import java.util.ArrayList;
import java.util.List;

class Example {

    public static String[] findWordsOfLength(String letters, int wordSize) {
        if(letters == null) {
            return null;
        }

        int size = letters.length();
        int wordMax = size - wordSize + 1;
        if(size < wordMax || wordMax <= 0) {
            return new String[0];
        }
        
        List<String> result = new ArrayList<>();

        for (int i = 0; i < wordMax; i++) {
            result.add(letters.substring(i, i + wordSize));
        }

        return result.toArray(new String[0]);
    }

    public static void main(String[] args) {
        String[] words = findWordsOfLength("fourgooddogsswam", 4);

        for(String word : words) {
            System.out.println(word);
        }
    }
}