public class DemoBlockEntity extends BlockEntity {
 
    public int number = 0;
 
    public DemoBlockEntity(BlockPos pos, BlockState state) {
        super(TutorialBlockEntityTypes.DEMO_BLOCK, pos, state);
    }
 
    @Override
    public void writeNbt(NbtCompound nbt, RegistryWrapper.WrapperLookup registries) {
        nbt.putInt("number", number);
 
        super.writeNbt(nbt, registries);
    }
    
    @Override
    public void readNbt(NbtCompound nbt, RegistryWrapper.WrapperLookup registries) {
        super.readNbt(nbt, registries);
 
        // for versions before 1.21.5, use nbt.getInt("number");
        number = nbt.getInt("number", 0);
    }
}