if you see the ObjectId.java (http://github.com/mongodb/mongo-java-
driver/blob/master/src/main/org/bson/types/ObjectId.java)

isValid(String) method it returns "true" for those strings whose
character length is between 18 and 24 and whose each character happens
to be a qualified hex character.

And then if you see the constructor code, you would notice following
piece of code:

byte b[] = new byte[12];
for ( int i=0; i<b.length; i++ )

{ b[b.length-(i+1)] = (byte)Integer.parseInt( s.substring( i*2 , i*2 + 2) , 16 ); }

Here it expects the string to be of 24 characters.

Hence sometimes though isValid method returns true it cannot create a
valid ObjectId. It throws java.lang.StringIndexOutOfBoundsException:
String index out of range.

This is an important issue as in some cases does not allow to store valid data.