class cast exception: java.lang.String incompatible with [Ljava.lang.Object;

Question

What is wrong with below code ?

List<Object[]> currencies = null;
List<String> currencyList = new ArrayList<String>();

//code to fetch currency
String currencySql = "select C.Currency from Currency C";
Query currencyQuery = this.em.createQuery(currencySql);

currencies = currencyQuery.getResultList();

for (Object[] currency : currencies) {    //getting runtime error here
   for (int i = 0; i < currency.length; i++) {
            currencyList.add(currency[i].toString());
        }
    }

Getting runtime error in first line of for loop as: java.lang.ClassCastException: java.lang.String incompatible with [Ljava.lang.Object;

The code compiles alright.

What is the issue ?

Solution

I'm not sure why you are thinking that the Query would return a list of Object arrays.

The query will most likely return a list of Strings (unless the Currency entity contains a Currency object, in which case I'd revisit the data design), so operating under the assumption that this is a scalar query, your code can be simplified to:

String currencySql = "select C.Currency from Currency C";
Query currencyQuery = this.em.createQuery(currencySql);

List<String> currencyList = (List<String>) currencyQuery.getResultList();

Comments

[Vicky]

changing currencies to List<String[]> threw java.lang.ClassCastException: java.lang.String incompatible with [Ljava.lang.String;

[dbf]

I mean not List<String[]>, but List<String>, because looks like your query returns one column, that is converted to list of strings.

[Donal Fellows]

And the result is definitely not a list of arrays of anything.

[Vicky]

So is it that hibernate decides from the SQL whether it will return list of strings or list of objects based on the type of things returned ???

[Donal Fellows]

It knows the types. It knows how many values there are in each row. It says “one value per row; row value is that value” rather than wastefully putting it inside an array of length 1.

[Sid Malani]

i am not sure what the getResultList returns. I would assume its a List. Or does it return Object Array?

[Vicky]

Thanks!! So if the query is for several columns, in that case getResultList will return list of Object arrays. But in this case it was just one column, which was String, it returned a list of Strings. Correct ???

[beny23]

It depends on the concrete implementation of your EntityManager, but it certainly looks that way.