You will need to register most content you add to the game. This helps with:

• Letting the game know your content exists
• Verifying game content between client & server
• Handling invalid content in a save
• Preventing conflicts between different mods
• Compression for client ↔ server communication and data saving
• Abstracting or hiding numerical IDs

When registering any type of content, you pass in an Identifier, which is a label for your addition. Identifiers– often abbreviated as IDs– have a namespace and path. In most cases, the namespace is the ID of your mod, and the path is the name of the content you’re registering. For example, the standard dirt block has the ID of minecraft:dirt.

Using custom content without registering it can lead to buggy behavior, such as missing textures, world save issues, and crashes. The game will usually let you know if you forget to register something.

When registering content, you need to specify which registry you are adding content to. The base game provides registries for all vanilla content, which can be found in Registry. Two examples of registries you may use include Registry.ITEM for items and Registry.BLOCK for blocks.

For a deeper overview and description of all available registries, read the registry types page.

Use Registry.register for adding content to registries:

public static <T> T register(Registry<? super T> registry, Identifier id, T entry) {
    return ((MutableRegistry)registry).add(id, entry);
}

registry - an instance of the registry you want to add content to. A list of all vanilla registries, located in Registry, can be found in the registry types page.

id - an identifying label for your content inside the registry. Standard convention is modid:name, as seen with minecraft:dirt.

entry - an instance of the content you want to register.

getId - returns the Identifier associated with an entry inside a registry. If the entry doesn’t exist, DefaultedRegistry returns the default registry value, and SimpleRegistry returns null.

@Nullable
public abstract Identifier getId(T entry);

getRawId - returns the internal integer ID associated with an entry inside a registry. If the entry doesn’t exist, DefaultedRegistry returns the raw ID of the default registry value, and SimpleRegistry returns -1.

public abstract int getRawId(@Nullable T entry);