Java - Stream Filter Operation

Introduction

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);

Demo

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);//
  }/*from w w  w . ja  va 2  s.  co  m*/
}

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;
  }
}

Result