What's the difference between ConcurrentHashMap and Collections.synchronizedMap(Map)?

Question

I have a Map which is to be modified by several threads concurrently.

There seem to be three different synchronized Map implementations in the Java API:

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Answer 1

ConcurrentHashMap is preferred when you can use it - though it requires at least Java 5.

It is designed to scale well when used by multiple threads. Performance may be marginally poorer when only a single thread accesses the Map at a time, but significantly better when multiple threads access the map concurrently.

I found a blog entry that reproduces a table from the excellent book Java Concurrency In Practice, which I thoroughly recommend.

Collections.synchronizedMap makes sense really only if you need to wrap up a map with some other characteristics, perhaps some sort of ordered map, like a TreeMap.