Java, convert hashMap to array or empty array

Question

Im a bit beginner. Funny or not, I dont know how to create an empty typed array. My class:

class Depot
{
}

storing them:

private final HashMap<Integer, Depot> depots;
depots = new HashMap<>();

and I want to return them:

public Depot[] getDepots()
{
    if (depots.values().isEmpty())
    {
        return (Depot[]) new Object[0]; ***
    }
    else
    {
        return depots.values().toArray(new Depot[depots.values().size()-1]);
    }
}

as an array. Now when there is no depots, them comes the tricky part. First of all, this line depots.values().size()-1 fails. The return (Depot[]) new Object[0]; seems to be OK, but still triggers an exception: java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Depot;

why not just new Depot[0]?

SomeJavaGuy
– SomeJavaGuy

2016-09-26 13:42:03 +00:00
Commented Sep 26, 2016 at 13:42 — SomeJavaGuy
– SomeJavaGuy, Commented Sep 26, 2016 at 13:42

Eran · Accepted Answer · 2016-09-26 13:54:17Z

2

There's no need to cast. Just create a Depot[] in the first place :

public Depot[] getDepots()
{
    if (depots.isEmpty())
    {
        return new Depot[0];
    }
    else
    {
        return depots.values().toArray(new Depot[depots.size()]);
    }
}

Also note that the length of the array in the else clause should be depots.values().size() (or depots.size()), not depots.values().size() - 1. If you pass a too small array to toArray(), that array will not be used by that method, and a new array will be created.

edited Sep 26, 2016 at 13:54

answered Sep 26, 2016 at 13:42

Eran

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2 Comments

Sergey Kalinichenko Over a year ago

Why not depots.values().size(), if OP prefers so? Once you drop minus one, do you even need a conditional?

Eran Over a year ago

@dasblinkenlight It would be pointless to pass a too small array to that method, since it will cause toArray() to ignore the passed array and create a large enough array. Therefore I assumed it wasn't intentional. And while it's true that depots.values().toArray(new Depot[depots.size()]); would also work for the empty case, perhaps it would be a little more efficient not to call values() and toArray() when the Map is empty (though it probably won't make a noticeable difference).

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