The string pool is the JVM's particular implementation of the concept of string interning:
In computer science, string interning
is a method of storing only one copy
of each distinct string value, which
must be immutable. Interning strings
makes some string processing tasks
more time- or space-efficient at the
cost of requiring more time when the
string is created or interned. The
distinct values are stored in a string
intern pool.
Basically, a string intern pool allows a runtime to save memory by preserving immutable strings in a pool so that areas of the application can reuse instances of common strings instead of creating multiple instances of it.
As an interesting side note, string interning is an example of the flyweight design pattern:
Flyweight is a software design
pattern. A flyweight is an object that
minimizes memory use by sharing as
much data as possible with other
similar objects; it is a way to use
objects in large numbers when a simple
repeated representation would use an
unacceptable amount of memory.
