Java – change classes and objects (other aproachs?)

I have several situations that I want to do This may be an example:

Consider the possible implementation of the chess game We define the abstract class' piece 'and the classes inherited from it:' Bishop 'peon' tower 'horse' Queen 'and so on

We may let our peon reach the end of the chessboard, and we may need a method to call the peon object to change the class of the object to "Queen", "horse" or anything else

So my question is, is there any way to do this in C or Java? If not, in other languages? In addition, is there any other way in this case?

Solution

You can use decorator pattern or strategy pattern

Decorated:

interface ChessPiece {
    Set<Move> getPossibleMoves();
    Image getAppearance();
}

class Pawn implements ChessPiece {
    ...
}

class Decorator implements ChessPiece {
    ChessPiece decorated;

    public Decorator(ChessPiece decorated_) {
        this.decorated = decorated_;
    }

    @override
    Set<Move> getPossibleMoves() {
        Set<Move> result = decorated.getPossibleMoves();
        // alter result
        return result;
    }
}

Replace the pawn instance with a decorative instance when necessary So it still involves replacing an instance with another instance mentioned in the opensause's answer

ChessPiece pawn = new Pawn;
...
pawn = new Decorated(pawn);

When you plan ahead, you can use a policy that still doesn't really change the class:

interface PossibleMovesStrategy {
    Set<Move> getPossibleMoves();
}

interface AppearanceStrategy {
    Image getAppearance();
}

class ChangingChessPiece extends ChessPiece {
    PossibleMovesStrategy posMoves;
    AppearanceStrategy appearance;

    @override
    Set<Move> getPossibleMoves() {
        return posMoves.getPossibleMoves();
    }

    @override
    Image getAppearance() {
        return appearance.getAppearance();
    }
}