Java streams zip two lists

问题

I have a HashSet of Persons. A person has a firstName, lastName and age like: Person("Hans", "Man", 36)

My task is to get a list of the persons who are older than 17, sort them by age and concat the firstName with the lastName like: ["Hans Man","another name", "another name"]

Im just allowed to import:

import java.util.stream.Stream; import java.util.stream.Collectors; import java.util.List; import java.util.ArrayList;

My idea was to sort them first, map the names in separate Streams and to zip them, but it doesn't work.

public static void getNamesOfAdultsSortedByAge(Stream<Person> stream){

    Stream<Person> result = stream;
    Stream<Person> result1 = result.filter(p -> p.getAge() >= 18)
                            .sorted((x, y) -> Integer.compare(x.getAge(),y.getAge()));


    Stream<String> firstName = result1.map(Person::getFirstName);
    Stream<String> lastName = result1.map(Person::getLastName);

    Stream<String> result3 = concat(firstName, lastName);

    List<String> result4 = result3.collect(Collectors.toList());

    System.out.println(result4);
}

thank you in advance

回答1:

You could do so using :

public static void getNamesOfAdultsSortedByAge(Stream<Person> stream) {
    List<String> sorted = stream.filter(p -> p.getAge() >= 18)
                                .sorted((x, y) -> Integer.compare(x.getAge(),y.getAge()))
                                .map(e -> e.getFirstName() + " " + e.getLastName())
                                .collect(Collectors.toList());
    System.out.println(sorted);
}

Here we just map the sorted stream by concatenating the first name and then the last name, after which we use the .collect() terminal operation to collect it to a list.

回答2:

This doesn't work:

Stream<String> firstName = result1.map(Person::getFirstName);
Stream<String> lastName = result1.map(Person::getLastName);

After the first map, the Stream is of type String which doesn't fit Person::getLastName. You need to concat within the first map() call itself.

回答3:

Inside getNamesOfAdultsSortedByAge method:

stream
    .filter(p -> p.getAge() > 17)
    .sorted(Comparator.comparingInt(Person::getAge))
    .map(person -> person.getFirstName() + " " + person.getLastName())
    .collect(Collectors.toList())

I suppose it should suit your needs. filter filters people that are only 18 or older, second line sorts given by getAge predicate, third just maps Person instance to a string containing %firstName% %lastName%, and fourth collects the result into a list.

回答4:

I would use the following and as you have strange rules regarding the imports, I hide that one for Comparator deliberately:

List<String> result = stream.filter(p -> p.getAge() >= 18)
                            .sorted(java.util.Comparator.comparingInt(Person::getAge))
                            .map(p -> p.getFirstName() + " " + p.getLastName())
                            .collect(Collectors.toList());

Note: I know of some rules that imply that no other dependency should be used than the one listed on the "allowed list of dependencies", but I've never heard of a rule that only allows to use like 4 imports but disallows one of the most important ones. If Comparator would be an internal interface/class I could understand it, but it's an essential interface. You even use it as a functional interface if you write (x, y) -> Integer.compare(x.getAge(),y.getAge()), so I really don't get why this import shouldn't be allowed. That being said: my solution does not require you to use an import to java.util.Comparator. Have fun using it. It really makes comparisons easier and less error-prone (just imagine two similarly named methods and you catched the wrong one by mistake... have fun finding that error ;-)).

回答5:

public static void main(String[] args) {
    final List<String> matching = Lists.newArrayList(
            new Person("Joey", "Tribiani", 28),
            new Person("Rachel", "Green", 12),
            new Person("Ross", "Geller", 114),
            new Person("Chandler", "Bing", 17)).stream()
            .filter(person -> person.getAge() > 17)
            .sorted(Comparator.comparing(Person::getAge))
            .map(person -> person.getFirstName() + " " + person.getLastName())
            .collect(toList());
}

If you need to change sorting order just reverse the comparator:

Comparator.comparing(Person::getAge).reversed()

来源：https://stackoverflow.com/questions/53537160/java-streams-zip-two-lists