java.lang.String cannot be cast to [Ljava.lang.String;

Question

I have the following and I get the error

java.lang.String cannot be cast to [Ljava.lang.String;

I have changed the Object[] to String[] because I faced the next error:

java.lang.Object cannot be cast to [Ljava.lang.String;

Any idea?

private Collection queryStatement(String SelectStatement) {

    int colcount = 0;
    int rowcount = 0;
    int rowcounter = 0;

    ArrayList a = new ArrayList();

    Query query = getEntityManager().createNativeQuery(SelectStatement);

    List<String[]> resultList = (List<String[]>) query.getResultList();

    if (!resultList.equals(Collections.emptyList())) {
        rowcount = resultList.size();
    }

    if (rowcount > 0) {
        colcount = ((String[]) query.getResultList().get(0)).length;
    }

    rows = rowcount;
    cols = colcount;

    String[][] array = new String[rowcount][colcount];

    for (String[] obj : resultList) {
        String[] record = new String[colcount];
        for (int colCounter = 0; colCounter < colcount; colCounter++) {
            record[colCounter] = safeValue(obj[colCounter]+"");
        }

        array[ rowcounter++] = (String[]) record;
    }
    a.add(array);
    return a;
}

Unquestionable

What is your selectstatement? can you write query? – Xerosis

Naman SELECT PASSWD FROM AUDITORS WHERE USERNAME = '1114' But as you userstand the query is parameter – Unquestionable

Why would that return a List<String[]> ? It looks like it's returning a List<String>, hence the error. – Zeitgeist

yes it will come List<String> because in query you have only one column PASSWD – Xerosis

Because the query is parameter (SelectStatement) is posssible to return List(String[]) – Unquestionable

Your code assumes you'll get a List<String[]> instead of treating that as one of the possibilities. – Orangeman

That would make sense for SELECT *, but you're only selecting 1 attribute. – Zeitgeist

Answer 1

java.lang.String cannot be cast to [Ljava.lang.String;

This error occurs when you try to cast a String to an array of String.

For example:

List list = new ArrayList<>();
list.add("foo");
String[] values = (String[])list.get(0); -> throws the exception

For me you get this error because query.getResultList() returns a List<String> or List<Object> instead of List<String[]> such that when you try to cast a value as a String[] you get this exception.

---

According to the Javadoc createNativeQuery(String) returns a result of type Object[] or a result of type Object if there is only one column in the select list.

Approach #1

One simple way to fix it, could be to rely on the raw type for the result (it is not the most elegant approach but the simplest one) then later you can check the type of the content of the list to cast it properly.

List result = query.getResultList();

Then to check the type you can proceed as next:

if (resultList.isEmpty() || resultList.get(0) instanceof Object[]) {
    // Several columns in the result
    List<Object[]> resultList = (List<Object[]>) result;
    // The rest of your current code here
} else {
    // Only one column in the result
    List<Object> resultList = (List<Object>) result;
    ...
}

Approach #2

A more elegant way could be to create an Entity class and use createNativeQuery(String sqlString, Class entityClass) to create your query, this way it will automatically map your columns with the fields of your Entity

Here is how it could look like

private Collection<T> queryStatement(String SelectStatement, Class<T> resultType) {
    ...
    Query query = getEntityManager().createNativeQuery(SelectStatement, resultType);
    List<T> resultList = (List<T>) query.getResultList();
    ...
}

Picardi

Nicolas Filotto you have right, but the Select query is parameter so on some time return List<String> if the SELECT have one column and List<String[][]> if have more than one. Any Idea how to fix that? – Unquestionable

@NicolasFilotto, the second statement you make This error occurs ... is not strictly correct. I think you mean something like: "This error occurs when you try to cast a String object to an array of String objects String[]". If you use the word List as you have then you implicitly mean java.util.List – Brandybrandyn

For approach #2, the POJO need to be an Entity class, or you will run into a MappingException. For native queries that don't return a mapped table or view, you'll need to go with approach #1 – Flaunty

Answer 2

At some point in your code you're trying to cast a String to String[]. Your stack trace will tell you where exactly.

Apart from that your code has plenty of other issues.

Ics

Answer 3

Do like this:

List<String> resultList = (List<String>) query.getResultList();
if (!resultList.equals(Collections.emptyList())) {
    rowcount = resultList.size();
}

if (rowcount > 0) {
     colcount = resultList.get(0).length;
}

Xerosis

Please see my comment at Nicolas Filotto you have right, but the Select query is parameter so on some time return List<String> if the SELECT have one column and List<String[]> if have more than one. – Unquestionable

you are right @nicolas but here he has mentioned his query. I have already asked him that what is the query and it contains only one select column only. – Xerosis

Answer 4

The line

ArrayList a = new ArrayList();

is a raw type and so (potentailly) all objects in the code could be treaded as Object.

Try

ArrayList<String[][]> a = new ArrayList<>();

Verine