@
  * (inclusive) to {@link #end} (exclusive).
  */
 public class IntervalWindow extends BoundedWindow implements Comparable<IntervalWindow> {
+
   /** Start of the interval, inclusive. */
   private final Instant start;
 
   /** End of the interval, exclusive. */
   private final Instant end;
 
+  // Cached hashCode. ints don't tear and access don't need to be synchronized.
+  // Stale reads if any will return 0 and will recalculate hashCode.
+  // ByteString and String hashCodes are cached similarly.
+  private int hashCode; // Default is 0.
+
   /** Creates a new IntervalWindow that represents the half-open time interval [start, end). */
   public IntervalWindow(Instant start, Instant end) {
     this.start = start;
@@ -103,10 +109,13 @@ public boolean equals(@Nullable Object o) {
 
   @Override
   public int hashCode() {
-    // The end values are themselves likely to be arithmetic sequence, which
-    // is a poor distribution to use for a hashtable, so we
-    // add a highly non-linear transformation.
-    return (int) (start.getMillis() + modInverse((int) (end.getMillis() << 1) + 1));
+    if (hashCode == 0) {
+      // The end values are themselves likely to be arithmetic sequence, which
+      // is a poor distribution to use for a hashtable, so we
+      // add a highly non-linear transformation.
+      hashCode = (int) (start.getMillis() + modInverse((int) (end.getMillis() << 1) + 1));
+    }
+    return hashCode;
   }
 
   /** Compute the inverse of (odd) x mod 2^32. */