Type of array based on parameter in Java-CodePudding

I have a byte array like this, that I do some operations on like increment and decrement.

byte[] cells = new byte[numCells]
....
....
cells[index]  ;

Now, I want to change the type of the array cells based on some user input parameter to byte, short, int or long.

So, I made a new class Cell

public class Cell<T extends Number> {

    T cell;

    Cell(T defaultValue){

        cell = defaultValue;
    }

    T get(){
        return cell;
    }

    void set(T t){
        cell = t;
    }
}

And I am trying to use it like this

ArrayList<Cell<?>> cells;

if(cellSize == 8)
    cells = new ArrayList<Cell<Byte>>(numCells);
else if(cellSize == 16)
    cells = new ArrayList<Cell<Short>>(numCells);
...
...

The compiler throws the following error

error: incompatible types: ArrayList<Cell<Byte>> cannot be converted to ArrayList<Cell<?>>
cells = new ArrayList<Cell<Byte>>(numCells);
        ^

How can I accomplish this?

CodePudding user response：

You have to obey the covariance/contravariance rules of the language.

ArrayList<Cell<Byte>>
// is not assignable to
ArrayList<Cell<?>>

but:

ArrayList<Cell<Byte>>
// is not assignable to
ArrayList<? extends Cell<?>>

CodePudding user response：

You don't need to decide for your generics on creation time:

List<Cell<?>> cells = new ArrayList<>(10);

cells.add(new Cell(Short.MAX_VALUE));
cells.add(new Cell(Byte.MAX_VALUE));

CodePudding user response：

You'd have to type the variable as ArrayList<? extends Cell<?>>.

But even if that gets it to compile, it might not help you much. Wildcards are useful as method parameters, otherwise they're mostly a hindrance.

You might want to redesign your code so that the class is used in another class which has a type variable shared with Cell<T>, such as Table<T> so you just work with T inside both classes, and use the actual type in your "main" code when instantiating a Table<Short> for example.