HowToDoInJava

Java Boxed Stream Example

By | 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 !!

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Feedback, Discussion and Comments

  1. 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

    • 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

    •  
 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

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