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);
        }
    }
}

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