Sort by multiple conditions using Java Stream

Recently, I've been writing Java for the first time in half a year. I used to use ** Java7 (!) ** in my business, but now I'm using Java8.

I use stream quite often, and I wrote it down because I learned the other day that it can be sorted by multiple conditions.

Java Stream API Stream (Java Platform SE 8 )

API that supports aggregation operations such as iterative processing and parallel processing.

By combining with a lambda expression (arrow function in JS), it can be described simply without using extended for syntax.

Sort by multiple conditions

This is possible by using the Comparator interface.

Sorting by multiple conditions is possible by specifying the sorting condition by the comparing () and thenComparing () methods and passing it to sorted () of stream.

Sample code

Please forgive me for the complexity of the design.

ʻArticle` class

Article.java

import java.time.LocalDate;

public class Article {

  /**Contributor*/
  private String contributor;

  /**Posted date*/
  private LocalDate date;

  /**Number of likes*/
  private int goodCount;

  /**constructor*/
  public Article(String contributor, LocalDate date, int goodCount) {
    this.contributor = contributor;
    this.date = date;
    this.goodCount = goodCount;
  }

  /** Getter, Setter */
  public String getContributor() {
    return this.contributor;
  }

  public void setContributor(String contributor) {
    this.contributor = contributor;
  }

  public LocalDate getDate() {
    return this.date;
  }

  public void setDate(LocalDate date) {
    this.date = date;
  }

  public int getGoodCount() {
    return this.goodCount;
  }

  public void setGoodCount(int goodCount) {
    this.goodCount = goodCount;
  }

}

Stream class

Stream.java

import java.time.LocalDate;
import java.util.Arrays;
import java.util.List;

public class Stream {
  public static void main(String[] args) {
    //Data generation
    Article a1 = new Article("Taro", LocalDate.of(2018, 7, 11), 15);
    Article a2 = new Article("Taro", LocalDate.of(2015, 11, 1), 53);
    Article a3 = new Article("Taro", LocalDate.of(2015, 10, 31), 54);

    Article a4 = new Article("Hanako", LocalDate.of(2017, 2, 5), 53);
    Article a5 = new Article("Hanako", LocalDate.of(2016, 11, 1), 160);
    Article a6 = new Article("Hanako", LocalDate.of(2012, 8, 20), 22);

    Article a7 = new Article("Pochi", LocalDate.of(2013, 3, 16), 38);
    Article a8 = new Article("Pochi", LocalDate.of(2011, 9, 25), 200);
    Article a9 = new Article("Pochi", LocalDate.of(2018, 4, 13), 10);

    //List creation
    List<Article> list = Arrays.asList(a1, a2, a3, a4, a5, a6, a7, a8, a9);
    list.stream().forEach(a ->
      System.out.println(a.getContributor() + " " + a.getDate() + " " + a.getGoodCount()));

  }
}

For the time being, if you output a list of ʻa1 to ʻa9

Output result

Taro 2018-07-11 15
Taro 2015-11-01 53
Taro 2015-10-31 54
Hanako 2017-02-05 53
Hanako 2016-11-01 160
Hanako 2012-08-20 22
Pochi 2013-03-16 38
Pochi 2011-09-25 200
Pochi 2018-04-13 10

Will be.

Sorting

sorted (Java Platform SE 8)

Pass Comparator as an argument and sort in the specified order.

Sort by single item

Sort by name in a dictionary.

Stream.java

//Comparator creation
Comparator<Article> comparator = Comparator.comparing(Article::getContributor);
//Sort by poster name
list.stream().sorted(comparator)
  .forEach(a ->
    System.out.println(a.getContributor() + " " + a.getDate() + " " + a.getGoodCount()));

Output result

Hanako 2017-02-05 53
Hanako 2016-11-01 160
Hanako 2012-08-20 22
Pochi 2013-03-16 38
Pochi 2011-09-25 200
Pochi 2018-04-13 10
Taro 2018-07-11 15
Taro 2015-11-01 53
Taro 2015-10-31 54

You can also sort by other items.

thenComparing thenComparing (Java Platform SE 8)

A method that allows you to chain comparison conditions. Use as follows.

comparator

Comparator.comparing(...).thenComparing(...).thenComparing(...);

If the previous conditions are the same (ʻequals`), make a further comparison.

Returns a lexicographic order comparator with the other comparator. If this Comparator considers the two elements to be equal (that is, compare (a, b) == 0), then other is used to determine the order.

Sort by multiple items

Sort by name-> post date

Stream.java

//Comparator creation
Comparator<Article> comparator =
  Comparator.comparing(Article::getContributor).thenComparing(Article::getDate);
//Sorting
list.stream().sorted(comparator)
  .forEach(a ->
    System.out.println(a.getContributor() + " " + a.getDate() + " " + a.getGoodCount()));

Output result

Hanako 2012-08-20 22
Hanako 2016-11-01 160
Hanako 2017-02-05 53
Pochi 2011-09-25 200
Pochi 2013-03-16 38
Pochi 2018-04-13 10
Taro 2015-10-31 54
Taro 2015-11-01 53
Taro 2018-07-11 15

Name-> Sort by Likes

Stream.java

//Comparator creation
Comparator<Article> comparator =
  Comparator.comparing(Article::getContributor).thenComparing(Article::getGoodCount);
//Sorting
list.stream().sorted(comparator)
  .forEach(a ->
    System.out.println(a.getContributor() + " " + a.getDate() + " " + a.getGoodCount()));

Output result

Hanako 2012-08-20 22
Hanako 2017-02-05 53
Hanako 2016-11-01 160
Pochi 2018-04-13 10
Pochi 2013-03-16 38
Pochi 2011-09-25 200
Taro 2018-07-11 15
Taro 2015-11-01 53
Taro 2015-10-31 54

If you want to sort in ascending order, add reversed ()

Stream.java

//Comparator creation
Comparator<Article> comparator =
  Comparator.comparing(Article::getContributor).thenComparing(Article::getGoodCount).reversed();
//Sorting
list.stream().sorted(comparator)
  .forEach(a ->
    System.out.println(a.getContributor() + " " + a.getDate() + " " + a.getGoodCount()));

Output result

Taro 2015-10-31 54
Taro 2015-11-01 53
Taro 2018-07-11 15
Pochi 2011-09-25 200
Pochi 2013-03-16 38
Pochi 2018-04-13 10
Hanako 2016-11-01 160
Hanako 2017-02-05 53
Hanako 2012-08-20 22

Impressions

• Sorting by multiple conditions was surprisingly easy
• Stream Insanely convenient (although now)
• It seems that we still need to learn how to use Comparator
• If you want to specify ascending / descending order in detail, it seems better to separate the Comparator itself and sort it multiple times.

reference

Stream (Java Platform SE 8 ) Comparator (Java Platform SE 8 )