HowToDoInJava

Java Boxed Stream Example

By Lokesh Gupta | Filed Under: Java 8

In Java 8, if you want to convert stream of objects to collection, then you can use one of the static methods in the Collectors class.

//It works perfect !!
List<String> strings = Stream.of("how", "to", "do", "in", "java")
    				.collect(Collectors.toList());

The same process doesn’t work on streams of primitives, however.

//Compilation Error !!
IntStream.of(1,2,3,4,5)
    .collect(Collectors.toList());

To convert a stream of primitives, you must first box the elements in their wrapper class and then collect them. This type of stream in called boxed stream.

1. IntStream – stream of ints

Example to convert int stream to List of Integers.

//Get the collection and later convert to stream to process elements
List<Integer> ints = IntStream.of(1,2,3,4,5)
				.boxed()
				.collect(Collectors.toList());
		
System.out.println(ints);

//Stream operations directly
Optional<Integer> max = IntStream.of(1,2,3,4,5)
				.boxed()
				.max(Integer::compareTo);

System.out.println(max);

Program Output:

[1, 2, 3, 4, 5]
5

2. LongStream – stream of longs

Example to convert long stream to List of Longs.

List<Long> longs = LongStream.of(1l,2l,3l,4l,5l)
				.boxed()
				.collect(Collectors.toList());
		
System.out.println(longs);

Output:

[1, 2, 3, 4, 5]

3. DoubleStream – stream of doubles

Example to convert double stream to List of Doubles.

List<Double> doubles = DoubleStream.of(1d,2d,3d,4d,5d)
				.boxed()
				.collect(Collectors.toList());
		
System.out.println(doubles);

Output:

[1.0, 2.0, 3.0, 4.0, 5.0]

Drop me your questions in comments section related to boxed streams or stream of primitives in Java Stream API.

Happy Learning !!

LinkedinRedditPocket

About Lokesh Gupta

Founded HowToDoInJava.com in late 2012. I love computers, programming and solving problems everyday. A family guy with fun loving nature. You can find me on Facebook, Twitter and Google Plus.

Feedback, Discussion and Comments

  1. Tim

    int[] answer = new int[] { 3, 2 }
List<Integer> expected = Arrays.stream(answer).boxed().collect(Collectors.toList());

Object[] objArrayOfIntArrays = new Object[] {
                new int[] { 1, 2 }, // add 2
                new int[] { 1, 2 }, // add 2
                new int[] { 3, 2 }, // query freq 2
                new int[] { 2, 2 }, // delete 2
                new int[] { 3, 2 }, // query freq 2
            }

    My question is, is there a way to turn objArrayOfIntArrays to List<List<Integer>> ?

    Reply

    • Praveen

      Hi Tim,

       
Object[] objArrayOfIntArrays1 = new Object[] { new int[] { 1, 2 }, // add 2
		new int[] { 1, 2 }, // add 2
		new int[] { 3, 2 }, // query freq 2
		new int[] { 2, 2 }, // delete 2
		new int[] { 3, 2 }, // query freq 2
};

List<List<Integer>> primitiveList = Arrays
					.stream(objArrayOfIntArrays1)
					.map(q -> {
						return Arrays.stream((int[]) q)
						        .boxed()
							.collect(Collectors.toList());
						}).collect(Collectors.toList());

      Hope it Helps,
      Praveen

      Reply

    • Praveen

       
 Object[] objArrayOfIntArrays1 = new Object[] {
             new int[] { 1, 2 }, // add 2
             new int[] { 1, 2 }, // add 2
             new int[] { 3, 2 }, // query freq 2
             new int[] { 2, 2 }, // delete 2
             new int[] { 3, 2 }, // query freq 2
         };
  	   
  	 List<List<Integer>> primitiveList=Arrays.stream(objArrayOfIntArrays1)
		       .map(q->{return Arrays.stream((int[]) q).boxed().collect(Collectors.toList());}).collect(Collectors.toList());
      Reply

Ask Questions & Share Feedback

Please do not submit a comment only to say "Thank you".

*Want to Post Code Snippets or XML content? Please use [java] ... [/java] tags otherwise code may not appear partially or even fully. e.g.
[java] 
public static void main (String[] args) {
...
} 
[/java]