See the following REPL session:
Welcome to Scala version 2.10.0-RC3 (OpenJDK 64-Bit Server VM, Java 1.7.0_09). Type in expressions to have them evaluated. Type :help for more information. scala> var name = "foo" + 1 name: String = foo1 scala> var s1 = Symbol(name) s1: Symbol = 'foo1 scala> s1 = null s1: Symbol = null scala> System.gc scala> val s2 = Symbol("foo1") s2: Symbol = 'foo1 scala> name = null name: String = null scala> System.gc scala> val s3 = Symbol("foo1") s3: Symbol = 'foo1 scala> s2 eq s3 res2: Boolean = false
The problem arises if the symbol object for a particular name is garbage collected (so the weak reference stored in Symbol's weak hash map is null), but the string for that name has not been garbage collected (so the key still exists in the weak hash map). If a new symbol object with the same name is added, the weak hash map key will be a different object from the name field of the symbol object. If that string is subsequently garbage collected, the weak hash map entry disappears, so if the symbol is then created for a third time, it will end up being a different object from the second one.
For additional background, see https://groups.google.com/d/topic/scala-user/mp_pSbfRECI/discussion