thread safety - Java ConcurrentHashMap and For Each Loop -

supposed have following concurrenthashmap:

concurrenthashmap<integer,string> indentificationdocuments = new concurrenthashmap<integer,string>();           indentificationdocuments.put(1, "passport");          indentificationdocuments.put(2, "driver's licence"); 

how safely iterate on map each loop , append value of each entry string?

iterators produced concurrenthashmap weakly consistent. is:

• they may proceed concurrently other operations
• they never throw concurrentmodificationexception
• they guaranteed traverse elements existed upon construction once, , may (but not guaranteed to) reflect modifications subsequent construction.

the last bullet-point pretty important, iterator returns view of map @ point since creation of iterator, quote different section of javadocs concurrenthashmap:

similarly, iterators, spliterators , enumerations return elements reflecting state of hash table @ point @ or since creation of iterator/enumeration.

so when loop through keyset following, need double check if item still exists in collection:

for(integer i: indentificationdocuments.keyset()){     // below line problem, get(i) may not exist anymore may still in view of iterator     // somestringbuilder.append(indentificationdocuments.get(i));     // next line work     somestringbuilder.append(identificationdocuments.getordefault(i, "")); } 

the act of appending strings stringbuilder safe, long doing on 1 thread or have encapsulated stringbuilder entirely in thread-safe manner.