The filter operation applies on an input stream to produce another stream.
The filtered stream contains all elements for which a predicate evaluates to true.
The filtered stream is of the same type as the input stream.
The filter operation produces a subset of the input stream.
If the predicate evaluates to false for all elements of the input stream, the filtered stream is an empty stream.
The following code uses a stream of people and filters in only females.
It maps the females to their names and prints them on the standard output.
Person.persons() .stream() .filter(Person::isFemale) .map(Person::getName) .forEach(System.out::println);
The following code applies two filter operations to print the names of all males having income more than 5000.0:
Person.persons() .stream() .filter(Person::isMale) .filter(p -> p.getIncome() > 5000.0) .map(Person::getName) .forEach(System.out::println);
You could have accomplished the same using the following statement that uses only one filter operation that includes both predicates for filtering into one predicate:
Person.persons() .stream() .filter(p -> p.isMale() && p.getIncome() > 5000.0) .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()// .stream()// .filter(Person::isFemale)// .map(Person::getName)// .forEach(System.out::println);// Person.persons()// .stream()// .filter(Person::isMale)// .filter(p -> p.getIncome() > 5000.0)// .map(Person::getName)// .forEach(System.out::println);// Person.persons()// .stream()// .filter(p -> p.isMale() && p.getIncome() > 5000.0)// .map(Person::getName)// .forEach(System.out::println);// }//w ww .j a va2s. com } 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; } }