Convert a Set to a Stream in Java

This post will discuss how to convert a set to stream in Java 8 and above. We will also learn how to apply a filter on a stream and convert it back to a set or list.

1. Convert a set to stream

Converting a set to stream is very simple. Set interface extends Collection interface and Collection has stream() method that returns a sequential stream of the collection.

import java.util.Arrays;

import java.util.HashSet;

import java.util.Set;

import java.util.stream.Stream;

class Main

{

// Generic method to convert a set to stream

private static <T> Stream<T> setToStream (Set<T> set) {

return set.stream();

}

// Program to convert a set to stream in Java 8 and above

public static void main(String[] args)

{

Set<Integer> set = new HashSet<>();

for (int i = 0; i < 5; i++) {

set.add(i + 1);

}

Stream<Integer> stream = setToStream(set);

System.out.println(Arrays.toString(stream.toArray()));

}

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Output:

[1, 2, 3, 4, 5]

2. Filter stream using a predicate

For filtering a stream, we can use filter(Predicate) that returns a stream consisting of elements that match the given predicate.

In the following example, we’ll create a stream of Integer objects using Collection.stream() and filter it to produce a stream containing only even numbers.

import java.util.HashSet;

import java.util.Set;

import java.util.function.Predicate;

class Main

{

// Program to convert a set to stream and filter it in Java 8 and above

public static void main(String[] args)

{

Set<Integer> set = new HashSet<>();

for (int i = 0; i < 5; i++) {

set.add(i + 1);

}

Predicate<Integer> removeOdd = new Predicate<Integer>() {

@Override

public boolean test(Integer i) {

return i % 2 == 0; // remove odd numbers

}

};

set.stream()

.filter(removeOdd)

.forEach(System.out::println);

}

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Output:

2
4

We can also specify a lambda expression or method reference in place of the predicate:

set.stream()

.filter(n -> n % 2 == 0) // print only even numbers

.forEach(System.out::println);

3. Convert stream back to set

We can accumulate the stream elements into a new set using a Collector returned by Collectors.toList().

import java.util.HashSet;

import java.util.Objects;

import java.util.Set;

import java.util.stream.Collectors;

import java.util.stream.Stream;

class Main

{

// Generic method to convert a set to stream

private static <T> Stream<T> setToStream (Set<T> set) {

return set.stream();

}

// Generic method to convert a stream to a set

private static <T> Set<T> streamToSet (Stream<T> stream)

{

return stream

.filter(Objects::nonNull) // optionally apply any filter

.collect(Collectors.toSet());

}

// Program to convert the stream to set in Java 8 and above

public static void main(String[] args)

{

Set<Integer> set = new HashSet<>();

for (int i = 0; i < 5; i++) {

set.add(i + 1);

}

// convert a set to stream

Stream<Integer> stream = setToStream(set);

// convert stream back to set

set = streamToSet(stream);

System.out.println(set);

}

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Output:

[1, 2, 3, 4, 5]

We can also accumulate the stream elements in a new list using Collectors.toList(), as shown below:

// Generic method to convert a stream to a list

private static <T> List<T> streamToList (Stream<T> stream)

{

return stream

.filter(Objects::nonNull) // optionally apply any filter

.collect(Collectors.toList());

}

That’s all about converting a Set to Stream in Java.

Convert a Set to a Stream in Java

1. Convert a set to stream

2. Filter stream using a predicate

3. Convert stream back to set

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