Lambda expression java 8 map method

Question

The syntax of map method in java 8 is :

<R> Stream<R> map(Function<? super T,? extends R> mapper)

but i can use a lambda expression :

personList.stream().filter(p -> p.getPersonType().equals("student"))
            .map(p -> new Student(p.getId(), p.getName()))
            .collect(Collectors.toList());

How does the argument in map method equates to a Function datatype.Please help me understand this .

Thanks

Answer 1

The function Function<? super T,? extends R> mapper of the map method basically represents any function taking one parameter and returning a value so in this specific case the lambda p -> new Student(p.getId(), p.getName()) is a function taking a Person p and returning a Student hence fits into that perfectly.

To look at it another way, the lambda is equivalent to:

.map(new Function<Person, Student>() {
     @Override
      public Student apply(Person p) {
            return new Student(p.getId(), p.getName());
      }
})