Added StorageFolder#getOrDefault and some cleanup

src/main/java/net/minestom/server/storage/StorageFolder.java

@@ -53,6 +53,7 @@ public class StorageFolder {
         DataType<T> dataType = DATA_MANAGER.getDataType(type);
         if (dataType == null)
             throw new NullPointerException("You can only save registered DataType type!");
+
         PacketWriter packetWriter = new PacketWriter();
         dataType.encode(packetWriter, object); // Encode
         byte[] encodedValue = packetWriter.toByteArray(); // Retrieve bytes
@@ -65,12 +66,21 @@ public class StorageFolder {
         if (dataType == null)
             throw new NullPointerException("You can only save registered DataType type!");
 
-        ByteBuf buffer = Unpooled.wrappedBuffer(get(key));
+        byte[] data = get(key);
+        if (data == null)
+            return null;
+
+        ByteBuf buffer = Unpooled.wrappedBuffer(data);
         PacketReader packetReader = new PacketReader(buffer);
         T value = dataType.decode(packetReader);
         return value;
     }
 
+    public <T> T getOrDefault(String key, Class<T> type, T defaultValue) {
+        T value;
+        return (value = get(key, type)) != null ? value : defaultValue;
+    }
+
     public void getAndCloneData(String key, DataContainer dataContainer) {
         synchronized (cachedData) {
 

src/main/java/net/minestom/server/storage/StorageManager.java

@@ -52,7 +52,7 @@ public class StorageManager {
 
     public void defineDefaultStorageSystem(Supplier<StorageSystem> storageSystemSupplier) {
         if (this.defaultStorageSystemSupplier != null) {
-            LOGGER.error("The default storage-system has been changed. This could lead to issues!");
+            LOGGER.warn("The default storage-system has been changed. This could lead to issues!");
         }
         this.defaultStorageSystemSupplier = storageSystemSupplier;
     }

src/main/java/net/minestom/server/storage/StorageSystem.java

@@ -2,14 +2,37 @@ package net.minestom.server.storage;
 
 public interface StorageSystem {
 
+    /**
+     * Called when a foler is opened with this StorageSystem
+     *
+     * @param folderName the name of the folder
+     */
     void open(String folderName);
 
+    /**
+     * @param key
+     * @return the retrieved data
+     */
     byte[] get(String key);
 
+    /**
+     * Set the specified data to the defined key
+     *
+     * @param key
+     * @param data
+     */
     void set(String key, byte[] data);
 
+    /**
+     * Delete the specified key from the database
+     *
+     * @param key
+     */
     void delete(String key);
 
+    /**
+     * Called when the folder is closed, generally during server shutdown
+     */
     void close();
 
 }