In this article, we will discuss Stream filter() method in details with examples
1. Stream filter() method :
- This Stream method is an intermediate operation which reads stream and returns new stream after applying given predicate (or condition)
- Predicate is Functional Interface so either we can pass Predicate class or lambda expression
- Predicate or lambda expression is applied to each of the elements in the Stream for evaluating for the given condition
- Stream filter() method is stateless which means it is non-interfering with other elements in the stream
- Another intermediate operation can be applied to returned stream like map(), sorted(), distinct(), etc. after filter() intermediate operation
- Note :- Number of elements returned in the new stream after filtering is either equal or less than the original stream elements
- Method signature :- Stream<T> filter(Predicate<? super T> predicate)
2. Stream filter() method examples :
2.1 To find all EVEN number from stream
- A list contains first 10 natural numbers
- We are applying condition to filter out only EVEN numbers using filter() method i.e.; Lambda expression
- Finally, printing to console using Stream’s forEach() method
package net.bench.resources.stream.filter.example;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Stream;
public class StreamFilterExample {
public static void main(String[] args) {
System.out.println("EVEN number from list of first 10 numbers : \n");
// Integer numbers
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
// filter only EVEN numbers
Stream<Integer> stream = numbers.stream().filter(i -> i%2 == 0);
// print to console using forEach
stream.forEach(s -> System.out.println(s));
}
}
Output:
EVEN number from list of first 10 numbers :
2
4
6
8
10
2.2 To find all names with length greater than 5
- List contains names of variable length
- We are applying condition to filter names with length greater than 5
- Finally, printing to console using Stream’s forEach() method
package net.bench.resources.stream.filter.example;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Stream;
public class StreamFilterNames {
public static void main(String[] args) {
System.out.println("Names with length greater than 5 : \n");
// String
List<String> names = Arrays.asList(
"Sachin",
"Rahul",
"Sehwag",
"Anil",
"Sourav",
"Sunil",
"Laxman"
);
// filter names with length greater than 5
Stream<String> stream = names.stream().filter(name -> name.length() > 5);
// print to console using forEach
stream.forEach(str -> System.out.println(str));
}
}
Output:
Names with length greater than 5 :
Sachin
Sehwag
Sourav
Laxman
2.3 Stream filter() and collect() method
- We are going to use same example which we used in last example 2.2 to filter names with length greater than 5
- But this time instead of printing directly to console, we are going to collect to the List using Stream’s collect() method
- Finally, iterate through new List using Stream’s forEach() method and print to console
package net.bench.resources.stream.filter.example;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class StreamFilterCollectNames {
public static void main(String[] args) {
System.out.println("Names with length greater than 5 : \n");
// String names
List<String> names = Arrays.asList(
"Sachin",
"Rahul",
"Sehwag",
"Anil",
"Sourav",
"Sunil",
"Laxman"
);
// filter names with length greater than 5
List<String> newList = names
.stream() // 1. get stream
.filter(name -> name.length() > 5) // 2. intermediate operation filtering
.collect(Collectors.toList()); // 3. collecting to List
// print to console using forEach
newList.stream().forEach(System.out::println);
}
}
Output:
Names with length greater than 5 :
Sachin
Sehwag
Sourav
Laxman
2.4 Stream filter() and map() method
- A list contains first 10 natural numbers
- We are applying condition to filter out only EVEN numbers using filter() method i.e.; Lambda expression
- Then another intermediate operation to map each even number to its cube value is applied using map() method and collecting it to new List using collect() method
- Finally, iterate through new List using forEach() method and print to console
package net.bench.resources.stream.filter.example;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class StreamFilterMapMethod {
public static void main(String[] args) {
System.out.println("Filter EVEN numbers and Cube it : \n");
// Integer numbers
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
// filter only EVEN numbers
List<Integer> newList = numbers
.stream() // 1. get stream
.filter(i -> i%2 == 0) // 2.1 intermediate operation - filtering even number
.map(n -> n * n * n) // 2.2 intermediate operation - Square
.collect(Collectors.toList()); // 3. collecting to List
// print to console using forEach
newList.forEach(s -> System.out.println(s));
}
}
Output:
Filter EVEN numbers and Cube it :
8
64
216
512
1000
2.5 Stream filter() with multiple conditions
- A list contains names of variable length
- We are applying multiple conditions to filter out names starting with capital alphabet S and having length greater than 5 using filter() method i.e.; Lambda expression
- Then terminal operation is used to collect to a new List i .e.; collect() method of Stream
- Finally, iterate through new List using forEach() method and print to console
package net.bench.resources.stream.filter.example;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class StreamMultipleFilters {
public static void main(String[] args) {
System.out.println("Names starting with 'S' and"
+ " having length greater than 5 : \n");
// String names
List<String> names = Arrays.asList(
"Sachin",
"Rahul",
"Sehwag",
"Anil",
"Sourav",
"Sunil",
"Laxman"
);
// filter names with length greater than 5
List<String> newList = names
.stream() // 1. get stream
.filter(s -> s.length() > 5 && s.startsWith("S")) // 2 filtering
.collect(Collectors.toList()); // 3. collecting to List
// print to console using forEach
newList.forEach(System.out::println);
}
}
Output:
Names starting with 'S' and having length greater than 5 :
Sachin
Sehwag
Sourav
References:
- https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html
- https://docs.oracle.com/javase/8/docs/api/java/util/Collection.html
- https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html
Happy Coding !!
Happy Learning !!
