Every type of block in Minecraft is represented by a singular Block instance. This makes it impossible to change a specific block's state by simply changing the Block instance's state, as every other block of that type will be affected! But, what if you do want to give a singular block state, so it can change based on some condition? This is what BlockStates are for. Say we wanted a block to have a hardness of 0.5 normally, but if we right click it before we try to break it, it would become harder and gain a hardness of 2.

First we define the boolean property of the block - whether or not it is hard:

public class MyBlock extends Block {
    public static final BooleanProperty HARDENED = BooleanProperty.of("hardened");
}

Then we need to register the property by overriding appendProperties:

public class MyBlock extends Block {
    [...]
    @Override
    protected void appendProperties(StateFactory.Builder<Block, BlockState> stateFactory) {
        stateFactory.add(HARDENED);
    }
 
}

Then we need to set the default state of our property in the block constructor:

public class MyBlock extends Block {
    [...]
    public MyBlock(Settings settings) {
        super(settings);
        setDefaultState(getStateFactory().getDefaultState().with(HARDENED, false));
    }
 
}

Now, to set the property we need to call

world.setBlockState(<block-position>, <block-instance>.getDefaultState().with(<property-name>, <new-value>):

(Replace MyBlocks.MY_BLOCK_INSTANCE with your block's instance)

public class MyBlock extends Block {
    [...]
    @Override
    public boolean activate(BlockState state, World world, BlockPos pos, PlayerEntity player, Hand hand, BlockHitResult blockHitResult) {
        world.setBlockState(pos, MyBlocks.MY_BLOCK_INSTANCE.getDefaultState().with(HARDENED, true));
        return true;
    }
}

And to use the property we call blockState.get(<our-property-name>):

public class MyBlock extends Block {
    [...]
    @Override
    public float getHardness(BlockState blockState, BlockView blockView, BlockPos pos) {
        boolean hardened = blockState.get(HARDENED);
        if(hardened) return 2.0f;
        else return 0.5f;
    }
}

You can also make the texture and model of your block change based on the state. This is done through a JSON file called a Blockstate JSON. All blocks need a blockstate JSON, whether they have multiple states or not, but the contents of the JSON can be as simple or complex as you like. Let's say you register an instance of MyBlock to the ID mymod:my_block. Minecraft would look for a file at the location src/main/resources/assets/mymod/blockstates/my_block.json to load the state from. If you don't want your block to change models between states, the blockstate JSON can be very simple. It would look something like this:

{
    "variants": {
        "": { "model": "mymod:block/my_block" }
    }
}

Let's break this simple example down. There are a couple important parts to this JSON:

- The “variants” block will be where all possible variations for your blockstate go. We'll explore variants more in a little. - A variant named “” will apply to every permutation of a blockstate. If you have a “” variant, you shouldn't have any other variants in the JSON, or Minecraft will get upset. - The object assigned to the “” variant can have various properties added to it like rotation or texture manipulation. Check out the Minecraft wiki(https://minecraft.gamepedia.com/Model#Block_states) for more documentation on what properties can be added.

Every possible state of a block is registered at the start of the game. This means that if you have 14 boolean properties, the block has 2^14 = 16384 different states and 2^14 states are registered. For this reason blocks should not contain too many blockstate properties. Rather, blockstates should be mostly reserved for visuals, and Block Entities should be used for more advanced state.