[JAVA] Stream API map method

Summary

map transforms all the elements stored in its stream object map method uses two type variables The Stream interface takes Stream <T> and the map method returns Stream <R>.

map method signature

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

The first line

Declaring the Stream interface. Here, the type variable T is specified to make a generic type interface. That is, any type can be handled.

2nd line

Defines the signature of the map method.

<R> -> Declaration of type variable -> The map method allows you to use two type variables, T and R.

Stream<R> -> Declaration of return value of map method -> The return value of the map method is Stream type

** Since the map method is defined in the Stream \ <T > interface, this method will be responsible for converting Stream <T> to Stream <R>. ** **

map -> Method name

Function<? super T, ? extends R> -> Argument type -> Function is a functional interface introduced from Java8 -> Function interface takes a T-type object and returns an R-type object -> ? Super and? Extends are notations called boundary wildcards -> This widens the range of functions that can be passed to the map method.

mapper -> Formal argument name

Example

sample.java

public List<String> collectEmpNames(List<Employee> employees) {
    return employees.stream()
                  .map(e -> e.getName())
                  .collect(Collectors.toList());
  }

The first line

The stream () method is called to convert the Employee List to Stream.

2nd line

You are calling the map method. Extract the name from each element in the Stream of Employee and convert it to Stream of string.

e -> e.getName() The map method takes a Function type function object as an argument. The above lambda expression is an object that implements the Function interface.

Specifically ... Employee is stored in Stream when the map method is called. That is, the type T is Employee. Therefore, you can call getName () of the Employee class on the right side of the expression. Since the return value of getName () is String, the return value of the map method will be Stream \ <String >.

3rd line

Calling the collect method to convert Stream to List.