How to convert hash Set into array using toArray() if the method toArray is not specified?

Question

Looking at the java api for java collections framework, I could not find toArray() method in HashSet, there is toArray() method in abstract class Set.

class Ideone {
    public static void main (String[] args) throws java.lang.Exception {
        Set x = new HashSet();
        x.add(4);
        //ArrayList<Integer> y = x.toArray(); this does not work !
        int[] y = x.toArray();//this does not work!

        System.out.println(x.toArray());//this gives some weird stuff printed : Ljava.lang.Object;@106d69c
    }
}

How do I convert hashset into array if there is no toArray() specified?

well, the toArray returns an Object[], just printing that is giving you the typical java output if the toString is not properly overwritten. And an Object[] is not implicitly convertible to int[]. — luk2302
– luk2302, Commented Nov 8, 2015 at 11:24
@luk2302 exactly. This is what i have written in my answer but its downvoted. I am not sure whats the issue in answer. Can anyone explain please so that i can improve — M Sach
– M Sach, Commented Nov 8, 2015 at 11:37
@MSach probably because you only explain the printed output, not why the first line does not work nor what would be the proper way - but that is of course only a guess — luk2302
– luk2302, Commented Nov 8, 2015 at 11:39
Thanks luk. i did not pay attention that there is one more question. Will explain that too — M Sach
– M Sach, Commented Nov 8, 2015 at 11:41

Eran · Accepted Answer · 2015-11-08 11:27:28Z

44

Of course HashSet implements toArray. It must implement it, since it implements the Set interface, which specifies this method. The actual implementation is in AbstractCollection which is the super class of AbstractSet which is the super class of HashSet.

First of all, you shouldn't use raw types.

Use :

Set<Integer> x = new HashSet<>();
x.add(4);

Then convert to array :

Integer[] arr = x.toArray(new Integer[x.size()]);

Using x.toArray() would give you an Object[].

edited Nov 8, 2015 at 11:27

answered Nov 8, 2015 at 11:24

Eran

395k57 gold badges726 silver badges793 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

ERJAN Over a year ago

yes, i just noticed that you can add string, int , char etc to unspecified hashSet!

SamTebbs33 Over a year ago

@ERJAN Yeah that's because it then uses the Object class, and the boxed versions of those primitives extend Object.

Sayyid Iskandar Khan Over a year ago

also this is to convert to Integer[] array, if developer is interested to convert to int[] then refer to my solution below. stackoverflow.com/a/74862173/17903735

SamTebbs33 · Accepted Answer · 2015-11-08 11:25:13Z

5

Make sure that you declare the generic for the HashSet

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

And convert it to an array like so:

int[] y = new int[x.size()];
int c = 0;
for(int x : x) y[c++] = x;

answered Nov 8, 2015 at 11:25

SamTebbs33

5,6473 gold badges24 silver badges47 bronze badges

Comments

M Sach · Accepted Answer · 2015-11-08 11:50:54Z

3

First Line

ArrayList y = x.toArray(); this does not work !

First of all you used Set x = new HashSet(); i.e raw type . Compiler does not know that s it going to contain integer object but with above line on left hand side you are saying its going to be arraylist of integer where actually its an array

Second line

int[] y = x.toArray();//this does not work!

with above line on left hand side you are saying its going to be array of integer where actually its an array of objects

This will work

Object[] y = x.toArray();

But this is not the right way . You should not use raw types

 Set<Integer> x = new HashSet<>();
 Integer[] intArray= x.toArray(new Integer[x.size()]);

System.out.println(x.toArray());//this gives some weird stuff printed : Ljava.lang.Object;@106d69c

Its printing toString representation of array object . Thats why you are seeing it as Ljava.lang.Object;@106d69c

If you want to print each element , iterate over it and then print it.

edited Nov 8, 2015 at 11:50

answered Nov 8, 2015 at 11:26

M Sach

34.6k78 gold badges232 silver badges317 bronze badges

1 Comment

M Sach Over a year ago

Downvoter it will be helpful to know whats wrong here ? I am not getting whats the issue in the answer

Pramod · Accepted Answer · 2019-11-29 06:31:27Z

2

It looks like you originally wanted to create an ArrayList rather than a simple Array. So, try this!

class Ideone 
{
        public static void main (String[] args) throws java.lang.Exception   
        {
            Set x = new HashSet();
            x.add(4);
            ArrayList<Integer> y = new ArrayList<>(x);
            System.out.println(y);
        }
}

edited Nov 29, 2019 at 6:31

Pramod

1,41613 silver badges22 bronze badges

answered Jan 21, 2017 at 7:29

paiego

3,7971 gold badge42 silver badges52 bronze badges

Comments

sjzack · Accepted Answer · 2017-08-30 14:41:18Z

Comparison in JDK 7 sorting a small map, using TreeSet, ArrayList and Array:

long start  = System.currentTimeMillis(); 
for(int i=0; i<10000000; i++){ 
   TreeSet a   = new TreeSet(payloads.keySet());                           
} 
System.out.println("TreeSet: "    + (System.currentTimeMillis()-start) + " ms.");
start       = System.currentTimeMillis(); 
for(int i=0; i<10000000; i++){ 
   ArrayList a = new ArrayList(payloads.keySet()); 
   Collections.sort(a);    
} 
System.out.println("ArrayList: "  + (System.currentTimeMillis()-start) + " ms.");
start       = System.currentTimeMillis(); 
for(int i=0; i<10000000; i++){ 
   String[] a = payloads.keySet().toArray(new String[payloads.size()]); 
   Arrays.sort(a);    
} 
System.out.println("Array: "  + (System.currentTimeMillis()-start) + " ms.");

Yields:

TreeSet: 1527 ms.
ArrayList: 943 ms.
Array: 485 ms.

user3133925 · Accepted Answer · 2019-03-15 07:39:43Z

0

We can iterate through the loop and store the values into the array.

int[] answer = new int[set1.size()];
int i = 0;
for (int num : set1) {
      answer[i++] = num;
}

answered Mar 15, 2019 at 7:39

user3133925

Comments

Sayyid Iskandar Khan · Accepted Answer · 2022-12-20 11:08:47Z

0

you can convert a hashset to arraylist then to arrays int[]. this approach is good if u want to dynamically convert it back to array.

this would be a one-liner solution.

Set<Integer> my_set = new HashSet<>();
my_set.add(4);

int[] result = new ArrayList<>(my_set).stream().mapToInt(i -> i).toArray();
System.out.println(Arrays.toString(result)); // [4]

answered Dec 20, 2022 at 11:08

Sayyid Iskandar Khan

1016 bronze badges

Collectives™ on Stack Overflow

How to convert hash Set into array using toArray() if the method toArray is not specified?

7 Answers 7

3 Comments

Comments

1 Comment

Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

7 Answers 7

3 Comments

Comments

1 Comment

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related