如何查找列表是否包含多个谓词Java Lambda

Question

I am trying to figure out how to do multiple predicate on a single filter in Java Lambda but not had much luck.

I have a list of Strings

List<String> namesList = new ArrayList(){};
namesList.add("John");
namesList.add("Jane");
namesList.add("Smith");
namesList.add("Roger");

I have two if statements below in pseudologic that i want to test but not sure how to do it with lambda (i can do it old school method but trying to learn here).

if nameslist contains John and Roger
   print "John & Roger"

if nameslist contains Jane and Smith
   print "Jane Smith"

Using Java lambda how can i test for both scenarios on the list?

Answer 1

Don't use streams: Just convert English to code:

if (namesList.contains("John") && namesList.contains("Roger"))
    System.out.println("John & Roger");

Or

if (namesList.containsAll(Arrays.asList("John", "Roger")))
    System.out.println("John & Roger");

It's easier to read and will likely perform as well or better than the stream-based approach.

A lambda is not the right approach.

Answer 2

I would do it as follows:

    if (namesList.stream()
            .filter(x -> (x.equals("John") || x.equals(("Roger"))))
            .collect(Collectors.toSet())
            .size() == 2) {
        System.out.print("John & Roger");
    }

    if (namesList.stream()
            .filter(x -> (x.equals("Jane") || x.equals(("Smith"))))
            .collect(Collectors.toSet())
            .size() == 2) {
        System.out.print("Hane Smith");
    }

Answer 3

You could combine distinct and count:

if (namesList.stream()
        .filter(s -> s.equals("John") || s.equals("Roger"))
        .distinct()
        .count() == 2) {
    System.out.print("John & Roger");
}