How to filter a List using Java 8 stream and startwith array of values

Question

I have 2 Lists, the one contains a list of numbers and the other a list of names. I have prefixed the names with a number from the first list, followed by an underscore. I want to filter the second list based on all the numbers found in the first list.

What I have tried.

List<String> numberList = new ArrayList<>();
numberList.add("1_");
numberList.add("9_");

List<String> nameList = new ArrayList<>();
nameList.add("1_John");
nameList.add("2_Peter");
nameList.add("9_Susan");

List<String> filteredList = Stream.of(numberList.toArray(new String[0]))
                .filter(str -> nameList.stream().anyMatch(str::startsWith))
                .collect(Collectors.toList());

The code above runs with no error, but the filteredList is empty. Clearly I am doing something wrong.

The filteredList should contain only:

1_John

9_Susan

Laevogyrate

To create a stream from a Collection just use the existing API as Stream.of(numberList.toArray(new String[0])) => numberList.stream(). While you're actually iterating on the incorrect list using the streams. It should be the second list to iterate on and first used to filter in. – Azine

Answer 1

You call startsWith on the wrong Strings (for example, you test if "1_".startsWith("1_John") instead of "1_John".startsWith("1_")).

You should stream over nameList and use numberList for the filtering:

List<String> filteredList = 
    nameList.stream()
            .filter(str -> numberList.stream().anyMatch(str::startsWith))
            .collect(Collectors.toList());

P.S. Stream.of(numberList.toArray(new String[0])) is redundant. Use numberList.stream() instead.

Airman

Answer 2

As an alternate to Eran's solution, you can also use a combination of removeIf and noneMatch as follows:

List<String> filteredList = new ArrayList<>(nameList);
filteredList.removeIf(str -> numberList.stream().noneMatch(str::startsWith));

Azine

Thanks, I will also give this a try to see it in action. – Laevogyrate

Answer 3

Another attempt to give a solution could be to use contains , though the actual implementation of startsWith and contains are different but in OP case both will fetch the same result.

List<String> strList = nameList.stream().filter(s->numberList.stream().anyMatch(s::contains)).collect(Collectors.toList());

Output:

[1_John, 9_Susan]

Cathern

Answer 4

List<String> numberList = new ArrayList<>();
numberList.add("1_");
numberList.add("9_");

List<String> nameList = new ArrayList<>();
nameList.add("1_John");
nameList.add("2_Peter");
nameList.add("9_Susan");

Following is the correct code:

Your code uses the following:

Lambard