Write java method

Let L be the language defined by: L-(S: S is in the form of A"B2n for some n 0) thus a string is in L if it starts with a sequence of A's and is followed by a sequence of twice as many B's. For example, the string AABBBB, the empty string, etc are in this language but the strings AABB, AB, etc. are not in the language. Write a recursive method that determines if the string str in in L. (12 pts)