having Obj class which in his constructor has System.out.println("Hello world!") ;
I create an array of this class using - Obj[] objArray = new Obj[10] ; and nothing printed , means - no instance of Obj has been called . Is there any way to create such array ,but with instances , beyond create them in for loop ?
Well, since you want to know a way apart from using a for loop, you can do this: -
Obj[] objArray = {new Obj(), new Obj(), new Obj()};
What happens here is, you are initializing your array reference directly with array elements. Now the type of actual array object is inferred from type of array reference on the LHS.
So, with that declaration, an array of size 3 (in the above case) is created, with each index in the array initialized with the instance of Obj class in the given order.
A better way that I would suggest is to use an ArrayList, in which case, you have double-braces initialization to initialize your List without for loop. And plus with an added advantage that you can anytime add new elements to it. As it dynamically increasing array.
List<Object> list = new ArrayList<Object>() {
{
add(new Obj());
add(new Obj());
add(new Obj());
}
}; // Note the semi-colon here.
list.add(new Obj()); // Add another element here.
Answers so far are good and helpful. I'm here just to remind you about
Obj[] objArray = new Obj[10];
Arrays.fill(objArray, new Obj());
Though, this will only assign one reference (to a new Obj()) to all of the elements of array.
When you do Obj[] objArray = new Obj[10] ;
You only create an array of references to point to the actual 'Obj` object.
But in your case the actual Obj object is never created.
for (int i = 0; i < objArray.length; i++) {
objArray[i] = new Obj();
}
Doing above will print the desired.
Finally do System.out.println(Arrays.deepToString(objArray)) to print toString() of all the Obj
Obj[] objArray = new Obj[10] ; just creates an array capable of holding 10 Objs. To place Objs into the array you need to either use Rohit's approach or write a simple for loop to initialize the array entries one at a time:
for (int i = 0; i < 10; i++) {
objArray[i] = new Obj();
}
Or, without a for loop:
int i = 0;
while (i < 10) {
objArray[i] = new Obj();
i++;
}
objArray[0] = ...; objArray[1] = ...; objArray[2]= ...; ... -- which is essentially what happens with {new Obj(), new Obj(), new Obj()}; under the covers.