why the following list can not be convert to string array in Java

Question

I have the following code in Java, when I run it, it give some error:

String[] stringArray1 = {"1","2","3"};
String[] stringArray2 = {"1","3","5"};
String[] stringArray3 = {"1","4","6"};

public String[] getString(int age) {

        List<String> list = new ArrayList<String>();

        switch (age) {
            case 1:

                Collections.addAll(list, stringArray1);
                return composeStrings(list);

            case 2:

                Collections.addAll(list, stringArray2);
                return composeStrings(list);

            case 3:
                Collections.addAll(list, stringArray3);
                return composeStrings(list);

            default:
                return new String[]{"Not find", "Please specify the age"};
            }
    }

public String[] composeStrings(List<String> list) {

        list.add("added a new string");

        return (String[]) list.toArray();
}

but when I run it, it says line: return (String[]) list.toArray();

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

what is the problem of this? and how to fix it?

Why are you returning String[].. why not return Collections.unmodifiableList(list); — user2591612
– user2591612, Commented Feb 3, 2014 at 15:23
This is very convoluted.. you create a List to just throw it away — user2591612
– user2591612, Commented Feb 3, 2014 at 15:27
Further reading on casting from Object[] to String[]: en.wikipedia.org/wiki/… — biddulph.r
– biddulph.r, Commented Feb 3, 2014 at 15:27

lance-java · Accepted Answer · 2014-02-03 15:39:11Z

5

Try:

public String[] composeStrings(List<String> list) {

    list.add("added a new string");

    return list.toArray(new String[list.size()]);
}

You can read the javadoc for toArray here

edited Feb 3, 2014 at 15:39

answered Feb 3, 2014 at 15:21

lance-java

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1 Comment

Joop Eggen Over a year ago

Won't need the cast (String[]). Generic types are erased in the compiled code. By passing an object to toArray the component type String can be determined and returned. If the size fits, that new array is used as return value, after filling it.

Jaffar Ramay · Accepted Answer · 2014-02-03 15:26:14Z

0

You have to pass String[] as argument to toArray() method otherwise it will return you a Object[]

public static void main(String[] args) {

        String[] stringArray1 = { "1", "2", "3" };
        String[] stringArray2 = { "1", "3", "5" };
        String[] stringArray3 = { "1", "4", "6" };

        List<String> list = new ArrayList<String>();

        Collections.addAll(list, stringArray1);
        Collections.addAll(list, stringArray2);
        Collections.addAll(list, stringArray3);

        String[] finalArray = list.toArray(new String[list.size()]);
        for (String item : finalArray) {
            System.out.println(item);
        }

    }

answered Feb 3, 2014 at 15:26

Jaffar Ramay

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1 Comment

Jaffar Ramay Over a year ago

I have added the example code as well, now you can see there is no casting at all.

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