All anagrams indices of pattern in a string

AnagramIndices.java

package code;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Main {

    public static Map<Character, Integer> getCharacterFrequency(String s) {
        Map<Character, Integer> m = new HashMap<Character, Integer>();
        for (int i = 0; i < s.length(); i++) {
            if (m.containsKey(s.charAt(i))) {
                m.put(s.charAt(i), m.get(s.charAt(i)) + 1);
            } else {
                m.put(s.charAt(i), 1);
            }
        }
        return m;
    }

    public static List<Integer> getAnagramIndices(String s, String p) {
        List<Integer> l = new ArrayList<>(); // result array

        if (p.length() > s.length()) {
            return l;
        }

        String current = "";

        // create initial window
        for (int i = 0; i < p.length(); i++) {
            current = current.concat(String.valueOf(s.charAt(i)));
        }

        Map<Character, Integer> currentWindowMap = getCharacterFrequency(current); // sliding window map
        Map<Character, Integer> patternMap = getCharacterFrequency(p); // input pattern map

        int i = p.length();

        do {
            if (currentWindowMap.equals(patternMap)) {
                l.add(i - p.length());
            }

            // character at beginning of sliding window
            char c = s.charAt(i - p.length());

            // reduce count of window's beginning character
            currentWindowMap.put(c, currentWindowMap.get(c) - 1);

            if (currentWindowMap.containsKey(c) && currentWindowMap.get(c) == 0) {
                // remove char from map if it's count becomes 0
                currentWindowMap.remove(c);
            }

            // slide the window
            if (currentWindowMap.containsKey(s.charAt(i))) {
                currentWindowMap.put(s.charAt(i), currentWindowMap.get(s.charAt(i)) + 1);
            } else {
                currentWindowMap.put(s.charAt(i), 1);
            }

            i += 1;

        } while (i < s.length());

        if (currentWindowMap.equals(patternMap)) {
            l.add(i-p.length());
        }

        return l;
    }

    public static void main(String[] args) {

        // eg. 1
        System.out.println("eg. 1");
        for (int i : getAnagramIndices("hello", "lol")) {
            System.out.println(i);
        }
        ;

        System.out.println("eg. 2");
        // eg. 2
        for (int i : getAnagramIndices("msississsippi", "siss")) {
            System.out.println(i);
        }

    }
}