Timer class to Map

Question

For example, I have some class Person

class Person {

    private String firstName;
    private String lastName;
    private String street;

    ...
}

Also, I have a List<Person> personList that contains some Person objects. The goal is to put these objects to Map<Person, Timer> personMap as keys and add new Timer() objects as values. So each Person has one Timer().
I'm, trying to do next:

personMap = personList.stream().collect(toMap(person -> person, new Timer()));

But compiler says: there is no instance(s) of type variable(s) T, U exist so that Timer conforms to Function<? super T, ? extends U>. I've been searching here Java 8 List into Map but it doesn't work for me.

What am I missing? What's wrong with Timer() class?

Change `new Timer()` to `person -> new Timer()` – ernest_k Aug 28 '18 at 11:35 — ernest_k, Aug 28 '18 at 11:35

score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

You have to provide a function for both key and vaue to the Collectors::toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper) where:

keyMapper - a mapping function to produce keys

valueMapper - a mapping function to produce values

You need:

Map<Person, Timer> personMap = personList.stream()
    .collect(Collectors.toMap(
        person -> person,           // Function to create a key
        person -> new Timer()));    // Function to create a value

For better understanding, here are lambdas represented as anonymous classes using the very same Collector:

personList.stream().collect(Collectors.toMap(
        new Function<Person, Person>() {                    // Function to create a key
            @Override
            public Person apply(Person person) {
                return person;                              // ... return self
            }},
        new Function<Person, Timer>() {                     // Function to create a value
            @Override
            public Timer apply(Person person) {
                return new Timer();                         // ... return a new Timer
            }}
    ));

score 1 · Answer 2 · answered Aug 28 '18 at 11:54

The second argument of toMap is a function (Function<? super Person, ? extends Timer> valueMapper in this case).

new Timer() is not a Function instance, it's a Timer instance (which explains the argument type mismatch error). You can fix the problem with:

Map<Person, Timer> map = personList.stream()
        .collect(Collectors.toMap(Function.identity(), person -> new Timer()));

person -> new Timer() is a lambda expression with the signature of the expected function.

Timer class to Map

2 Answers2