Java 插口：三类不合作

Question

Say I've got three classes: GUI, OpenGui and BanGUIMain. GUI:

public class GUI {

    private BanGUIMain main = new BanGUIMain(); /*line 16*/
    public void createOptionsGUI(Player p) {}
}

OpenGui:

public class OpenGui implements CommandExecutor {
    @Override
    public boolean onCommand(CommandSender sender, Command command, String label, String[] args) {
        if (sender instanceof Player) {
            if (sender.hasPermission("bangui.opengui")) {
                GUI gui = new GUI(); /*line 14*/
                gui.createOptionsGUI((Player) sender);
            }
        }
        return false;
    }
}

BanGUIMain:

public final class BanGUIMain extends JavaPlugin { /*this is line 12*/
// much stuff that does not matter
}

And well, I am coding a Spigot Plugin. Basically, when I am calling an /opengui command (I registered it), I get this error:

[16:29:55 ERROR]: null
org.bukkit.command.CommandException: Unhandled exception executing command 'opengui' in plugin BanGUIMain v1.0.0
        at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46) ~[server.jar:git-Spigot-db6de12-18fbb24]
        at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:141) ~[server.jar:git-Spigot-db6de12-18fbb24]
        at org.bukkit.craftbukkit.v1_8_R3.CraftServer.dispatchCommand(CraftServer.java:641) ~[server.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.PlayerConnection.handleCommand(PlayerConnection.java:1162) [server.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.PlayerConnection.a(PlayerConnection.java:997) [server.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.PacketPlayInChat.a(PacketPlayInChat.java:45) [server.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.PacketPlayInChat.a(PacketPlayInChat.java:1) [server.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.PlayerConnectionUtils$1.run(SourceFile:13) [server.jar:git-Spigot-db6de12-18fbb24]
        at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_271]
        at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_271]
        at net.minecraft.server.v1_8_R3.SystemUtils.a(SourceFile:44) [server.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.MinecraftServer.B(MinecraftServer.java:715) [server.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.DedicatedServer.B(DedicatedServer.java:374) [server.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.MinecraftServer.A(MinecraftServer.java:654) [server.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:557) [server.jar:git-Spigot-db6de12-18fbb24]
        at java.lang.Thread.run(Unknown Source) [?:1.8.0_271]
Caused by: java.lang.IllegalArgumentException: Plugin already initialized!
        at org.bukkit.plugin.java.PluginClassLoader.initialize(PluginClassLoader.java:122) ~[server.jar:git-Spigot-db6de12-18fbb24]
        at org.bukkit.plugin.java.JavaPlugin.<init>(JavaPlugin.java:66) ~[server.jar:git-Spigot-db6de12-18fbb24]
        at me.roughlyunderscore.plugins.premium.bangui.main.BanGUIMain.<init>(BanGUIMain.java:12) ~[?:?]
        at me.roughlyunderscore.plugins.premium.bangui.main.gui.GUI.<init>(GUI.java:16) ~[?:?]
        at me.roughlyunderscore.plugins.premium.bangui.main.commands.OpenGui.onCommand(OpenGui.java:14) ~[?:?]
        at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44) ~[server.jar:git-Spigot-db6de12-18fbb24]
        ... 15 more
Caused by: java.lang.IllegalStateException: Initial initialization
        at org.bukkit.plugin.java.PluginClassLoader.initialize(PluginClassLoader.java:125) ~[server.jar:git-Spigot-db6de12-18fbb24]
        at org.bukkit.plugin.java.JavaPlugin.<init>(JavaPlugin.java:66) ~[server.jar:git-Spigot-db6de12-18fbb24]
        at me.roughlyunderscore.plugins.premium.bangui.main.BanGUIMain.<init>(BanGUIMain.java:12) ~[?:?]
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[?:1.8.0_271]
        at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source) ~[?:1.8.0_271]
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source) ~[?:1.8.0_271]
        at java.lang.reflect.Constructor.newInstance(Unknown Source) ~[?:1.8.0_271]
        at java.lang.Class.newInstance(Unknown Source) ~[?:1.8.0_271]
        at org.bukkit.plugin.java.PluginClassLoader.<init>(PluginClassLoader.java:76) ~[server.jar:git-Spigot-db6de12-18fbb24]
        at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:131) ~[server.jar:git-Spigot-db6de12-18fbb24]
        at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:329) ~[server.jar:git-Spigot-db6de12-18fbb24]
        at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:251) ~[server.jar:git-Spigot-db6de12-18fbb24]
        at org.bukkit.craftbukkit.v1_8_R3.CraftServer.loadPlugins(CraftServer.java:292) ~[server.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.DedicatedServer.init(DedicatedServer.java:198) ~[server.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:525) ~[server.jar:git-Spigot-db6de12-18fbb24]
        ... 1 more

That's probably what is going on:

1. Calling opengui command is going to the main class BanGUIMain, where it finds the registered OpenGui.
2. Goes to the OpenGui, where finds the new GUI() line.
3. Goes to the GUI class, where finds the new BanGUIMain() line and probably tries to register the command again or something.
   
   Well, my code doesn't work anyways. Probably it just loops through all the classes. But why can't I do that? Should I like do an anti-"new GUI()" after finishing using it? I am quite new to Java.

Answer 1

Upon searching, you probably have the same problem as this . In there, it is told to remove the piece of code where it is creating a new instance of the main class.

In your situation, try removing this part in your GUI class:

private BanGUIMain main = new BanGUIMain();

The server is returning the Plugin already initialized! error because the plugin is loading its main class when the plugin class loader has already loaded it, which is why you should remove the code mentioned above.

---

Edit:

If you want to access the methods and variables in the main class, you can do so by declaring a field variable of the main class in your GUI class and assigning or initializing its value at the constructor, like this:

public class GUI {

    BanGUIMain plugin;
    
    GUI(BanGUIMain plugin) {
        this.plugin = plugin;
    }
}

Then, if you want to use it in your main class, for example, you would do this:

GUI gui = new GUI(this);

But in your code, you used GUI in the OpenGui class, so you would also need to add a constructor in the OpenGui class:

public class OpenGui implements CommandExecutor {

    BanGUIMain plugin;

    OpenGui(BanGUIMain plugin) {
        this.plugin = plugin;
    }

    /* few lines skipped */

    GUI gui = new GUI(plugin);

}