ConcurrentHashMap operations

Question

Following are some lines from the java docs of ConcurrentHashMap

This class obeys the same functional specification as Hashtable, and includes versions of methods corresponding to each method of Hashtable. However, even though all operations are thread-safe, retrieval operations do not entail locking, and there is not any support for locking the entire table in a way that prevents all access.

What is the meaning of the statement

though all operations are thread-safe

from above paragraph? Can anyone explain with any example of put() or get() methods?

yes i want thread safety explained to me but with respect to get() and put() in this case. — a Learner, Feb 06 '14 at 10:05
There is nothing specific to get() and put(). They are two thread-safe operations. — Marko Topolnik, Feb 06 '14 at 10:05
thats what i want to ask that what is the meaning when we say they are two thread safe operations. — a Learner, Feb 06 '14 at 10:07
Tried to google it yet? Here's a link: http://stackoverflow.com/questions/261683/what-is-meant-by-thread-safe-code — Marko Topolnik, Feb 06 '14 at 10:07

score 1 · Answer 1 · answered Feb 06 '14 at 10:02

1

The ConcurrentHashMap allows concurrent modification of the Map from several threads without the need to block them. Collections.synchronizedMap(map) creates a blocking Map which will degrade performance, albeit ensure consistency (if used properly).

Use the second option if you need to ensure data consistency, and each thread needs to have an up-to-date view of the map. Use the first if performance is critical, and each thread only inserts data to the map, with reads happening less frequently.

answered Feb 06 '14 at 10:02

Cupple Kay

155
2
14

Was it helpful in any way? – Cupple Kay Feb 06 '14 at 10:03
your answer clear some points but it doesnot answer my question. – a Learner Feb 06 '14 at 10:18
That's all i can help with i wish there was more, sorry. – Cupple Kay Feb 06 '14 at 12:19

score 0 · Answer 2 · answered Feb 06 '14 at 16:55

Your question is odd. If you understand what "thread safety" means then you would be able to understand how it applies to get() and put() on your own. If you don't understand thread safety then there is no point to explain it specifically in relation to get() and put(). Are you sure this isn't a homework question?

However, answering your question anyway, the fact that ConcurrentHashMap is thread safe means that if you have several threads executing put()s on the same map at the same time, then: a) no damage will occur to the internal data structures of the map and: b) some other thread doing a get() will see all of the values put in by the other threads. With a non-thread safe Map such as HashMap neither of those are guaranteed.

ConcurrentHashMap operations

2 Answers2