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Easy Learn Java: Programming Articles, Examples and Tips - Page 129


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I know that Hashmap implementation is not synchronized. How can I synchronize it

Go to all tips in General Java

Question: I know that Hashmap implementation is not synchronized. How can I synchronize it?

Answer: Really good answer often can be found in API!
"If multiple threads access this map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with a key that an instance already contains is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the map. If no such object exists, the map should be "wrapped" using the Collections.synchronizedMap method. This is best done at creation time, to prevent accidental unsynchronized access to the map:

Map m = Collections.synchronizedMap(new HashMap(...));

The iterators returned by all of this class's "collection view methods" are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs."

From Java API (v 1.4.1)

--
John

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7 comments | Printer Friendly Page  Send to a Friend | Score: 4
Posted by jalex on Wednesday, January 14, 2004 (00:00:00) (14521 reads)

News: IBM Toolkit for MPEG-4 (version 1.1.1)

Go to all tips in Sound, multimedia
IBM Toolkit for MPEG-4 consists of a set of Java classes and APIs with three sample applications: a cross-platform playback application and two tools for generating MPEG-4 content for use with MPEG-4-compliant devices.

Version 1.1.1: Video and audio improvements; fixed problems with RTSP and odd-sized video; and other minor enhancements and bug fixes.


29 comments | Printer Friendly Page  Send to a Friend | News | Score: 0
Posted by Anonymous on Tuesday, January 13, 2004 (00:00:00) (3444 reads)

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