String.intern in Java 6, 7 and 8 – string pooling

• Stay away from String.intern() method on Java 6 due to a fixed size memory area (PermGen) used for JVM string pool storage.
• Java 7 and 8 implement the string pool in the heap memory. It means that you are limited by the whole application memory for string pooling in Java 7 and 8.
• Use -XX:StringTableSize JVM parameter in Java 7 and 8 to set the string pool map size. It is fixed, because it is implemented as a hash map with lists in the buckets. Approximate the number of distinct strings in your application (which you intend to intern) and set the pool size equal to some prime number close to this value multiplied by 2 (to reduce the likelihood of collisions). It will allow String.intern to run in the constant time and requires a rather small memory consumption per interned string (explicitly used Java WeakHashMap will consume 4-5 times more memory for the same task).
• The default value of -XX:StringTableSize parameter is 1009 in Java 6 and Java 7 until Java7u40. It was increased to 60013 in Java 7u40 (same value is used in Java 8 as well).
• If you are not sure about the string pool usage, try -XX:+PrintStringTableStatistics JVM argument. It will print you the string pool usage when your program terminates.

Ref: http://java-performance.info/string-intern-in-java-6-7-8/