The possibility of deduplication
Granted, this wasn't an initial motivation for why Strings should be immutable, but when one is looking at the rational of why immutable Strings are a good thing, this is certainly something to consider.
Anyone who has worked with Strings a bit knows that they can suck memory. This is especially true when you're doing things like pulling data from databases that sticks around for awhile. Many times with these stings, they are the same string over and over again (once for each row).
Many large-scale Java applications are currently bottlenecked on memory. Measurements have shown that roughly 25% of the Java heap live data set in these types of applications is consumed by String objects. Further, roughly half of those String objects are duplicates, where duplicates means string1.equals(string2) is true. Having duplicate String objects on the heap is, essentially, just a waste of memory. ...
With Java 8 update 20, JEP 192 (motivation quoted above) is being implemented to address this. Without getting into the details of how string deduplication works, it is essential that the Strings themselves are immutable. You can't deduplicate StringBuilders because they can change and you don't want someone changing something from under you. Immutable Strings (related to that String pool) means that you can go through and if you find two strings that are the same, you can point one string reference to the other and let the garbage collector consume the newly unused one.
