PermissionHandlers: add default methods for trivial duplications

- Add easy default methods on PermissionHandler interface (override whenever there's a better way!)
- Change getGroups() signature to return a Collection instead of a List

src/main/java/fr/xephi/authme/permission/PermissionsManager.java

@@ -19,8 +19,8 @@ import org.bukkit.plugin.PluginManager;
 
 import javax.annotation.PostConstruct;
 import javax.inject.Inject;
-import java.util.ArrayList;
-import java.util.List;
+import java.util.Collection;
+import java.util.Collections;
 
 /**
  * <p>
@@ -255,12 +255,12 @@ public class PermissionsManager implements Reloadable {
      *
      * @param player The player.
      *
-     * @return Permission groups, or an empty list if this feature is not supported.
+     * @return Permission groups, or an empty collection if this feature is not supported.
      */
-    public List<String> getGroups(Player player) {
+    public Collection<String> getGroups(Player player) {
         // If no permissions system is used, return an empty list
         if (!isEnabled())
-            return new ArrayList<>();
+            return Collections.emptyList();
 
         return handler.getGroups(player);
     }
@@ -389,7 +389,7 @@ public class PermissionsManager implements Reloadable {
             return false;
 
         // Get a list of current groups
-        List<String> groupNames = getGroups(player);
+        Collection<String> groupNames = getGroups(player);
 
         // Remove each group
         return removeGroups(player, groupNames.toArray(new String[groupNames.size()]));

src/main/java/fr/xephi/authme/permission/handlers/BPermissionsHandler.java

@@ -9,6 +9,12 @@ import org.bukkit.entity.Player;
 import java.util.Arrays;
 import java.util.List;
 
+/**
+ * Handler for bPermissions.
+ *
+ * @see <a href="https://dev.bukkit.org/projects/bpermissions">bPermissions Bukkit page</a>
+ * @see <a href="https://github.com/rymate1234/bPermissions/">bPermissions on Github</a>
+ */
 public class BPermissionsHandler implements PermissionHandler {
 
     @Override
@@ -49,19 +55,6 @@ public class BPermissionsHandler implements PermissionHandler {
         return Arrays.asList(ApiLayer.getGroups(player.getWorld().getName(), CalculableType.USER, player.getName()));
     }
 
-    @Override
-    public String getPrimaryGroup(Player player) {
-        // Get the groups of the player
-        List<String> groups = getGroups(player);
-
-        // Make sure there is any group available, or return null
-        if (groups.isEmpty())
-            return null;
-
-        // Return the first group
-        return groups.get(0);
-    }
-
     @Override
     public PermissionsSystemType getPermissionSystem() {
         return PermissionsSystemType.B_PERMISSIONS;

src/main/java/fr/xephi/authme/permission/handlers/PermissionHandler.java

@@ -2,9 +2,10 @@ package fr.xephi.authme.permission.handlers;
 
 import fr.xephi.authme.permission.PermissionNode;
 import fr.xephi.authme.permission.PermissionsSystemType;
+import fr.xephi.authme.util.Utils;
 import org.bukkit.entity.Player;
 
-import java.util.List;
+import java.util.Collection;
 
 
 public interface PermissionHandler {
@@ -48,7 +49,9 @@ public interface PermissionHandler {
      * @return True if the player is in the specified group, false otherwise.
      * False is also returned if groups aren't supported by the used permissions system.
      */
-    boolean isInGroup(Player player, String group);
+    default boolean isInGroup(Player player, String group) {
+        return getGroups(player).contains(group);
+    }
 
     /**
      * Remove the permission group of a player, if supported.
@@ -80,7 +83,7 @@ public interface PermissionHandler {
      *
      * @return Permission groups, or an empty list if this feature is not supported.
      */
-    List<String> getGroups(Player player);
+    Collection<String> getGroups(Player player);
 
     /**
      * Get the primary group of a player, if available.
@@ -89,7 +92,13 @@ public interface PermissionHandler {
      *
      * @return The name of the primary permission group. Or null.
      */
-    String getPrimaryGroup(Player player);
+    default String getPrimaryGroup(Player player) {
+        Collection<String> groups = getGroups(player);
+        if (Utils.isCollectionEmpty(groups)) {
+            return null;
+        }
+        return groups.iterator().next();
+    }
 
     /**
      * Get the permission system that is being used.

src/main/java/fr/xephi/authme/permission/handlers/PermissionsBukkitHandler.java

@@ -12,6 +12,11 @@ import org.bukkit.plugin.PluginManager;
 import java.util.ArrayList;
 import java.util.List;
 
+/**
+ * Handler for PermissionsBukkit.
+ *
+ * @see <a href="https://dev.bukkit.org/projects/permbukkit">PermissionsBukkit Bukkit page</a>
+ */
 public class PermissionsBukkitHandler implements PermissionHandler {
 
     private PermissionsPlugin permissionsBukkitInstance;
@@ -39,13 +44,6 @@ public class PermissionsBukkitHandler implements PermissionHandler {
         return false;
     }
 
-    @Override
-    public boolean isInGroup(Player player, String group) {
-        List<String> groupNames = getGroups(player);
-
-        return groupNames.contains(group);
-    }
-
     @Override
     public boolean removeFromGroup(Player player, String group) {
         return Bukkit.dispatchCommand(Bukkit.getConsoleSender(), "permissions player removegroup " + player.getName() + " " + group);
@@ -65,20 +63,6 @@ public class PermissionsBukkitHandler implements PermissionHandler {
         return groups;
     }
 
-    @Override
-    public String getPrimaryGroup(Player player) {
-        // Get the groups of the player
-        List<String> groups = getGroups(player);
-
-        // Make sure there is any group available, or return null
-        if (groups.isEmpty()) {
-            return null;
-        }
-
-        // Return the first group
-        return groups.get(0);
-    }
-
     @Override
     public PermissionsSystemType getPermissionSystem() {
         return PermissionsSystemType.PERMISSIONS_BUKKIT;

src/main/java/fr/xephi/authme/permission/handlers/PermissionsExHandler.java

@@ -10,6 +10,12 @@ import ru.tehkode.permissions.bukkit.PermissionsEx;
 import java.util.ArrayList;
 import java.util.List;
 
+/**
+ * Handler for PermissionsEx.
+ *
+ * @see <a href="https://dev.bukkit.org/projects/permissionsex">PermissionsEx Bukkit page</a>
+ * @see <a href="https://github.com/PEXPlugins/PermissionsEx">PermissionsEx on Github</a>
+ */
 public class PermissionsExHandler implements PermissionHandler {
 
     private PermissionManager permissionManager;
@@ -72,17 +78,6 @@ public class PermissionsExHandler implements PermissionHandler {
         return user.getParentIdentifiers(null);
     }
 
-    @Override
-    public String getPrimaryGroup(Player player) {
-        PermissionUser user = permissionManager.getUser(player);
-
-        List<String> groups = user.getParentIdentifiers(null);
-        if (groups.isEmpty())
-            return null;
-
-        return groups.get(0);
-    }
-
     @Override
     public PermissionsSystemType getPermissionSystem() {
         return PermissionsSystemType.PERMISSIONS_EX;

src/main/java/fr/xephi/authme/permission/handlers/VaultHandler.java

@@ -10,6 +10,12 @@ import org.bukkit.plugin.RegisteredServiceProvider;
 import java.util.Arrays;
 import java.util.List;
 
+/**
+ * Handler for permissions via Vault.
+ *
+ * @see <a href="https://dev.bukkit.org/projects/vault">Vault Bukkit page</a>
+ * @see <a href="https://github.com/milkbowl/Vault">Vault on Github</a>
+ */
 public class VaultHandler implements PermissionHandler {
 
     private Permission vaultProvider;

src/main/java/fr/xephi/authme/permission/handlers/ZPermissionsHandler.java

@@ -6,10 +6,15 @@ import org.bukkit.Bukkit;
 import org.bukkit.entity.Player;
 import org.tyrannyofheaven.bukkit.zPermissions.ZPermissionsService;
 
-import java.util.ArrayList;
-import java.util.List;
+import java.util.Collection;
 import java.util.Map;
 
+/**
+ * Handler for zPermissions.
+ *
+ * @see <a href="https://dev.bukkit.org/projects/zpermissions">zPermissions Bukkit page</a>
+ * @see <a href="https://github.com/ZerothAngel/zPermissions">zPermissions on Github</a>
+ */
 public class ZPermissionsHandler implements PermissionHandler {
 
     private ZPermissionsService zPermissionsService;
@@ -42,11 +47,6 @@ public class ZPermissionsHandler implements PermissionHandler {
             return false;
     }
 
-    @Override
-    public boolean isInGroup(Player player, String group) {
-        return getGroups(player).contains(group);
-    }
-
     @Override
     public boolean removeFromGroup(Player player, String group) {
         return Bukkit.dispatchCommand(Bukkit.getConsoleSender(), "permissions player " + player.getName() + " removegroup " + group);
@@ -58,9 +58,9 @@ public class ZPermissionsHandler implements PermissionHandler {
     }
 
     @Override
-    public List<String> getGroups(Player player) {
+    public Collection<String> getGroups(Player player) {
         // TODO Gnat008 20160631: Use UUID not name?
-        return new ArrayList<String>(zPermissionsService.getPlayerGroups(player.getName()));
+        return zPermissionsService.getPlayerGroups(player.getName());
     }
 
     @Override