.

core-bukkit-api/src/main/java/de/kentoj/scrow/bukkit/ScrowAPI.java

@@ -3,14 +3,13 @@ package de.kentoj.scrow.bukkit;
 import com.mongodb.reactivestreams.client.MongoDatabase;
 import de.kentoj.scrow.bukkit.services.EconomyService;
 import de.kentoj.scrow.bukkit.services.command.CommandManager;
+import de.kentoj.scrow.bukkit.services.friends.FriendsService;
 import lombok.AccessLevel;
 import lombok.Getter;
 import lombok.NoArgsConstructor;
-import org.bukkit.Bukkit;
 import org.jetbrains.annotations.ApiStatus;
 import org.jetbrains.annotations.Nullable;
 import reactor.core.scheduler.Scheduler;
-import reactor.core.scheduler.Schedulers;
 
 @NoArgsConstructor(access = AccessLevel.PRIVATE)
 public class ScrowAPI {
@@ -18,11 +17,13 @@ public class ScrowAPI {
     @Getter
     private static @Nullable MongoDatabase database;
     @Getter
-    private static EconomyService economyService;
-    @Getter
     private static CommandManager commandManager;
     @Getter
     private static Scheduler minecraftScheduler;
+    @Getter
+    private static EconomyService economyService;
+    @Getter
+    private static FriendsService friendsService;
 
     @ApiStatus.Internal
     public static void setDatabase(@Nullable MongoDatabase database) {
@@ -43,5 +44,10 @@ public class ScrowAPI {
     public static void setMinecraftScheduler(Scheduler minecraftScheduler) {
         ScrowAPI.minecraftScheduler = minecraftScheduler;
     }
+
+    @ApiStatus.Internal
+    public static void setFriendsService(FriendsService friendsService) {
+        ScrowAPI.friendsService = friendsService;
+    }
 }
 

core-bukkit-api/src/main/java/de/kentoj/scrow/bukkit/services/command/execution/CommandExecutor.java

@@ -1,6 +1,8 @@
 package de.kentoj.scrow.bukkit.services.command.execution;
 
+import de.kentoj.scrow.bukkit.services.command.exception.CommandInvocationException;
+
 public interface CommandExecutor {
 
-    void execute(CommandContext ctx);
+    void execute(CommandContext ctx) throws CommandInvocationException;
 }

core-bukkit-api/src/main/java/de/kentoj/scrow/bukkit/services/command/type/PlayerArgumentType.java

@@ -57,7 +57,7 @@ public class PlayerArgumentType {
                 try {
                     player = Bukkit.getOfflinePlayer(UUID.fromString(rawInput));
                 } catch (IllegalArgumentException ex) {
-                    throw new CommandInvocationException("Not a UUID or name of an online player");
+                    throw new CommandInvocationException("Not a UUID or name of an online player -- " + rawInput);
                 }
             }
             return player;
@@ -75,6 +75,7 @@ public class PlayerArgumentType {
 
         @Override
         public void checkValue(CommandContext ctx, OfflinePlayer value, String rawInput) throws CommandInvocationException {
+            if (!value.hasPlayedBefore()) throw new CommandInvocationException("Player has never played before -- " + rawInput);
         }
     };
 

core-bukkit-api/src/main/java/de/kentoj/scrow/bukkit/services/friends/FriendRequestUsageException.java

@@ -0,0 +1,12 @@
+package de.kentoj.scrow.bukkit.services.friends;
+
+public class FriendRequestUsageException extends RuntimeException {
+
+    public FriendRequestUsageException(String message) {
+        super(message);
+    }
+
+    public FriendRequestUsageException(String message, Throwable cause) {
+        super(message, cause);
+    }
+}

core-bukkit-api/src/main/java/de/kentoj/scrow/bukkit/services/friends/FriendsService.java

@@ -0,0 +1,33 @@
+package de.kentoj.scrow.bukkit.services.friends;
+
+import org.jetbrains.annotations.NotNull;
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
+
+import java.util.UUID;
+
+public interface FriendsService {
+
+    Flux<@NotNull Friendship> getFriendships(UUID playerId);
+
+    /**
+     * @param initiator id of player who wished to end the friendship
+     * @param other     id of the player the initiator wants to end the friendship with
+     * @throws FriendRequestUsageException
+     */
+    Mono<@NotNull Boolean> removeFriend(UUID initiator, UUID other);
+
+    /**
+     * @param playerId
+     */
+    Flux<@NotNull FriendRequest> getIncomingFriendRequests(UUID playerId);
+
+    Flux<@NotNull FriendRequest> getOutgoingFriendRequests(UUID playerId);
+
+    /**
+     * @param from id of player sending the friend request
+     * @param to id of the player the initiator wants to befriend
+     * @throws FriendRequestUsageException
+     */
+    Mono<@NotNull Friendship> sendFriendRequest(UUID from, UUID to);
+}

core-bukkit-api/src/main/java/de/kentoj/scrow/bukkit/services/friends/Friendship.java

@@ -8,4 +8,9 @@ import java.util.UUID;
 public interface Friendship {
     Tuple2<UUID, UUID> getPlayerIds();
     Instant getCreatedAt();
+
+    default UUID getOther(UUID self) {
+        final var ids = getPlayerIds();
+        return ids.getFirst() != self ? ids.getFirst() : ids.getSecond();
+    }
 }