Java filter map

last modified March 8, 2024

In this article we show how to filter a map in Java.

HashMap is a container that stores key-value pairs. Each key is associated with one value. Keys in a HashMap must be unique.

HashMap can be filtered with the filter method of the Java Stream API.

Filter map by values

In the first example, we filter the values of a map.

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

void main() {

    Map<String, String> capitals = new HashMap<>();

    capitals.put("svk", "Bratislava");
    capitals.put("ger", "Berlin");
    capitals.put("hun", "Budapest");
    capitals.put("czk", "Prague");
    capitals.put("pol", "Warsaw");
    capitals.put("ita", "Rome");

    Map<String, String> filteredCapitals = capitals.entrySet().stream()
        .filter(e -> e.getValue().startsWith("B"))
        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

    filteredCapitals.entrySet().forEach(System.out::println);
}

We have a map of capitals. We get the entries of a map with entrySet. Then we turn the set into a stream with stream.

Map<String, String> filteredCapitals = capitals.entrySet().stream()
    .filter(e -> e.getValue().startsWith("B"))
    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

We filter the map to contain only pairs whose values start with B. We get the value from the current entry with getValue.

$ java Main.java
hun=Budapest
ger=Berlin
svk=Bratislava

Filter map by keys

In the next example, we filter the map by its keys.

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

void main() {

    Map<String, String> capitals = new HashMap<>();

    capitals.put("svk", "Bratislava");
    capitals.put("ger", "Berlin");
    capitals.put("hun", "Budapest");
    capitals.put("czk", "Prague");
    capitals.put("pol", "Warsaw");
    capitals.put("ita", "Rome");

    Map<String, String> filteredCapitals = capitals.entrySet().stream()
        .filter(e -> e.getKey().endsWith("k"))
        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

    filteredCapitals.entrySet().forEach(System.out::println);
}

The example filters all entries whose keys end with letter k.

Map<String, String> filteredCapitals = capitals.entrySet().stream()
    .filter(e -> e.getKey().endsWith("k"))
    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

We get the current key of an entry with getKey.

$ java Main.java
czk=Prague
svk=Bratislava

Filter map by keys and values

In the next example, we filter a map by the keys and values.

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

void main() {

    Map<Integer, String> users = new HashMap<>();

    users.put(1, "John Doe");
    users.put(2, "Roger Roe");
    users.put(3, "Jane Doe");
    users.put(4, "Jack Drake");
    users.put(5, "Peter Morgan");
    users.put(6, "Robert Melnik");

    Map<Integer, String> filtered = users.entrySet().stream()
            .filter(e -> e.getKey() % 2 == 0)
            .filter(e -> e.getValue().split(" ")[1].startsWith("D"))
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

    filtered.entrySet().forEach(System.out::println);
}

We have a map of users. We filter out all users whose keys are even and whose lastnames start with letter D.

$ java Main.java
4=Jack Drake

Filter by age

We have a map of users. We filter the map by the users' age.

import java.util.Map;
import java.util.stream.Collectors;
import java.time.LocalDate;
import java.time.Period;
import java.time.format.DateTimeFormatter;
import java.util.HashMap;

void main() {

    var dft = DateTimeFormatter.ofPattern("yyyy-MM-dd");

    Map<Integer, User> users = new HashMap<>();
    users.put(1, new User("John Doe", "gardener", LocalDate.parse("1973-09-07", dft)));
    users.put(2, new User("Roger Roe", "driver", LocalDate.parse("1963-03-30", dft)));
    users.put(3, new User("Kim Smith", "teacher", LocalDate.parse("1980-05-12", dft)));
    users.put(4, new User("Joe Nigel", "artist", LocalDate.parse("1983-03-30", dft)));
    users.put(5, new User("Liam Strong", "teacher", LocalDate.parse("2009-03-06", dft)));
    users.put(6, new User("Robert Young", "gardener", LocalDate.parse("1978-11-16", dft)));
    users.put(7, new User("Liam Strong", "teacher", LocalDate.parse("1986-10-23", dft)));

    Map<Integer, User> olderThanForty = users.entrySet().stream().filter(e -> {
        LocalDate dob = e.getValue().dob();
        int age = Period.between(dob, LocalDate.now()).getYears();
        return age > 40;
    }).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

    olderThanForty.entrySet().forEach(System.out::println);
}

record User(String name, String occupation, LocalDate dob) {
}

We filter out all users that are older than forty.

Map<Integer, User> olderThanForty = users.entrySet().stream().filter(e -> {
    LocalDate dob = e.getValue().dob();
    int age = Period.between(dob, LocalDate.now()).getYears();
    return age > 40;
}).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

We retrieve the date of birth from the user object with e.getValue().dob(). Then we calculate the user's age with Period.between(dob, LocalDate.now()).getYears(). The filter condition is age > 40;

$ java Main.java
1=User[name=John Doe, occupation=gardener, dob=1973-09-07]
2=User[name=Roger Roe, occupation=driver, dob=1963-03-30]
3=User[name=Kim Smith, occupation=teacher, dob=1980-05-12]
6=User[name=Robert Young, occupation=gardener, dob=1978-11-16]

Source

Java HashMap - language reference

In this article we have showed how to filter a map in Java.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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