Fibonacci Number

Write a method that returns the nth element of the Fibonacci Sequence
The Fibonacci Sequence is the series of numbers: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34,...
The next number is found by adding up the two numbers before it.
Assume that indexes start at zero, e.g., fib(0) = 0, fib(1) = 1, ...

public Integer fibonacci(Integer n) {
}