CODEWITHSUNDEEP
javastringobjectarraylist
Squeez
Squeez

Reputation: 959

ClassCastException when I do ObjectList = StringList

I created list using method Arrays.asList() with one String. Then I assigned it to List<Object>, and I get ClassCastException. What did I do wrong?

The code is: 

class A {
    Object value;

    public <T> T getAValue() {
        return (T) value;
    }
}

And then I do

A a = new A();
a.value = "abc";

List<Object> list = Arrays.asList(a.getAValue());

Upvotes: 0

Views: 71

Answers (2)

user180100
user180100

Reputation:

If you really cannot add type info on A (i.e. use class A<T>), then a simple cast solves your issue, like this (demo):

A a = new A();
a.value = "abc";
List<Object> list = Arrays.asList((Object) a.getAValue());

This is safe, because everything is an Object, but the clean way is to use type info on A, as stated in another answer (demo):

A<String> a = new A<String>();
a.value = "abc";
List<String> list = Arrays.asList(a.getAValue());

Also note that you should use a setter for value (i.e. no a.value = "...").

Some reading: https://docs.oracle.com/javase/tutorial/java/generics/

Upvotes: 1

pczeus
pczeus

Reputation: 7867

Try this:

    import java.util.Arrays;
    import java.util.List;

    class A<T>{
        T value;

        public T getAValue(){
            return value;
        }

        public static void main(String... args){
            A a = new A();
            a.value = "abc";

            List<Object> list = Arrays.asList(a.getAValue());
            System.out.println(list);
            System.out.println(a.value.getClass());
        }
    }

Note the simplified syntax by declaring the generic type 'T' when declaring class A<T> T's type is bound to String and it is unnecessary to cast the return value in the getAValue(), generics take care of that for you.

Upvotes: 1

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