Cool downs.java

// Imports, I just wrote this up quick in Gist

public class Cooldowns extends JavaPlugin {
  
  private Map<UUID, Long> cooldowns = new HashMap<>();
  private long cooldownDurationMillis = 3000; // 3 seconds (1000 millis = 1 second)
  
  @Override public void onEnable() {
    
  }
  
  @Override public void onCommand(CommandSender sender, Command command, String label, String[] args) {
    if(!(sender instanceof Player)) {
      sender.sendMessage("You must be a player to do this."); 
      return;
    }
    
    UUID senderUid = ((Player) sender).getUniqueId();
    
    if(command.getName().equalsIgnoreCase("cooldown")) {
      long cooldownStartTime = cooldowns.get(senderUid);
      if(!cooldowns.containsKey(senderUid) || System.currentTimeMillis() >= cooldownStartTime) {
        // The cooldown has expired
        sender.sendMessage("You won't be able to use this command for another 3 seconds."); 
        cooldowns.put(senderUid, System.currentTimeMillis());
        return;
      }
      sender.sendMessage("Still cooling down!");
    }
  }
  
}