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

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[].

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;

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.