Java 8 Stream API filter() examples

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.

Stream filter(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:

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:

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:

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:

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.