Java Stream filter
last modified July 8, 2024
In this article we show how to filter Java streams using filtering operations.
Java Stream is a sequence of elements from a source that supports aggregate operations. Streams do not store elements; the elements are computed on demand. Elements are consumed from data sources such as collections, arrays, or I/O resources.
Stream aggregate operations are similar to SQL operations. We can apply filtering, mapping, reducing, matching, searching, or sorting operations on streams. Streams allow chaining of multiple stream operations. Unlike collections which use external iterations, streams are iterated internally.
The filter method
Java Stream filter method is an intermediate operation, which returns elements of the stream that match the given predicate. A predicate is a function that returns a boolean value.
Filter by string length
The following example filters a list of strings.
import java.util.List;
void main() {
List<String> words = List.of("pen", "custom", "orphanage",
"forest", "bubble", "butterfly");
List<String> result = words.stream().filter(word -> word.length() > 5).toList();
result.forEach(System.out::println);
}
We have a list of words. We filter the list to include only strings whose length is bigger than five.
List<String> result = words.stream().filter(word -> word.length() > 5).toList();
With the stream method, we create a Java Stream from a list of
strings. On this stream, we apply the filter method. The
filter method accepts an anonymous functions that returns a boolean
true for all elements of the stream whose length is bigger that five.
result.forEach(System.out::println);
We go through the result with the forEach method and print all its elements
to the console.
$ java Main.java custom orphanage forest bubble butterfly
Filter null values
The next example filters out null values.
import java.util.Arrays;
import java.util.List;
import java.util.Objects;
import java.util.stream.Collectors;
void main() {
List<String> words = Arrays.asList("cup", null, "forest",
"sky", "book", null, "theatre");
List<String> result = words.stream().filter(Objects::nonNull)
.collect(Collectors.toList());
System.out.println(result);
}
We have a list of words. With the Stream filtering operation, we create a new
list with null values removed.
List<String> result = words.stream().filter(Objects::nonNull)
.collect(Collectors.toList());
In the body of the lambda expression, we check that the value is not
null. The collect method is a terminal operation that
creates a list from the filtered stream.
Multiple filter operations
It is possible to apply multiple filter operations on a stream.
import java.util.Arrays;
import java.util.function.IntConsumer;
void main() {
int[] inums = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 };
IntConsumer icons = i -> System.out.print(i + " ");
Arrays.stream(inums).filter(e -> e < 6 || e > 10)
.filter(e -> e % 2 == 0).forEach(icons);
}
In the example, we apply multiple filter operations on a stream of integers.
int[] inums = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 };
We have an array of integer values.
IntConsumer icons = i -> System.out.print(i + " ");
IntConsumer is an operation that accepts a single integer value
argument and returns no result.
Arrays.stream(inums).filter(e -> e < 6 || e > 10)
.filter(e -> e % 2 == 0).forEach(icons);
A stream is created from the array with the Arrays.stream method.
Multiple filtering operations are performed.
Filter objects
The next example shows how to filter objects.
import java.util.List;
void main() {
List<User> persons = List.of(
new User("Jack", "jack234@gmail.com"),
new User("Peter", "pete2@post.com"),
new User("Lucy", "lucy17@gmail.com"),
new User("Robert", "bob56@post.com"),
new User("Martin", "mato4@imail.com")
);
List<User> result = persons.stream()
.filter(person -> person.email().matches(".*post\\.com"))
.toList();
result.forEach(p -> System.out.println(p.name()));
}
record User(String name, String email) {
}
The example creates a stream of User objects. It filters those
which match a specific regular expression.
List<User> result = persons.stream()
.filter(person -> person.email().matches(".*post\\.com"))
.toList();
In the filter predicate, we choose emails that match the
.*post\\.com pattern.
Filter map by keys
In the following example, we filter a map by its keys.
import java.util.HashMap;
import java.util.Map;
void main() {
Map<String, String> hmap = new HashMap<>();
hmap.put("de", "Germany");
hmap.put("hu", "Hungary");
hmap.put("sk", "Slovakia");
hmap.put("si", "Slovenia");
hmap.put("so", "Somalia");
hmap.put("us", "United States");
hmap.put("ru", "Russia");
hmap.entrySet().stream().filter(map -> map.getKey().startsWith("s"))
.forEach(System.out::println);
}
The example filters domain names starting with s letter.
Filter map by values
In the following example, we filter a map by its values.
import java.util.HashMap;
import java.util.Map;
void main() {
Map<String, String> hmap = new HashMap<>();
hmap.put("de", "Germany");
hmap.put("hu", "Hungary");
hmap.put("sk", "Slovakia");
hmap.put("si", "Slovenia");
hmap.put("so", "Somalia");
hmap.put("us", "United States");
hmap.put("ru", "Russia");
hmap.entrySet().stream().filter(map -> map.getValue().equals("Slovakia")
|| map.getValue().equals("Slovenia"))
.forEach(System.out::println);
}
In the example, we filter out two countries from the map.
Source
In this article we have have worked with Java Stream filtering operations.
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