KAFKA-8736: Streams performance improvement, use isEmpty() rather than size() == 0 (#7164)

Reviewers: A. Sophie Blee-Goldman <sophie@confluent.io>, Matthias J. Sax <matthias@confluent.io>, Guozhang Wang <guozhang@confluent.io>

streams/src/main/java/org/apache/kafka/streams/state/internals/NamedCache.java

@@ -266,6 +266,10 @@ class NamedCache {
         return cache.size();
     }
 
+    public boolean isEmpty() {
+        return cache.isEmpty();
+    }
+
     synchronized Iterator<Map.Entry<Bytes, LRUNode>> subMapIterator(final Bytes from, final Bytes to) {
         return cache.subMap(from, true, to, true).entrySet().iterator();
     }

streams/src/main/java/org/apache/kafka/streams/state/internals/ThreadCache.java

@@ -193,7 +193,7 @@ public class ThreadCache {
         }
         return new MemoryLRUCacheBytesIterator(cache.allIterator());
     }
-    
+
     public long size() {
         long size = 0;
         for (final NamedCache cache : caches.values()) {
@@ -235,7 +235,7 @@ public class ThreadCache {
             // a put on another cache. So even though the sizeInBytes() is
             // still > maxCacheSizeBytes there is nothing to evict from this
             // namespaced cache.
-            if (cache.size() == 0) {
+            if (cache.isEmpty()) {
                 return;
             }
             cache.evict();