compute ProtocolVersion::supportedVersions once immutably to avoid object generation

patch by Matt Byrd; reviewed by  for CASSANDRA-21199

Author: Matt Byrd

CHANGES.txt

@@ -1,4 +1,5 @@
 7.0
+ * compute ProtocolVersion::supportedVersions once immutably to avoid object generation (CASSANDRA-21199)
  * Avoid using ObjectUtils.getFirstNonNull in Schema (CASSANDRA-21394)
  * Allow nodetool garbagecollect to take a user defined list of SSTables (CASSANDRA-16767)
  * Add a guardrail for misprepared statements (CASSANDRA-21139)

src/java/org/apache/cassandra/transport/ProtocolVersion.java

@@ -24,6 +24,8 @@
 import java.util.List;
 import java.util.Optional;
 
+import com.google.common.collect.ImmutableList;
+
 import org.apache.commons.lang3.ArrayUtils;
 
 import org.apache.cassandra.cql3.FunctionContext;
@@ -73,6 +75,7 @@ public enum ProtocolVersion implements Comparable<ProtocolVersion>, FunctionCont
 
     /** All supported versions, published as an enumset */
     public final static EnumSet<ProtocolVersion> SUPPORTED = EnumSet.copyOf(Arrays.asList(ArrayUtils.addAll(SUPPORTED_VERSIONS)));
+    private final static ImmutableList<String> SUPPORTED_VERSION_STRINGS = SUPPORTED.stream().map(ProtocolVersion::toString).collect(ImmutableList.toImmutableList());
 
     /** Old unsupported versions, this is OK as long as we never add newer unsupported versions */
     public final static EnumSet<ProtocolVersion> UNSUPPORTED = EnumSet.complementOf(SUPPORTED);
@@ -84,12 +87,9 @@ public enum ProtocolVersion implements Comparable<ProtocolVersion>, FunctionCont
     public final static ProtocolVersion CURRENT = V5;
     public final static Optional<ProtocolVersion> BETA = Optional.of(V6);
 
-    public static List<String> supportedVersions()
+    public static ImmutableList<String> supportedVersions()
     {
-        List<String> ret = new ArrayList<>(SUPPORTED.size());
-        for (ProtocolVersion version : SUPPORTED)
-            ret.add(version.toString());
-        return ret;
+        return SUPPORTED_VERSION_STRINGS;
     }
 
     public static List<ProtocolVersion> supportedVersionsStartingWith(ProtocolVersion smallestVersion)