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
Best Answer
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>() {@Overridepublic Student apply(Person p) {return new Student(p.getId(), p.getName());}})You can write like this Function<Person,Student> function = p -> new Student(p.getId(), p.getName())
so as you see it is represent a function.
personList.stream().filter(p -> p.getPersonType().equals("student")).map(function::apply) // p -> function.apply(p).collect(Collectors.toList());In the context of your Stream<Person> (assuming personList is a List<Person>), the lambda expression p -> new Student(p.getId(), p.getName()) implements a functional interface that accepts a Person instance (an element of your Stream) and returns a Student instance.
Hence it matches the Function<Person,Student> functional interface, which is the interface required by the map() method.
At runtime, the following lambda expression:
p -> new Student(p.getId(), p.getName())will be represented by the class that implements Function<T, R> interface.
An instance of this functional interface can be passed as a parameter to Stream.map(...) method:
<R> Stream<R> map(Function<? super T, ? extends R> mapper);In order to get a better understanding of how this works, one could replace a lambda with a good old anonymous class that implements the corresponding interface.
.map(p -> new Student(p.getId(), p.getName())) is equivalent to:
.map(new Function<Person, Student>() {@Overridepublic Student apply(Person p) {return new Student(p.getId(), p.getName());}})Same as @Ousmane D. but instead of this:
.map(new Function<Person, Student>() {@Overridepublic Student apply(Person p) {return new Student(p.getId(), p.getName());}})Try this:
Function<Person, Student> f = p -> new Student(p.getId(), p.getName());and use it inside your stream with .map(f).