避免使用重载mutator的instanceof

Question

I have two classes that inherit from Player: SimplePlayer and InterleavedPlayer.

Class B has these methods: setPlayer(SimplePlayer player) and setPlayer(InterleavedPlayer player)

Class A has a Player field, which is designed to be either a SimplePlayer or InterleavedPlayer. Class A wants to call B.setPlayer() on these players, without concerning itself with what subclass of Player the object is.

I overloaded B.setPlayer() for the subclasses in order to avoid using instanceof in a generalized B.setPlayer(Player player) method.

But on A's call to B.setPlayer(), I get a symbol not found error referring to B.setPlayer(Player).

What design approach should I use here to resolve the issue? (A actually creates the players, so it in theory "knows" what it's passing to B. However, because of the way A indiscriminately treats it's player it seems to make sense just having a Player that it does stuff to, rather than having to distinguish between SimplePlayer and InterleavedPlayer).

Answer 1

Using instanceof is not necessarily evil. Like all design decisions, it involves a trade off. I think the question here to ask is, is the code in question more coupled with A or B?

For example, what happens if a new subclass of Player is created? Will you have to modify B to add a new setPlayer method? If so, it makes sense to have a generic method in B that dispatches via instanceof since you'll have to modify B anyway.

Answer 2

The reason you are getting the error is that we do not have run time dispatching on the method parameters in java, just on the object on which it is being called. Meaning that the actual run-time class of the object you call it on ( B ) affects which function is picked, however the only the compile time class of the reference to the parameter ( Player ) affects the choice of function.

The answer to fix this would depend on why B needs 2 functions, and if the difference can be moved into the individual player classes , and if that makes design sense. ie the does the extra computation happen because of B or because of the player classes?

Answer 3

Have overloaded methods in A:

private Player;

public void setPlayer(SimplePlager player) {
    this.player = player;
    b.setPlayer(player);
}

// similar for InterleavedPlayer

Or if the call comes from within the constructor, you can overload that too.

Answer 4

you'll need to post code to get it fixed... You should use mediator, because you are flip-flopping calls between two objects.

I have two classes that inherit from Player: SimplePlayer and InterleavedPlayer

ok

class Player{}
class SimplePlayer extends Player {}
class InterleavedPlayer extends Player{}

Class A has a Player field, which is designed to be either a SimplePlayer or InterleavedPlayer. Class A wants to call B.setPlayer() on these players, without concerning itself with what subclass of Player the object is.

ok

class A { Player field; }

I overloaded B.setPlayer() for the subclasses in order to avoid using instanceof in a generalized B.setPlayer(Player player) method.

ok

class B { 
    void setPlayer(InterleavedPlayer player) {  out.println( player );  }  
}

"But on A's call to B.setPlayer(), I get a symbol not found error referring to B.setPlayer(Player)." ok.

B requires an interleaved player right? in A where it calls B.setPlayer( (InterleavedPlayer) thePlayer ), cast the Player to an Interleaved player, you can get around this a lot of different ways but without code to see, it can be a lot of words....

particularly, the code needed is Player::setPlayer, and Interleaved::setPlayer