如何在java 8中使用stream过滤两个列表对象并将值设置为新的List

Question

I using java 8 and i have two object look like :

Person class :

public class Person {

    private String id;
    private String name;

    public Person() {
    }

    public Person(String id, String name) {
        this.id = id;
        this.name = name;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Person{" +
                "id='" + id + '\'' +
                ", name='" + name + '\'' +
                '}';
    }
}

Person1 Class :

public class Person1 {
    private String id;
    private String name;

    public Person1() {
    }

    public Person1(String id, String name) {
        this.id = id;
        this.name = name;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Person1{" +
                "id='" + id + '\'' +
                ", name='" + name + '\'' +
                '}';
    }
}

I have two list look like :

 List<Person> persons= Arrays.asList(new Person("1","A"),new Person("2","B"));
  List<Person1> persons1 = Arrays.asList(new Person1("3","C"),new Person1("1","F"));

Now i want using stream java 8 loop two list and compare. If any object in list persons equal with persons1 , i will create new list and set it with new value. Example : If Person1("1","F") equal Person("1","A") because i using id compare it, i get name from Person1 set to Person. Result : Person("1,"F") and add it two new list.

My code when i using for:

 for (Person person : persons) {
            for (Person1 person1 : persons1 ) {
                if (person1.getId().equals(person.getId())) {
                    person.setName(person1.getId());
                    break;
                }
            }
        }

Now i want convert it to Stream with new list look like :

 List<Person> personList =
                list.stream()
                        .filter (
                                person -> list1.stream()
                                        .anyMatch(person1 -> person1.getId()== person.getId()))
                        .collect(Collectors.toList());

But it only filter person. I want compare two list object. If same id, i want set name from object in persons1 to persons. I know we can using map in stream java 8 but i can't how to do that. Please help

Answer 1

Some tasks are best accomplished with streams, and others with iteration. Many tasks are best accomplished by combining the two approaches. In this solution streams are used to construct the map and then iteration is used to update matching person's name. Your solution runs in Quadratic time whereas this solution runs in linear time complexity.

Map<String, String> idToNameMap = persons1.stream()
    .collect(Collectors.toMap(Person1::getId, Person1::getName, (a, b) -> a));
for (Person person : persons) {
    if (idToNameMap.containsKey(person.getId())) {
        person.setName(idToNameMap.get(person.getId()));
    }
}

Answer 2

It's not the most beautiful answer etc but i guess it's gonna help u get idea how its work.

List<Person> collect = persons.stream()
        .filter(person -> persons1.stream().anyMatch(person1 -> person1.getId() == person.getId()))
        .map(person -> {
            Person1 getted = persons1.stream()
                .filter(person1 -> person1.getId() == person.getId())
                .findAny()
                .orElseGet(null);
            if (getted == null) throw new IllegalStateException("Should be Impossible");
            person.setName(getted.getName());
            return person;
        })
        .collect(Collectors.toList());