如何使用 java 中的 stream api 在两个列表中添加每个元素

Question

partnersName =["partner1","partner2"];

totalDates = ["2022-05-18","2022-05-19"];

both the lists store string only

here is the following code which gives output that I want

 for (int i = 0; i < partnersName.size(); i++) {
        if (totalDates.isEmpty()) {
            totalDates.add(partnersName.get(i));
        } else {
            for (int j = 0; j < totalDates.size(); j++) {
                totalDates.set(j, partnersName.get(i) +"/" +totalDates.get(j));
            }
        }
    }

output =["partner1/2022-05-18","partner2/2022-05-18","partner1/2022-05-19","partner2/2022-05-19"];

but I want to reduce the line of code using stream. Anyone could help to get stream API code

Answer 1

Assuming the result [partner1/2022-05-18, partner1/2022-05-19, partner2/2022-05-18, partner2/2022-05-19] is what you want:

List<String> result = partnersName.stream().<String>mapMulti((s, stringConsumer) -> totalDates.forEach(d -> stringConsumer.accept(s + "/" + d))).toList();

Answer 2

Another way can be

List<String> result = partnersName.stream()
                        .flatMap(x -> totalDates.stream().map(y -> x.concat("/"+y)))
                        .collect(Collectors.toList());

//output: [partner1/2022-05-18, partner1/2022-05-19, partner2/2022-05-18, partner2/2022-05-19]

Answer 3

Here is one way.

List<String> partnerName = List.of("partner1","partner2");
List<String> totalDates = List.of("2022-05-18","2022-05-19");

List<String> result = partnerName.stream()
        .flatMap(name -> totalDates.stream()
                .map(date->String.join("/", name, date)))
        .toList();
    
System.out.println(result);

prints

[partner1/2022-05-18, partner1/2022-05-19, partner2/2022-05-18, partner2/2022-05
-19]

• The above uses nested streams to process each List.
• String.join - takes a delimiter and a variable number of arguments and concatenates them, separated by the delimiter.
• flatMap flattens the nested streams into one for conversion to a list.