Java 8 - Difference between Optional.flatMap and Optional.map

Question

What\'s the difference between these two methods: Optional.flatMap() and Optional.map()?

An example would be appreciated.

Answer 1

Note:- below is the illustration of map and flatmap function, otherwise Optional is primarily designed to be used as a return type only.

As you already may know Optional is a kind of container which may or may not contain a single object, so it can be used wherever you anticipate a null value(You may never see NPE if use Optional properly). For example if you have a method which expects a person object which may be nullable you may want to write the method something like this:

void doSome(Optional person){
  /*and here you want to retrieve some property phone out of person
    you may write something like this:
  */
  Optional phone = person.map((p)->p.getPhone());
  phone.ifPresent((ph)->dial(ph));
}
class Person{
  private String phone;
  //setter, getters
}

Here you have returned a String type which is automatically wrapped in an Optional type.

If person class looked like this, i.e. phone is also Optional

class Person{
  private Optional phone;
  //setter,getter
}

In this case invoking map function will wrap the returned value in Optional and yield something like:

Optional> 
//And you may want Optional instead, here comes flatMap

void doSome(Optional person){
  Optional phone = person.flatMap((p)->p.getPhone());
  phone.ifPresent((ph)->dial(ph));
}

PS; Never call get method (if you need to) on an Optional without checking it with isPresent() unless you can't live without NullPointerExceptions.