如何使某些东西与数组和集合兼容？

Question

It sounds a stupid question. However, in the API that I am using, version 1.7.2 has got the method Bukkit.getServer().getOnlinePlayers() returning a Player[] , and version 1.7.10 has got Bukkit.getServer().getOnlinePlayers() returning a Collection<Player> . I need to make my plugin compatible with both.

I have got the API's for both, but aside from creating separate plugins, I currently have got no idea how to do this.

I currently just convert the collection into an array anyway. So, is there any way to (i can already get the version) if the version is less than 1.7.9, not use the .toArray() but since it already returns an array?

Answer 1

You could do it with reflection. Something like this:

List<Player> getPlayers() {
    try {
        Method method = getMethod(Server.class, "getOnlinePlayers");
        Object result = method.invoke(Bukkit.getServer());
        if(result instanceof Player[])
            return Arrays.asList((Player[])result);
        else
            return (List<Player>)result;

    } catch(ReflectiveOperationException e) {

        // something went wrong! If you have a better way to handle problems, do that instead
        throw new RuntimeException(e);
    }
}

Answer 2

There are actually two ways to do this. @immibis's answer gives the first way. The second way involves creating a "version adapter" API with multiple plugin implementations for different versions of (in this case) Bukkit; eg

public interface BukkitVersionAdapter {
    Collection<Player> getOnlinePlayers();
    ...
}

public class BukkitVersionAdapterV1dot7 implements BukkitVersionAdapter {
    public Collection<Player> getOnlinePlayers() {
        return Arrays.asList(Bukkit.getServer().getOnlinePlayers());
    }
    ...
}

public class BukkitVersionAdapterV1dot8 implements BukkitVersionAdapter {
    public Collection<Player> getOnlinePlayers() {
        return Bukkit.getServer().getOnlinePlayers();
    }
    ...
}

The version-specific adapter classes then need to be compiled against the Bukkit API jars for the respective Bukkit versions.

Then when you start your main application (or Bukkit plugin I guess), you do something like this:

String version = // get the Bukkit version
String className = // map the version to a class name; e.g.
                   // "pkg.BukkitVersionAdapterV1dot7" or
                   // "pkg.BukkitVersionAdapterV1dot8"
Class clazz = Class.forName(className);
BukkitVersionAdapter adapter = 
         (BukkitVersionAdapter) clazz.newInstance();

// Then ...
Collection<Player> players = adapter.getOnlinePlayers();

---

This is all rather cumbersome, but it has two benefits compared with the approach of using reflection to make the calls:

1. The logic that is specific to different Bukkit versions is now isolated to one part of the codebase ... rather than being scattered (potentially) all over the place.

2. The overheads of reflection are only incurred at startup. After that, all of the method calls to Bukkit made via the adapter API are regular Java method calls.

Depending on the context, these may make the adapter approach more appropriate.

Answer 3

If the class org.bukkit.Server just changed the return type of the method getOnlinePlayers() rather than making it deprecated and introducing a new method (yack!!!!! who dares to change the already published interface in such a way?), you have no decent way how to call the method in one single code. You simply cannot write something like

Server server = Bukkit.getServer();
// One of the branches will not be compilable
if (version <= xxxx) {
    Player[] players = server.getOnlinePlayers();
}
else {
    Collection<Player> players = server.getOnlinePlayers();
}

I'm afraid the only way is to use reflection like in the meantime others recommended.

Answer 4

You are doing well. Unfortunately there is no way to create strongly typed method that can deal with both collection and array.

(You can create method that accepts Object and then examine it using instanceof , cast and deal with both collection and arrays but this solution is ugly.)