Java Map: putIfAbsent
I think it is pretty much impossible to write a good program without using, at least once, aMap, what other languages call hash.
I tend to use a lot hashes in my applications, and often I find myself writing a pattern like the following one:
if ( ! cache.containsKey( wanted ) )
cahce.put( wanted, value );
value = cache.get( wanted );
The idea is quite simple: if the value is already in map just get it, otherwise insert into the map and get it out. In this way, the map can work as a cache for a following elaboration.
Then a colleague of mine yelded about Map.putIfAbsent() method, that apparently does what I do, but in a single line of code.
Emphasises on apparently does.
There is a clear difference in my approach, which is not a rocket-science and the
putIfAbsent: the value to be stored could be obtained by a complex computation, and therefore the following two blocks of code are not the same even if they look they are:
cache.putIfAbsent( wanted, Database.doVeryLongQuery( wanted ) );
value = cache.get( wanted );
if ( ! cache.containsKey( wanted ) )
cahce.put( wanted, Database.doVeryLongQuery( wanted ) );
value = cache.get( wanted );
In fact, in the first snippet of code, the one that exploits the
putIfAbsent, the complex database query is executed every time, while in the second example it is executed only if really required.
There could be solutions that involves a lazy evaluation of the second argument to
putIfAbsent, but they need to be manually crafted (i.e., the Map does not support them in its interface) and the ending result is to write a lot more code than the initial example.