Java passing variable - downcasting

Question

I have a function e.g.

helloworld(list<object> names)

I have the following code :

List<CustomClass> newMe = new ArrayList<CustomClass>();

Now, if i want to pass newMe into helloworld(newMe);. This is not possible because im down casting. How can i overcome this issue? Do i downcast my list to (Object) and then try to upcast it? is there another way? would appreciate an example.

thanks

score 2 · Accepted Answer · answered Nov 18 '14 at 16:46

2

Change the definition of helloworld to

public void helloworld(List<?> names) {
    //method implementation...
}

Take into account that your method won't be able to add or remove elements from the list parameter.

answered Nov 18 '14 at 16:46

Luiggi Mendoza

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1

You can remove elements from the list parameter. You can also add a null reference but that's not very useful. – user2336315 Nov 18 '14 at 16:50
1

@user2336315 you meant `remove(int)`. Yes, you're right but I don't think it's proper to call `remove(int)` in this kind of methods. – Luiggi Mendoza Nov 18 '14 at 16:52
So if i pass it as a generic > , i wont be able to directly modify? – Rory Lester Nov 18 '14 at 17:27
@RoryLester exactly. You cannot modify it since at compile time you don't know which type the list is. – Luiggi Mendoza Nov 18 '14 at 17:28

score 0 · Answer 2 · answered Nov 18 '14 at 17:06

Just use a ? as generic type in your parameter list. Example:

public class Foobar {
    public static void helloworld(List<?> names) {

    }
    public static void main(String[] args) {
        List<CustomClass> newMe = new ArrayList<>();

        helloworld(newMe);
    }
}

Java passing variable - downcasting

2 Answers2