8239055: Wrong implementation of VMState.hasListener

Correct the VMState.hasListener implementation to return WeakReference<VMListener> type

Reviewed-by: sspitsyn, poonam

src/jdk.jdi/share/classes/com/sun/tools/jdi/VMState.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1999, 2018, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1999, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -174,7 +174,14 @@ synchronized void addListener(VMListener listener) {
     }
 
     synchronized boolean hasListener(VMListener listener) {
-        return listeners.contains(listener);
+        Iterator<WeakReference<VMListener>> iter = listeners.iterator();
+        while (iter.hasNext()) {
+            WeakReference<VMListener> ref = iter.next();
+            if (listener.equals(ref.get())) {
+                return true;
+            }
+        }
+        return false;
     }
 
     synchronized void removeListener(VMListener listener) {