Example

public class ClassName implements CommandExecutor {


    @Override
    public boolean onCommand(CommandSender sender, Command cmd, String label, String[] args) {
        if (!(sender instanceof Player)) return false;

        Player p = (Player) sender;

        if(args.length == 1){
                Player target = Bukkit.getPlayer(args[0]);
                if(target == null){
                    p.sendMessage(ChatUtil.getPrefix()+OFFLINE_PLAYER);
                } else {

                    ItemStack item = new ItemStack(Material.DIAMOND, 1);
                    target.getInventory().addItem(item);
                    target.updateInventory();
                }
            }
        return true;
    }


}