public class TestPlugin extends JavaPlugin {

    private static TestConfig testConfig;

    @Override
    public void onEnable() {
        testConfig = ConfigAPI.init(
            TestConfig.class, // Class of config's interface
            NameStyle.UNDERSCORE, // Style of fields' name in YAML file
            CommentStyle.INLINE, // Style of comments in YAML file
            true, // Automatic translation of '&' based colors
            this // Instance of plugin
        );

        // You can simply access data from the config by getters
        System.out.println(testConfig.getExampleMessage());
        Bukkit.getPlayer("mikigal").getInventory().addItem(testConfig.getAward());

        // After calling setter new data are automatically saved to file
        testConfig.setAward(new ItemStack(Material.DIRT));

        // If you want to do something manually you can access instance of YamlConfiguration
        testConfig.getBukkitConfiguration();
    }

    public static TestConfig getTestConfig() {
        return testConfig;
    }
}

@ConfigName("test.yml") // Name of YAML file
public interface TestConfig extends Config {

    @Comment("This comment will be saved to YAML file!")
    default String getExampleMessage() {
        // Getter method should return default value of the field
        return "&cIt's default value of example message";
    }

    default ItemStack getAward() {
        return Item.of(Material.DIAMOND_SWORD)
            .name("&cAward")
            .lore("&aFirst line", "&cSecond line")
            .toItem();
    }

    public void setAward(ItemStack award);

    // Key of Map must be String
    default Map<String, Integer> getValues() {
        Map<String, Integer> map = new HashMap<>();
        map.put("a", 1);
        map.put("b", 2);

        return map;
    }

    default List<Location> getSpawnPoints() {
        World world = Bukkit.getWorld("world");
        return Arrays.asList(
            new Location(world, 0, 100, 0),
            new Location(world, 10, 90, 10, 90f, 0f),
            new Location(world, 20, 80, 20, 0f, 180f)
        );
    }
}

public class User implements Serializable { // It must implement Serializable interface
    private String username;
    private int kills;
    private transient String temporary; // Transient fields will not be serialized!

    public User() { // It MUST have no-args constructor!

    }

    public User(String username, int kills, String temporary) {
        this.username = username;
        this.kills = kills;
        this.temporary = temporary;
    }

    // Getters and setters...
}

@ConfigName("config")
public interface MyConfig extends Config {

    void setUser(User user);

    default User getTest() {
        return new User("mikigal", 1, "some text");
    }
}

public class PotionEffectSerializer extends Serializer<PotionEffect> {

    @Override
    protected void saveObject(String path, PotionEffect object, BukkitConfiguration configuration) {
        // In saveObject() method you have to set data of object to config. You can use set() method to set another object which need serialization too
        configuration.set(path + ".type", object.getType().getName());
        configuration.set(path + ".duration", object.getDuration());
        configuration.set(path + ".amplifier", object.getAmplifier());
    }

    @Override
    public PotionEffect deserialize(String path, BukkitConfiguration configuration) {
        // In deserialize() method you have to load data from config and return instance of object
        PotionEffectType type = PotionEffectType.getByName(configuration.getString(path + ".type"));
        int duration = configuration.getInt(path + ".duration");
        int amplifier = configuration.getInt(path + ".amplifier");

        if (type == null) {
            throw new InvalidConfigFileException("Invalid PotionEffect type (path: " + path + ")");
        }

        return new PotionEffect(type, duration, amplifier);
    }
}

public class TestPlugin extends JavaPlugin {

    @Override
    public void onEnable() {
        // Remember to register you Serializer before use!
        ConfigAPI.registerSerializer(PotionEffect.class, new PotionEffectSerializer());

        // Init your configs...

    }
}