How to generate an n-gram in Java

import java.util.*;

class Ngrams {
  public static List<String> ngrams(int n, String str) {
    List<String> ngrams = new ArrayList<String>();
    for (int i = 0; i < str.length() - n + 1; i++)
        // Add the substring or size n
        ngrams.add(str.substring(i, i + n));
        // In each iteration, the window moves one step forward
        // Hence, each n-gram is added to the list

    return ngrams;
  }

  public static void main( String args[] ) {
      String s = "abcdef";
      List<String> ngrams = ngrams(3, s);
      for (String ngram : ngrams){
        System.out.println(ngram);
      }
  }
}

import java.util.Iterator;
class NgramIterator implements Iterator<String> {
    private final String str;
    private final int n;
    int pos = 0;
    public NgramIterator(int n, String str) {
        this.n = n;
        this.str = str;
    }
    public boolean hasNext() {
        return pos < str.length() - n + 1;
    }
    public String next() {
        return str.substring(pos, pos++ + n);
    }
    public static void main( String args[] ) {
        String s = "abcdef";
        new NgramIterator(3, s).forEachRemaining(System.out::println);
    }
}