done (untested code)

src/main/java/rip/iwakura/civilcore/PlayerHandler.java

@@ -15,7 +15,7 @@ import rip.iwakura.civilcore.types.DbPlayer;
 
 public class PlayerHandler implements Listener {
 
-  private CivilCore civilCore = null;
+  private final CivilCore civilCore;
 
   public PlayerHandler(CivilCore civilCore) {
     this.civilCore = civilCore;
@@ -33,7 +33,7 @@ public class PlayerHandler implements Listener {
     String prefix = (inTeamChat ? ChatColor.DARK_BLUE + "TC " + ChatColor.RESET : "") 
       + (dbPlayer.team.prefix == null ?
         "" : 
-        ChatColor.translateAlternateColorCodes('&', dbPlayer.team.prefix) + " ");
+        ChatColor.translateAlternateColorCodes('&', "[" + dbPlayer.team.prefix + "]") + " ");
     String message = String.format("%s%s: %s", prefix, player_name, e.getMessage()); 
 
     if (!inTeamChat) {

src/main/java/rip/iwakura/civilcore/commands/TeamChatCommand.java

@@ -1,61 +1,39 @@
 package rip.iwakura.civilcore.commands;
 
-import java.sql.SQLException;
-
 import org.bukkit.Bukkit;
 import org.bukkit.command.Command;
 import org.bukkit.command.CommandExecutor;
 import org.bukkit.command.CommandSender;
 import org.bukkit.entity.Player;
-import org.bukkit.event.player.PlayerEvent;
 
 import net.md_5.bungee.api.ChatColor;
+import org.jetbrains.annotations.NotNull;
 import rip.iwakura.civilcore.Utils;
 import rip.iwakura.civilcore.CivilCore;
-import rip.iwakura.civilcore.types.DbPlayer;
+
-import rip.iwakura.civilcore.types.Team;
+import java.util.Objects;
 
 public class TeamChatCommand implements CommandExecutor {
-  private CivilCore civilCore = null;
+  private final CivilCore civilCore;
 
   public TeamChatCommand(CivilCore civilCore) {
     this.civilCore = civilCore;
   }
 
   @Override
-  public boolean onCommand(CommandSender sender, Command command, String label, String[] args) {
+  public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, String[] args) {
-    args = Utils.parser(args).toArray(new String[0]);
+    args = Objects.requireNonNull(Utils.parser(args)).toArray(new String[0]);
-
-    if (!(sender instanceof Player)) return true;
-
-    Player p = (Player) sender;
-
-    /*if (civilCore.teamChatRegister.contains(p)) {
-      civilCore.teamChatRegister.remove(p);
-      p.sendMessage(ChatColor.GREEN + "You are now in the " + ChatColor.GOLD + "GLOBAL" + ChatColor.GREEN + " channel");
-      return true;
-    }
-
-    if (civilCore.db.getPlayerByName(p.getName()).team.name == null) {
-      p.sendMessage(ChatColor.RED + "You aren't in a team :(");
-      return true;
-    }
-    civilCore.teamChatRegister.add(p);
-    p.sendMessage(ChatColor.GREEN + "You are now in the " + ChatColor.GOLD + "TEAM" + ChatColor.GREEN + " channel");*/
-
-    // /c [msg] -> if team chat toggled: broadcast to global chat, if global chat toggled send to team
-    // /tc or /c -> switch between global chat & team chat
 
+    if (!(sender instanceof Player p)) return true;
     if (args.length == 0) {
       if (civilCore.teamChatRegister.contains(p)) {
         civilCore.teamChatRegister.remove(p);
         p.sendMessage(ChatColor.GREEN + "You are now in the " + ChatColor.GOLD + "GLOBAL" + ChatColor.GREEN + " channel");
-        return true;
       } else {
         civilCore.teamChatRegister.add(p);
         p.sendMessage(ChatColor.GREEN + "You are now in the " + ChatColor.GOLD + "TEAM" + ChatColor.GREEN + " channel");
-        return true;
       }
+        return true;
     }
 
     String message = String.join(" ", args);