How to get ArrayList Type and create an ArrayList based off that?-CodePudding

I have this method to read a txt file. First it converts the .txt file to an Object, and then checks if it's an ArrayList. If it is I want to create an ArrayList of whatever type obj is.

    public static readFile(String fileName) {
        Object obj = null;
        
        try {
            obj = deserialize(fileName);
        } catch (IOException e) {
            return null;
        } catch (ClassNotFoundException e) {
            return null;
        }

        ArrayList<?> list;

        if (obj instanceof ArrayList<?>) {
            list = (ArrayList<?>) obj;
        } else {
            return null;
        }

        return null;
    }

I know all elements of the ArrayList will be of the same type in my scenario so I was thinking about using list.get(0).getClass() I tried doing ArrayList<(list.get(0).getClass()> but it doesn't work so I don't know where to go from here.

CodePudding user response：

I was thinking about using list.get(0).getClass() I tried doing ArrayList<(list.get(0).getClass()>

Generics is a compile time mechanism. Its purpose is to allow the compiler to verify that manipulations in your code are type-safe.

And during the compilation ArrayList<Something> turn into ArrayList<Object>, this process is called generic type erasure.

You can't provide a generic type through a method call, like list.get(0).getClass().

If you want to benefit from generics, then you need to provide the Generic type explicitly, and can't be dynamic.

CodePudding user response：

You can't do that!

Think about that, if you can do that, it same to this:

ArrayList<Integer> instanceof ArrayList<?> or ArrayList<Integer> instanceof ArrayList<Number>

Integer is a Number, that is ok, but as generics is invariance, ArrayList<Integer> is not a ArrayList<Number>. This contradicts the above.