Find all occurences of an Element in List-CodePudding

I want to write a method with the following body:

public static List<Integer> collectAllIndices(List<String> ls, String elem) {

        List<Integer> list = new ArrayList<>();

        return list;
    }

I have to save the Position(index) of every occurrence in a new List How can I do this? if I'm not wrong, indexof() only returns the first occurrence?

CodePudding user response：

With Java 8:

public static <T> List<Integer> collectAllIndices(List<T> list, T element) {
    List<Integer> results = IntStream.range(0, ls.size())
        .boxed()
        .filter(i -> list.get(i).equals(element))
        .collect(Collectors.toList());
    return results;
}

CodePudding user response：

You could simply iterate through the list and save them. Something like:

public static List<Integer> collectAllIndices(List<String> ls, String elem) {

        List<Integer> list = new ArrayList<>();

        for(int i = 0; i < ls.size(); i  ) {
            if(elem.equals(ls.get(i)) {
                list.add(i);
            }
        }

        return list;
}

EDIT: it seems like a commenter posted the same solution at the same time

CodePudding user response：

You can just iterate and increment an index counter, if the element in the list matches, store it in an index list. Also this looks like a generic piece of code, so you can use generic type <T>, and it will work with any type that support equals, not only strings.

Example:

public static <T> List<Integer> collectAllIndices(List<T> ls, T elem) {
    List<Integer> results = new ArrayList<>();
    int index = 0;
    for (var i : ls) {
        if (i.equals(elem)) {
            results.add(index);
        }
        index  ;
    }
    return results;
}