In the previous tutorial, we have created a block entity. Now we try to modify the block entity data. The DemoBlockEntity class should be written like this:

DemoBlockEntity.class

public class DemoBlockEntity extends BlockEntity {
 
    public int number = 0;
 
    public DemoBlockEntity(BlockPos pos, BlockState state) {
        super(ExampleMod.DEMO_BLOCK_ENTITY, pos, state);
    }
 
    @Override
    public void writeNbt(NbtCompound nbt, RegistryWrapper.WrapperLookup registryLookup) {
        nbt.putInt("number", number);
 
        super.writeNbt(nbt, registryLookup);
    }
 
    @Override
    public void readNbt(NbtCompound nbt, RegistryWrapper.WrapperLookup registryLookup) {
        super.readNbt(nbt);
 
        number = nbt.getInt("number", registryLookup);
    }
}

Make sure the number field is public, as we are going to change it in the DemoBlock class. You could also make getter and setter methods, but we just expose the field here for simplicity..

This gets the BlockEntity at the right-clicked the block's position and if it's of the type DemoBlockEntity, increments its number field and sends a chat message to the player.

DemoBlock.class

public class DemoBlock extends BlockWithEntity {
 
    [...]
 
    @Override
    public ActionResult onUse(BlockState state, World world, BlockPos pos, PlayerEntity player, BlockHitResult hit) {
        if (!world.isClient){
            BlockEntity blockEntity = world.getBlockEntity(pos);
            if (blockEntity instanceof DemoBlockEntity demoBlockEntity) {
                demoBlockEntity.number++;
                player.sendMessage(Text.literal("Number is " + demoBlockEntity.number));
 
                return ActionResult.SUCCESS;
            }
        }
 
        return ActionResult.PASS;
    }
}