CODEWITHSUNDEEP
javajava-8java-stream
Ravi
Ravi

Reputation: 971

Filter the objects from a given list with common criteria using Java Streams

I have a requirement to fetch the sublist from a given super list as explained below.

Let's say I have a super list of "Person" objects as follows:

List<Person> personsList = new ArrayList<>();
personsList.add(new Person("John", 28));
personsList.add(new Person("Paul", 29));
personsList.add(new Person("Adam", 30));
personsList.add(new Person("Peter", 31));
personsList.add(new Person("Kate", 32));
personsList.add(new Person("John", 67));
personsList.add(new Person("Paul", 68));
personsList.add(new Person("Adam", 69));

Let the Person class be defined as follows:

class Person {
    public String name;
    public int age;

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

    @Override
    public String toString() {
        return name + ": " + age;
    }
}

By using Java Streams API, how do I fetch a sub list with the entries like below (note that the repeated instances of objects with same name have to be fetched):

John : 28
John : 67
Paul : 29
Paul : 68
Adam : 30
Adam : 69

Can someone please help me in achieving this by using Java 8 Streams API?

P.S : I would not pre handedly know what values are there as names for those objects.

Upvotes: 2

Views: 157

Answers (1)

Yassin Hajaj
Yassin Hajaj

Reputation: 21975

The following operation will give you your awaited result. You first group by the name all your persons, from the resulting list, you exclude all the ones with a size less than 2 and flatMap everything, sort it and print everything

personsList.stream()      
       .collect(Collectors.groupingBy(Person::getName))
       .values()
       .stream()
       .filter(list -> list.size() > 1)
       .flatMap(List::stream)
       .forEach(System.out::println);

Output

Adam: 30
Adam: 69
John: 28
John: 67
Paul: 29
Paul: 68

Upvotes: 6

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