I have a question regarding the initiation of ArrayList in Java.

As far as I know I have to specify the type of my ArrayList when initialize it. For example:

ArrayList<String> names = new ArrayList<String>();

or

ArrayList<String> names = new ArrayList<>();

However, when I want to write a code to return an ArrayList of List, which is from the values of a map Map<String, List> map = new HashMap<>(); If I tried to use the following, error happens

return new ArrayList<>(map.values())

or

return new ArrayList<List<String>>(map.values())

Instead, I can only use the one, without the <>.

return new ArrayList(map.values())

Could anyone let me know when I should or should not use the <> when I initialize an ArrayList(or any other objects)?

The original code is as written below

    public List<List<String>> groupAnagrams(String[] strs) {
        if (strs.length == 0) return new ArrayList<>();
        Map<String, List> res = new HashMap<>();
        for (String s : strs) {
            char[] charArray = s.toCharArray();
            Arrays.sort(charArray);
            String key = String.valueOf(charArray);
            if (!res.containsKey(key)) res.put(key, new ArrayList());
            res.get(key).add(s);
        }
        
        return new ArrayList<List<String>>(res.values());
    }

CodePudding user response：

public List<List<String>> groupAnagrams(String[] strs) {
  ...
  Map<String, List> res = new HashMap<>();
  ...
  res.put(key, new ArrayList());
  ...
  return new ArrayList<List<String>>(res.values());
}

The problem here is that res.values() isn't a Collection<List<String>>, which is what would be required: it's a Collection<List>.

The elements of res.values() are raw types: you've used List without specifying the type parameters.

Raw types disable type checking, meaning you can end up putting things into collections with types you don't want.

Every time you use a generic type, make sure it has type parameters (or diamonds, if allowed:

public List<List<String>> groupAnagrams(String[] strs) {
  ...
  Map<String, List<String>> res = new HashMap<>();
  ...
  res.put(key, new ArrayList<>());
  ...
  return new ArrayList<>(res.values());
}

CodePudding user response：

Because the type of map value is List , but you define the type of list value is string.

And suggest use guava Multimap instead of Map<String ,List>