Java string pool and intern

Two ways of creating string are

1) String s ="hello"; No. of string literal = 1 i.e. "hello" - No. of String objects on heap = 0

2) String s= new String("hello"); - No. of string literals =1 and No. of string objects =1

Java maintains a string pool of "LITERALS" and not of objects.

Advantage of string pooling: 1) Reduced memory usage*(PermGenSpace Issue) 2) Faster Comparision i.e == comparision 3) Faster lookup

Disadvantages: 1) Overhead maintaining pool

How to pool String objects? Use Intern() on a string to add it to the pool. Downside of interning: 1) You may forget to intern some strings and compare them by == leading to unexpected results.

*Interning large number of strings is a problem again. 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.