A map operation applies a function to each element of the input stream to produce another stream.
You can map a stream of T to a stream of type S, where T and S may be the same or different types.
You can map a stream of Person to a stream of Person ID.
You can apply the map operation on a stream using one of the following methods of the Stream<T> interface:
<R> Stream<R> map(Function<? super T,? extends R> mapper) DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper) IntStream mapToInt(ToIntFunction<? super T> mapper) LongStream mapToLong(ToLongFunction<? super T> mapper)
The map operation takes a function as an argument.
Each element from the input stream is passed to the function.
The following code creates an IntStream whose elements are integers from 1 to 5, maps the elements of the stream to their squares, and prints the mapped stream on the standard output.
IntStream.rangeClosed(1, 5) .map(n -> n * n) .forEach(System.out::println);
The following code maps the elements of a stream of people to their names and prints the mapped stream.
Person.persons() .stream() .map(Person::getName) .forEach(System.out::println);
import java.time.LocalDate; import java.time.Month; import java.util.Arrays; import java.util.List; public class Main { public static void main(String[] args) { Person.persons()// w ww.ja v a2s . co m .stream() .map(Person::getName) .forEach(System.out::println); } } class Person { // An enum to represent the gender of a person public static enum Gender { MALE, FEMALE } private long id; private String name; private Gender gender; private LocalDate dob; private double income; public Person(long id, String name, Gender gender, LocalDate dob, double income) { this.id = id; this.name = name; this.gender = gender; this.dob = dob; this.income = income; } public long getId() { return id; } public void setId(long id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Gender getGender() { return gender; } public boolean isMale() { return this.gender == Gender.MALE; } public boolean isFemale() { return this.gender == Gender.FEMALE; } public void setGender(Gender gender) { this.gender = gender; } public LocalDate getDob() { return dob; } public void setDob(LocalDate dob) { this.dob = dob; } public double getIncome() { return income; } public void setIncome(double income) { this.income = income; } public static List<Person> persons() { Person ken = new Person(1, "Java", Gender.MALE, LocalDate.of(1970, Month.MAY, 4), 6020.0); Person jeff = new Person(2, "Jack", Gender.MALE, LocalDate.of(1970, Month.JULY, 15), 7320.0); Person donna = new Person(3, "Javascript", Gender.FEMALE, LocalDate.of(1972, Month.JULY, 29), 8720.0); Person chris = new Person(4, "XML", Gender.MALE, LocalDate.of(1993, Month.DECEMBER, 16), 5800.0); Person laynie = new Person(5, "Json", Gender.FEMALE, LocalDate.of(2002, Month.DECEMBER, 13), 0.0); Person lee = new Person(6, "Database", Gender.MALE, LocalDate.of(2011, Month.MAY, 9), 2400.0); // Create a list of persons List<Person> persons = Arrays.asList(ken, jeff, donna, chris, laynie, lee); return persons; } @Override public String toString() { String str = String.format("(%s, %s, %s, %s, %.2f)", id, name, gender, dob, income); return str; } }