A quick and practical guide to Java 8 Stream filter() method. How to use Stream API filter and simple example programs.
1. Overview
In this Stream tutorial, We will learn today what is Stream filter introduced in Java 8. Java 8 has a Stream API and it has a filter() method which takes the Predicate Functional Interface as an argument. filter() method is a part of Intermediate Operations which is used in the middle of streams and returns a Stream as output.
2. Stream API filter() method syntax
Below is the syntax from API.
Streamfilter(Predicate predicate)
This method returns a stream consisting of the elements of this stream that match the given predicate. Predicate means just a condition that applied to all the values in the Stream. We are still eligible to perform many operations on the returned Stream.
3. Java 8 Stream filter() examples
Now, We will see various example programs on the filter() method. In all the examples, filter stream will be converted to a List using Collectors.toList() utility method.
3.1 Print all odd numbers - Lambda expression
Java example program to iterate over a stream of integers and pick only odd numbers from it.
package com.java.w3schools.blog.java.program.to.java8;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;
/**
*
* Java program to print only odd numbers using Stream filter() api.
*
* @author JavaProgramTo.com
*
*/
public class StreamFilterLambda {
public static void main(String[] args) {
// creating a stream from 1 to 10 numbers.
Stream stream = Stream.of(1, 2, 3, 4, 5, 6, 7, 8, 9, 0);
// fitering only odd numbers
List oddNumbers = stream.filter(num -> num % 2 == 1).collect(Collectors.toList());
// printing
oddNumbers.forEach(value -> System.out.println(value));
}
}
Output:
1 3 5 7 9
3.2 Print all odd numbers - Custom Predicate Functional Interface
Let us create a Predicate Functional interface that checks the given number is odd or not. This can be used in many places rather than writing this logic in multiple places.
package com.java.w3schools.blog.java.program.to.java8;
import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;
import java.util.stream.Stream;
/**
*
* Java program to print only odd numbers with predicate using Stream filter()
* api.
*
* @author JavaProgramTo.com
*
*/
public class StreamFilterPredicate {
public static void main(String[] args) {
// creating a stream from 1 to 10 numbers.
Stream stream = Stream.of(1, 2, 3, 4, 5, 6, 7, 8, 9, 0);
// Custom predicate to check odd number.
Predicate oddPredicate = new Predicate() {
@Override
public boolean test(Integer t) {
if (t % 2 == 1) {
return true;
}
return false;
}
};
// filtering only odd numbers using custom predicate
List oddNumbers = stream.filter(oddPredicate).collect(Collectors.toList());
// printing
oddNumbers.forEach(value -> System.out.println(value));
}
}
Output:
1 3 5 7 9
3.3 Filtering odd numbers and generating square numbers
package com.java.w3schools.blog.java.program.to.java8;
import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;
import java.util.stream.Stream;
/**
*
* Java program to find odd numbers and print its squares.
*
* @author JavaProgramTo.com
*
*/
public class StreamFilterSqueres {
public static void main(String[] args) {
// creating a stream from 1 to 10 numbers.
Stream stream = Stream.of(1, 2, 3, 4, 5, 6, 7, 8, 9, 0);
// Custom predicate to check odd number.
Predicate oddPredicate = new Predicate() {
@Override
public boolean test(Integer t) {
if (t % 2 == 1) {
return true;
}
return false;
}
};
// filtering only odd numbers using custom predicate and generating square
// numbers.
List oddNumbers = stream.filter(oddPredicate).map(value -> value * value).collect(Collectors.toList());
// printing
oddNumbers.forEach(value -> System.out.println(value));
}
}
Output:
1 9 25 49 81
3.4 Filtering odd numbers and generating its cubic values
package com.java.w3schools.blog.java.program.to.java8;
import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;
import java.util.stream.Stream;
/**
*
* Java program to find odd numbers and print its cubic values.
*
* @author JavaProgramTo.com
*
*/
public class StreamFilterCubic {
public static void main(String[] args) {
// creating a stream from 1 to 10 numbers.
Stream stream = Stream.of(1, 2, 3, 4, 5, 6, 7, 8, 9, 0);
// filtering only odd numbers using custom predicate and generating square
// numbers.
List oddNumbers = stream.filter(n -> n % 2 == 1).map(value -> value * value * value)
.collect(Collectors.toList());
// printing
oddNumbers.forEach(value -> System.out.println(value));
}
}
Output:
1 27 125 343 729
4. Conclusion
In this article, We have seen what is Java 8 Stream filter and a few example programs using filter() method. And how to collect the output of filter into List.
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