Convert a List to a Stream in Java

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

1. Convert a list to Stream

Converting a list to stream is very simple. As List extends the Collection interface, we can use the Collection.stream() method that returns a sequential stream of elements in the list.

import java.util.Arrays;

import java.util.List;

import java.util.stream.Stream;

class Main

{

// Generic method to convert a list to stream

private static <T> Stream<T> listToStream (List<T> list) {

return list.stream();

}

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

public static void main(String[] args)

{

List<String> cities = Arrays.asList("New York","Tokyo","New Delhi");

Stream<String> stream = listToStream(cities);

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

}

Download Run Code

Output:

[New York, Tokyo, New Delhi]

2. Filter stream using a Predicate

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

In the following example, we’ll create a stream of string objects using Collection.stream() and filter it to produce a stream containing strings starting with “N”.

import java.util.Arrays;

import java.util.List;

import java.util.function.Predicate;

import java.util.stream.Stream;

class Main

{

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

public static void main(String[] args)

{

List<String> cities = Arrays.asList("New York", "Tokyo", "New Delhi");

Predicate<String> predicate = new Predicate<String>() {

@Override

public boolean test(String s) {

// filter cities that start with `N`

return s.startsWith("N");

}

};

cities.stream()

.filter(predicate)

.forEach(System.out::println);

}

Download Run Code

Output:

New York
New Delhi

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

list.stream()

.filter(s -> s.startsWith("N"))

.forEach(System.out::println);

3. Convert stream back to list

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

import java.util.Arrays;

import java.util.List;

import java.util.Objects;

import java.util.stream.Collectors;

import java.util.stream.Stream;

class Main

{

// Generic method to convert a list to stream

private static <T> Stream<T> listToStream (List<T> list) {

return list.stream();

}

// 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());

}

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

public static void main(String[] args)

{

List<String> cities = Arrays.asList("New York","Tokyo","New Delhi");

// convert a list to stream

Stream<String> stream = listToStream(cities);

// convert stream back to list

cities = streamToList(stream);

System.out.println(cities);

}

Download Run Code

Output:

[1, 2, 3, 4, 5]

We can also accumulate the stream elements into a set using Collectors.toSet(), as shown below:

// Generic method to convert a stream to a set

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

return stream.collect(Collectors.toSet());

}

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

Convert a List to a Stream in Java

1. Convert a list to Stream

2. Filter stream using a Predicate

3. Convert stream back to list

Share this: