Java Stream API in 5 minutes

This article is intended for people who can't read the Java Stream API.

Recommended for the following people

--I used to write Java, but I haven't written it recently --I quit coding because I couldn't read the Stream API ――I just want to know the point because I don't have time anyway

At first

The Java Stream API has nothing to do with I / O classes such as InputStream and OutputStream. A mechanism that processes a collection object such as List or Set, or a loopable element such as an array only once in a loop.

• Stream cannot be reused once it is processed. However, Stream can perform multiple processes by chaining.

The following two points are often used when using the Stream API.

--Convert entity (map method) --Filter the collection (filter method)

Here is the Java code.

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class StreamTest {
    public static void main(String[] args) {
        List<String> csvList = new ArrayList<>();
        csvList.add("Naoki Tanaka,MAN");
        csvList.add("Kazuki Kitamura,MAN");
        csvList.add("Shizuka Kato,WOMAN");
        csvList.add("Ono no Imoko,UNKNOWN");
        // １.Convert an entity.(map)
        List<Person> persons = csvList.stream().map(Person::convertPersonFromCSV).collect(Collectors.toList());

        // 2.Filter entities.(filter)
        List<Person> mens = persons.stream().filter(p -> p.getGender() == Gender.MAN).collect(Collectors.toList());

        System.out.println("Print all persons");
        persons.forEach(System.out::println);

        System.out.println("Print mens only");
        mens.forEach(p -> System.out.println(p.getName()));
    }

    enum Gender {
        MAN,
        WOMAN,
        UNKNOWN;
    }

    static class Person {
        private String name;
        private Gender gender;

        public Person(String name, Gender gender) {
            this.name = name;
            this.gender = gender;
        }

        @Override
        public String toString() {
            return String.format("My name is %s . \r\nMy gender is %s.", name, gender);
        }

        public String getName() {
            return this.name;
        }

        public Gender getGender() {
            return this.gender;
        }

        public static Person convertPersonFromCSV(String csv) {
            String[] csvStrings = csv.split(",");
            if (csvStrings.length != 2) throw new IllegalArgumentException("csv is illegal format.");
            return new Person(csvStrings[0], Gender.valueOf(csvStrings[1]));
        }
    }
}

Let's look at the above in order

Convert an entity

When converting an entity, use the map method. Use the map method to convert, and the collect method to convert Stream to Collection type again. </ b>

csvList.stream().map(Person::convertPersonFromCSV).collect(Collectors.toList());
csvList.stream().map(csv -> Person.convertPersonFromCSV(csv)).collect(Collectors.toList());
csvList.stream().map(csv -> {
                         Person person = Person.convertPersonFromCSV(csv);
                         return person;
                     }).collect(Collectors.toList());

persons.stream().filter(p -> p.getGender() == Gender.MAN).collect(Collectors.toList());