On one hand, it is true that you can remove String duplicates by
internalizing them. The problem is that the internalized strings go to
the Permanent Generation, which is an area of the JVM that is reserved
for non-user objects, like Classes, Methods and other internal JVM
objects. The size of this area is limited, and is usually much smaller
than the heap. Calling intern() on a String has the effect of moving
it out from the heap into the permanent generation, and you risk
running out of PermGen space.
In JDK 7, interned strings are no longer allocated in the permanent generation of the Java heap, but are instead allocated in the main part of the Java heap (known as the young and old generations), along with the other objects created by the application. This change will result in more data residing in the main Java heap, and less data in the permanent generation, and thus may require heap sizes to be adjusted. Most applications will see only relatively small differences in heap usage due to this change, but larger applications that load many classes or make heavy use of the String.intern() method will see more significant differences.
更新：内部字符串存储在Java 7以后的主堆中。 http://www.oracle.com/technetwork/java/javase/jdk7-relnotes-418459.html#jdk7changes
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