67

I have an array of int:

int[] a = {1, 2, 3};

I need a typed set from it:

Set<Integer> s;

If I do the following:

s = new HashSet(Arrays.asList(a));

it, of course, thinks I mean:

List<int[]>

whereas I meant:

List<Integer>

This is because int is a primitive. If I had used String, all would work:

Set<String> s = new HashSet<String>(
    Arrays.asList(new String[] { "1", "2", "3" }));

How to easily, correctly and succinctly go from:

A) int[] a...

to

B) Integer[] a ...

Thanks!

Improve this question
1

8 Answers 8

Reset to default
79

Using Stream: 

// int[] nums = {1,2,3,4,5}
Set<Integer> set = Arrays.stream(nums).boxed().collect(Collectors.toSet())
Improve this answer
Sign up to request clarification or add additional context in comments.

1 Comment

wow. let's not make it any easier huh?
25

The question asks two separate questions: converting int[] to Integer[] and creating a HashSet<Integer> from an int[]. Both are easy to do with Java 8 streams:

int[] array = ...
Integer[] boxedArray = IntStream.of(array).boxed().toArray(Integer[]::new);
Set<Integer> set = IntStream.of(array).boxed().collect(Collectors.toSet());
//or if you need a HashSet specifically
HashSet<Integer> hashset = IntStream.of(array).boxed()
    .collect(Collectors.toCollection(HashSet::new));
Improve this answer

1 Comment

I think it would be worthwhile to mention this line technique is slower. I used IntStream.of(nums).boxed().collect(Collectors.toSet()) instead of just looping over the elements, which increased my runtime from 18ms to 25ms, on average.
10

Some further explanation. The asList method has this signature

public static <T> List<T> asList(T... a)

So if you do this:

List<Integer> list = Arrays.asList(1, 2, 3, 4)

or this:

List<Integer> list = Arrays.asList(new Integer[] { 1, 2, 3, 4 })

In these cases, I believe java is able to infer that you want a List back, so it fills in the type parameter, which means it expects Integer parameters to the method call. Since it's able to autobox the values from int to Integer, it's fine.

However, this will not work

List<Integer> list = Arrays.asList(new int[] { 1, 2, 3, 4} )

because primitive to wrapper coercion (ie. int[] to Integer[]) is not built into the language (not sure why they didn't do this, but they didn't).

As a result, each primitive type would have to be handled as it's own overloaded method, which is what the commons package does. ie.

public static List<Integer> asList(int i...);
Improve this answer

2 Comments

I believe you can also call Arrays.<Integer>asList(1, 2, 3, 4)
The title of the question is Java int[] array to HashSet<Integer> and you don't have HashSet anywhere in your answer.
5

Or you could easly use Guava to convert int[] to List<Integer>:

Ints.asList(int...)

asList

public static List<Integer> asList(int... backingArray)

Returns a fixed-size list backed by the specified array, similar to Arrays.asList(Object[]). The list supports List.set(int, Object), but any attempt to set a value to null will result in a NullPointerException.

The returned list maintains the values, but not the identities, of Integer objects written to or read from it. For example, whether list.get(0) == list.get(0) is true for the returned list is unspecified.

Improve this answer

1 Comment

Yea I mean the standard libraries just don't have this.
2

Just add elements from array to Set with the below snippet

public class RemoveDuplicateElements {

    public static void main(String args[]){
        int array[] =  {0,1,2,3,4,5,6,7,8,9,1,2,3,4,5};
        Set <Integer> abc = new HashSet <Integer>();
        for (Integer t:array){  
            abc.add(t); 
        }       
        System.out.println("sampleSet"+abc);  
    }

}
Improve this answer

1 Comment

OP is asking JAVA8 way.
2

You can use ArrayUtils in Apache Commons:

int[] intArray  = { 1, 2, 3 };
Integer[] integerArray = ArrayUtils.toObject(intArray);
Improve this answer

3 Comments

If Commons has a method to do this, it means there wasn't a way to do it with the base JDK... thanks for the pointer!
Commons library takes up a lot of method references on Android, it's highly recommended to avoid using it. This makes sense and works well with ordinary Java.
Apache Commons link is not working.....and how to use arrayUtils?
1

Another option would be to use a primitive set from Eclipse Collections. You can easily convert an int[] to a MutableIntSet to a Set<Integer> or Integer[] as shown below, or you can use the MutableIntSet as is which will be much more memory efficient and performant.

int[] a = {1, 2, 3};
MutableIntSet intSet = IntSets.mutable.with(a);
Set<Integer> integerSet = intSet.collect(i -> i);  // auto-boxing
Integer[] integerArray = integerSet.toArray(new Integer[]{});

If you want to go directly from the int array to the Integer array and preserve order, then this will work.

Integer[] integers = 
        IntLists.mutable.with(a).collect(i -> i).toArray(new Integer[]{});

Note: I am a committer for Eclipse Collections

Improve this answer

Comments

-2

No need for looping :
Just you will convert the array to a List
Then converting this List to a hash set.
Ex:
List list = Arrays.asList(your_array);
Set set = new HashSet<>(list);


This worked perfect for me .

Improve this answer

2 Comments

Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center.
this code is not working.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.