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

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.

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));