AnagramGrouper.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;

public class AnagramGrouper {

    public static List<List<String>> groupAnagrams(String[] words) {
        if (words == null || words.length == 0) {
            return new ArrayList<>();
        }

        Map<String, List<String>> anagramGroups = new LinkedHashMap<>();

        for (String word : words) {
            if (word == null) {
                continue;
            }

            String key = createAnagramKey(word);

            anagramGroups
                    .computeIfAbsent(key, ignored -> new ArrayList<>())
                    .add(word);
        }

        return new ArrayList<>(anagramGroups.values());
    }

    private static String createAnagramKey(String word) {
        char[] characters = word.toLowerCase().toCharArray();
        Arrays.sort(characters);

        return new String(characters);
    }

    public static void main(String[] args) {
        String[] words = {
                "eat",
                "tea",
                "tan",
                "ate",
                "nat",
                "bat"
        };

        List<List<String>> groupedAnagrams = groupAnagrams(words);

        groupedAnagrams.forEach(System.out::println);
    }
}