package fr.minuskube.inv.content;

import org.bukkit.entity.Player;

public interface InventoryProvider {

    void init(Player player, InventoryContents contents);
    void update(Player player, InventoryContents contents);

}

package fr.minuskube.inv.content;

import org.bukkit.entity.Player;

public interface InventoryProvider {

    void init(Player player, InventoryContents contents);
    default void update(Player player, InventoryContents contents) {}

}

Make InventoryProvider#update(Player, InventoryContents) a default method (#85)