How to convert a Set<Set> to an ArrayList<ArrayList>

arraylist, collections, java

Solution

With Java 8, you can do:

return sameAnagramsSet.stream()
       .map(ArrayList::new)
       .collect(toList());

although it returns a `List<ArrayList<String>>`.

What it does:

- `.stream()` returns a `Stream<Set<String>>`

- `.map(ArrayList::new)` is equivalent to `.map(set -> new ArrayList(set))` and basically replaces each set by an array list

- `collect(toList())` places all the newly created lists in one list

Problem

How would I add all the elements of a `Set<<Set<String>>` var to an `ArrayList<<ArrayList<String>>`? Of course I'm aware of the naive approach of just adding them. ``` private static ArrayList<ArrayList<String>> groupAnagrams(ArrayList<String> words){ ArrayList<ArrayList<String>> groupedAnagrams = new ArrayList<>(); AbstractMap<String, String> sortedWords = new HashMap<>(); Set<Set<String>> sameAnagramsSet = new HashSet<>(); for(String word : words){ char[] wordToSort = word.toCharArray(); Arrays.sort(wordToSort); sortedWords.put(word, new String(wordToSort)); } for(Map.Entry<String, String> entry: sortedWords.entrySet() ){ Set<String> sameAnagrams = new HashSet<>(); sameAnagrams.add(entry.getKey()); for(Map.Entry<String, String> toCompare : sortedWords.entrySet()){ if(entry.getValue().equals(toCompare.getValue())){ sameAnagrams.add(toCompare.getKey()); } } if(sameAnagrams.size()>0){ sameAnagramsSet.add(sameAnagrams); } } //-->this line does not work! return new ArrayList<ArrayList<String>>(sameAnagramsSet); } ```

Original source