How to convert Stream to a Map in Java 8

Almost every developer has come into the following situation: You have a list of objects with IDs and you want to create a map to quickly access the right object with an ID.

As is so often the case, the Person class with ID, Firstname, and Lastname is used here:

public class Person 
{
    private String id;
    private String firstName;
    private String lastName;

    public Person(String id, String firstName, String lastName) {
        this.id = id;
        this.firstName = firstName;
        this.lastName = lastName;
    }
    ... Getter, Setter, toString, etc...
}

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If you want to further process a list of objects, the Stream API is available from Java 8. To “collect” the elements of a stream into a collection, there is the collect() method. This method accepts a collector. The static utility class java.util.stream.Collectors offer a selection of different standard collectors.

In the following example, a list of people is created first. In many cases such a list will probably come from a database.

List<Person> persons = new ArrayList<>();

persons.add(new Person("1", "Alex", "Itomir"));
persons.add(new Person("2", "Thomas", "Seintross"));
persons.add(new Person("3", "Emily", "Deggy"));

Map<String, Person> map = persons
        .stream()
        .collect(Collectors.toMap(p -> p.getId(), p -> p));

System.out.println(map.get("2"));

Output:

[2, Thomas, Seintross]

A stream can be created directly from the list and processed immediately with collect(). The factory method Collectors.toMap() has two arguments: a function that returns the key and a function for the corresponding value. In our case, we want the ID of the person as the key, which we get from the lambda expression p -> p.getId(). We want the person himself as the associated value, i.e. p -> p.

The result is a map which contains the IDs as keys and the corresponding objects as values.
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