Is Java's String Intern a flyweight?

Question

Does the implementation of Java's String memory pool follows flyweight pattern?

Why I have this doubt is, I see that there is no extrinsic state involved in Intern. In GoF I read that there should be a right balance between intrinsic and extrinsic state. But in intern everything is intrinsic.

Or shall we say there is no strict rule with respect to attributes and just sharing objects to reduce memory is sufficient to call it a flyweight.

Please help me understand.

Answer 1

Yes the String.intern() implementation follows the flyweight pattern.

As the javadoc says

Returns a canonical representation for the string object. A pool of strings, initially empty, is maintained privately by the class String.

When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.

It follows that for any two strings s and t, s.intern() == t.intern() is true if and only if s.equals(t) is true.

All literal strings and string-valued constant expressions are interned. String literals are defined in §3.10.5 of the Java Language Specification

The internalized strings reside in the "Perm Gen" space and on string objects returned by .intern() you can use the operator == because .intern() returns always the same object for equal values.

Then remember that .intern() method does not produce leaks, because the JVM today is able garbage the pool.

Try to read this article too.