Use ConcurrentMap as interface instead of Map.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1734799 13f79535-47bb-0310-9956-ffa450edef68

java/org/apache/coyote/http2/Http2UpgradeHandler.java

@@ -30,6 +30,7 @@
 import java.util.TreeSet;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.ConcurrentMap;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicReference;
 
@@ -138,7 +139,7 @@ public class Http2UpgradeHandler extends AbstractStream implements InternalHttpU
     private final PingManager pingManager = new PingManager();
     private volatile int newStreamsSinceLastPrune = 0;
     // Tracking for when the connection is blocked (windowSize < 1)
-    private final Map<AbstractStream,int[]> backLogStreams = new ConcurrentHashMap<>();
+    private final ConcurrentMap<AbstractStream,int[]> backLogStreams = new ConcurrentHashMap<>();
     private long backLogSize = 0;