Split alphabetically sorted list in alphabetical based sublists in java

Question

I have a sorted list in java, I just want to split this list into sub-lists based on the first alphabet in each index of the list. For example, the list contains

{
calculator,
catch,
doll,
elephant
}

I want sublists to be

{calculator,catch}
{doll}
{elephant}

I do not want to give statements if 26 if there is an effective and correct way to do this. The list is already sorted alphabetically. Any help would be greatly appreciated.

Answer 1

@SilverNak's solution works well with Java-8, you can use this as well if you are not using Java-8:

public static void main(String[] args) {
    String list[] = {"calculator", "catch", "doll", "elephant"};
    Map<Character, List<String>> map = new HashMap<>();

    List<String> lst;
    for (String str : list) {
        //if the key exit then add str to your list else add a new element
        if (map.containsKey(str.charAt(0))) {
            map.get(str.charAt(0)).add(str);
        } else {
            lst = new ArrayList<>();
            lst.add(str);
            map.put(str.charAt(0), lst);
        }
    }
}

Answer 2

Java 8. , . list , .

Map<Character, List<String>> collect =
        list.stream().collect(Collectors.groupingBy(elem -> elem.charAt(0)));

Answer 3

If you are open to using a third-party library, Eclipse Collections 7.x will work with Java 6. MutableListYou can call the use of Eclipse collections groupByas follows:

MutableList<String> list = 
        Lists.mutable.with("calculator", "catch", "doll", "elephant");
Multimap<Character, String> multimap = 
        list.groupBy(StringFunctions.firstLetter());
System.out.println(multimap); 
// Prints {d=[doll], e=[elephant], c=[calculator, catch]}

If you need to use java.util.Listfor strings, you can use a class ListAdapter.

List<String> list =
        Arrays.asList("calculator", "catch", "doll", "elephant");
Multimap<Character, String> multimap =
        ListAdapter.adapt(list).groupBy(StringFunctions.firstLetter());

Note. I am a committer for Eclipse collections.

Answer 4

Something like this might work.

private List<List<String>> subListOnFirstLetter(List<String> list) {
    List<List<String>> result = new ArrayList<>();
    char first = list.get(0).charAt(0);
    List<String> subList = new ArrayList<>();
    for (String element : list) {
        char next = element.charAt(0);
        if (next != first) {
            result.add(subList);
            subList = new ArrayList<>();
        } else {
            subList.add(element);
        }
        first = next;
    }
    return result;
}