CODEWITHSUNDEEP
javainheritancedowncast
Zac Sanchez
Zac Sanchez

Reputation: 71

How to downcast an array in Java?

I currently have a Tile class that extends a Node class, and want to downcast an array of Nodes to an array of Tiles like so:

class Node<T> {
  Node[] neighbours;
  private final T value;
}

class Tile extends Node {

  public Tile(Point value) {
    super(value);
  }

  Tile[] neighbours = (Tile[]) this.getNeighbours;
}

At the moment the compiler is throwing A ClassCastException and I do not know how to fix it.

I am not 100% familiar with inheritance, but I thought since Tile is a subclass this should be a safe casting from Nodes to Tiles.

Upvotes: 1

Views: 1331

Answers (2)

El Mahdi Benzekri
El Mahdi Benzekri

Reputation: 61

You could use a collection, for example something like:

class Node<T> {
    List<Node> neighbours;
    private T value;
}

class Tile extends Node {

    public Tile(String value) {
        super(value);
    }

    List<Tile> neighbours = this.neighbours.stream().filter(item -> item instanceof Tile).map(item -> (Tile) item).collect(Collectors.toList());
}

Upvotes: 0

Eran
Eran

Reputation: 393836

If Tile is a sub-class of Node, all Tiles a Nodes, but not all Nodes a Tiles.

Therefore casting a Node[] to Tile[] is wrong, since not all Node arrays are Tile arrays.

For example, the following will throw ClassCastException:

Node[] nodes = new Node[10];
Tile[] tiles = (Tile[]) nodes;

On the other hand, the following will work:

Node[] nodes = new Tile[10];
Tile[] tiles = (Tile[]) nodes;

Upvotes: 3

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