There are a number of errors preventing the conversion. The most important error is in line 29 when you call the method calculateShannonEntropy and pass a string variable, but line 4 says that the method is expecting a ' List of String variables'

I suspect that you have not come accross Lists before so I have converted the method to accept an array of Strings, then lines 22-29 define a string then creates an array of "two letter" values which is then passed to the method.

1. import java.util.*;
2. 
   
3. public double calculateShannonEntropy(String[] values) {
4.   Map<String, Integer> map = new HashMap<String, Integer>();
5.   // count the occurrences of each value
6.   for (String sequence : values) {
7.     if (!map.containsKey(sequence)) {
8.       map.put(sequence, 0);
9.     }
10.     map.put(sequence, map.get(sequence) + 1);
11.   }
12.   // calculate the entropy
13.   double result = 0.0d;
14.   for (String sequence : map.keySet()) {
15.     double frequency = (double) map.get(sequence) / values.length;
16.     result -= frequency * (Math.log(frequency) / Math.log(2));
17.   }
18.   return result;
19. }
20. 
    
21. void setup() {
22.   String s = "to be or not to be that is the question";
23.   String [] sa = new String[s.length() - 1];
24.   println("THe collection of values we want to");
25.   println("calculate the entropy for");
26.   for (int i = 0; i < sa.length; i++) {
27.     sa[i] = s.substring(i, i+2);
28.     println(sa[i]);
29.   }
30.   // The method returns a double need to cast to a float
31.   float entropy = (float) calculateShannonEntropy(sa);
32.   println("Entropy = " + entropy);
33. }