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

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. — Naman
– Naman, Commented May 28, 2019 at 6:36

Eran · Accepted Answer · 2019-05-28 06:35:08Z

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.

Naman · Accepted Answer · 2019-05-29 01:26:41Z

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));

Joka Lee · Accepted Answer · 2019-05-28 10:29:02Z

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]

Anshul Singhal · Accepted Answer · 2019-05-29 09:11:04Z

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:

Collectives™ on Stack Overflow

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

4 Answers 4

Comments

1 Comment

Comments

Comments

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

1 Comment

Comments

Comments

Related