TOC

Java 8 - Exemples de filtrage d’une carte

Java 8 - Filtrer des exemples de carte

Quelques exemples Java pour vous montrer comment filtrer unMap avec l'API de flux Java 8.

Avant Java 8: 

    Map map = new HashMap<>();
    map.put(1, "linode.com");
    map.put(2, "heroku.com");

    String result = "";
    for (Map.Entry entry : map.entrySet()) {
        if("something".equals(entry.getValue())){
            result = entry.getValue();
        }
    }

Avec Java 8, vous pouvez convertir unMap.entrySet() en unstream, suivi d'unfilter() et d'uncollect() it. 

    Map map = new HashMap<>();
    map.put(1, "linode.com");
    map.put(2, "heroku.com");

    //Map -> Stream -> Filter -> String
    String result = map.entrySet().stream()
        .filter(x -> "something".equals(x.getValue()))
        .map(x->x.getValue())
        .collect(Collectors.joining());

    //Map -> Stream -> Filter -> MAP
    Map collect = map.entrySet().stream()
        .filter(x -> x.getKey() == 2)
        .collect(Collectors.toMap(x -> x.getKey(), x -> x.getValue()));

    // or like this
    Map collect = map.entrySet().stream()
        .filter(x -> x.getKey() == 3)
        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

1. Java 8 - Filtrer une carte

Un exemple complet pour filtrer une carte par valeurs et renvoyer une chaîne.

TestMapFilter.java

package com.example;

import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;

public class TestMapFilter {

    public static void main(String[] args) {

        Map HOSTING = new HashMap<>();
        HOSTING.put(1, "linode.com");
        HOSTING.put(2, "heroku.com");
        HOSTING.put(3, "digitalocean.com");
        HOSTING.put(4, "aws.amazon.com");

        // Before Java 8
        String result = "";
        for (Map.Entry entry : HOSTING.entrySet()) {
            if ("aws.amazon.com".equals(entry.getValue())) {
                result = entry.getValue();
            }
        }
        System.out.println("Before Java 8 : " + result);

        //Map -> Stream -> Filter -> String
        result = HOSTING.entrySet().stream()
                .filter(map -> "aws.amazon.com".equals(map.getValue()))
                .map(map -> map.getValue())
                .collect(Collectors.joining());

        System.out.println("With Java 8 : " + result);

        // filter more values
        result = HOSTING.entrySet().stream()
                .filter(x -> {
                    if (!x.getValue().contains("amazon") && !x.getValue().contains("digital")) {
                        return true;
                    }
                    return false;
                })
                .map(map -> map.getValue())
                .collect(Collectors.joining(","));

        System.out.println("With Java 8 : " + result);

    }

}

Sortie 

Before Java 8 : aws.amazon.com
With Java 8 : aws.amazon.com
With Java 8 : linode.com,heroku.com

2. Java 8 - Filtrer une carte # 2

Encore un autre exemple pour filtrer unMap par clé, mais cette fois retournera unMap

TestMapFilter2.java

package com.example;

import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;

public class TestMapFilter2 {

    public static void main(String[] args) {

        Map HOSTING = new HashMap<>();
        HOSTING.put(1, "linode.com");
        HOSTING.put(2, "heroku.com");
        HOSTING.put(3, "digitalocean.com");
        HOSTING.put(4, "aws.amazon.com");

        //Map -> Stream -> Filter -> Map
        Map collect = HOSTING.entrySet().stream()
                .filter(map -> map.getKey() == 2)
                .collect(Collectors.toMap(p -> p.getKey(), p -> p.getValue()));

        System.out.println(collect); //output : {2=heroku.com}

        Map collect2 = HOSTING.entrySet().stream()
                .filter(map -> map.getKey() <= 3)
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

        System.out.println(collect2); //output : {1=linode.com, 2=heroku.com, 3=digitalocean.com}

    }

}

Sortie 

{2=heroku.com}
{1=linode.com, 2=heroku.com, 3=digitalocean.com}

3. Java 8 - Filtrer une carte # 3 - Prédicat

Cette fois, essayez les nouveaux Java 8Predicate

TestMapFilter3.java

package com.example;

import java.util.HashMap;
import java.util.Map;
import java.util.function.Predicate;
import java.util.stream.Collectors;

public class TestMapFilter3 {

    // Generic Map filterbyvalue, with predicate
    public static  Map filterByValue(Map map, Predicate predicate) {
        return map.entrySet()
                .stream()
                .filter(x -> predicate.test(x.getValue()))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
    }

    public static void main(String[] args) {

        Map HOSTING = new HashMap<>();
        HOSTING.put(1, "linode.com");
        HOSTING.put(2, "heroku.com");
        HOSTING.put(3, "digitalocean.com");
        HOSTING.put(4, "aws.amazon.com");
        HOSTING.put(5, "aws2.amazon.com");

        //  {1=linode.com}
        Map filteredMap = filterByValue(HOSTING, x -> x.contains("linode"));
        System.out.println(filteredMap);

        // {1=linode.com, 4=aws.amazon.com, 5=aws2.amazon.com}
        Map filteredMap2 = filterByValue(HOSTING, x -> (x.contains("aws") || x.contains("linode")));
        System.out.println(filteredMap2);

        // {4=aws.amazon.com}
        Map filteredMap3 = filterByValue(HOSTING, x -> (x.contains("aws") && !x.contains("aws2")));
        System.out.println(filteredMap3);

        // {1=linode.com, 2=heroku.com}
        Map filteredMap4 = filterByValue(HOSTING, x -> (x.length() <= 10));
        System.out.println(filteredMap4);

    }

}

Sortie 

{1=linode.com}
{1=linode.com, 4=aws.amazon.com, 5=aws2.amazon.com}
{4=aws.amazon.com}
{1=linode.com, 2=heroku.com}

Références

  1. Traitement des données avec les flux Java SE 8

  2. Collectionneurs Java JavaDoc

  3. Exemples de filtres de flux Java 8

Related