Add hashmap-vs-concurrenthashmap module: JDK 25 sizeCtl/treeify internals, forced and measured lost updates

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_017ic4b7rnC7jppGGdHGESvy

hashmap-vs-concurrenthashmap/output/07-null-handling.txt

@@ -0,0 +1,9 @@
+null keys and values: HashMap allows, ConcurrentHashMap rejects
+===============================================================
+
+HashMap.put(null, ..) accepted; the null key lives in table[0]: true
+HashMap.get(null) = for-null-key
+HashMap.get("present-but-null") = null   containsKey = true
+HashMap.get("absent")           = null   containsKey = false   <- get() alone cannot tell these two apart
+ConcurrentHashMap put(null, v), put(k, null), get(null), containsKey(null), merge(k, null, f): all throw NullPointerException
+compute(k, (k,v) -> null) removes the entry instead: containsKey(k) = false