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.
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