Cast a list of concrete type to a list of its interfaces in Java

Question

Is there a way to cast a list of concrete types to a list of its interfaces in Java?

For example:

public class Square implements Shape { ... }

public SquareRepository implements Repository {

  private List<Square> squares;

  @Override
  public List<Shape> getShapes() {
    return squares; // how can I return this list of shapes properly cast?
  }

}

Thanks in advance,

Caps

Answer 1

You can make use of the generic wildcards to allow for derived lists to be used as base lists:

public List<? extends Shape> getShapes() { ... }

Note that the returned list cannot have non-null items added to to it . (As Mr. Sauer points out, you can add null and deletion is fine as well.) That's the trade-off, though hopefully that won't matter in your case.

Since getShapes() is an override, you would need to update the return type in Repository as well.

Answer 2

If you really want to do this something like the below might work

@Override
public List<Shape> getShapes() {
   return new ArrayList<Shape>(squares); 
}

Answer 3

If you're in control of the Repository interface, I suggest you refactor it to return something of the type List<? extends Shape> List<? extends Shape> instead.

This compiles fine:

interface Shape { }

class Square implements Shape { }

interface Repository {
    List<? extends Shape> getShapes();
}

class SquareRepository implements Repository {
    private List<Square> squares;

    @Override
    public List<? extends Shape> getShapes() {
        return squares;
    }
}