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];
	    }
 
	    int j = 0;
	    String[] result = new String[wordMax];
 
	    for (int i = 0; i < wordMax; i++) {
	        result[j ++] = letters.substring(i, i + wordSize);
	    }
 
	    return result;
	}
 
    public static void main(String[] args) {
        String[] words = findWordsOfLength("fourgooddogsswam", 4);
 
        for(String word : words) {
            System.out.println(word);
        }
    }
}

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