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;
}

Naman SELECT PASSWD FROM AUDITORS WHERE USERNAME = '1114' But as you userstand the query is parameter — Giorgos
– Giorgos, Commented Sep 22, 2016 at 11:58
Why would that return a List<String[]> ? It looks like it's returning a List<String>, hence the error. — Jorn Vernee
– Jorn Vernee, Commented Sep 22, 2016 at 12:01
yes it will come List<String> because in query you have only one column PASSWD — Nimesh
– Nimesh, Commented Sep 22, 2016 at 12:01
Because the query is parameter (SelectStatement) is posssible to return List(String[]) — Giorgos
– Giorgos, Commented Sep 22, 2016 at 12:03

Nicolas Filotto · Accepted Answer · 2020-10-19 09:29:57Z

24

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();
    ...
}

edited Oct 19, 2020 at 9:29

answered Sep 22, 2016 at 12:10

Nicolas Filotto

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3 Comments

Giorgos Over a year ago

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?

Steve C Over a year ago

@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

Tobi Nonymous Over a year ago

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

DaImmi · Accepted Answer · 2016-09-22 12:08:07Z

1

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.

answered Sep 22, 2016 at 12:08

DaImmi

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Comments

Nimesh · Accepted Answer · 2016-09-22 12:15:55Z

1

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;
}

answered Sep 22, 2016 at 12:15

Nimesh

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2 Comments

Giorgos Over a year ago

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.

Nimesh Over a year ago

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.

OldCurmudgeon · Accepted Answer · 2016-09-22 12:05:14Z

0

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<>();

answered Sep 22, 2016 at 12:05

OldCurmudgeon

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