Getting Started with Capabilities: a practical example

MyCapability.java

public class MyCapability {

    public static final Capability<MyCapabilityInterface> INSTANCE = CapabilityManager.get(new CapabilityToken<>() {});

    public static void register(RegisterCapabilitiesEvent event) {
        event.register(MyCapabilityInterface.class);
    }

    private MyCapability() {
    }
}

MyCapabilityAttacher.java

public class MyCapabilityAttacher {

    private static class MyCapabilityProvider implements ICapabilityProvider, INBTSerializable<CompoundTag> {

        public static final ResourceLocation IDENTIFIER = new ResourceLocation("mod_id", "myCap");

        private final MyCapabilityInterface backend = new MyCapabilityImplementation();
        private final LazyOptional<MyCapabilityInterface> optionalData = LazyOptional.of(() -> backend);

        @NotNull
        @Override
        public <T> LazyOptional<T> getCapability(@NotNull Capability<T> cap, @Nullable Direction side) {
            return MyCapability.INSTANCE.orEmpty(cap, this.optionalData);
        }

        void invalidate() {
            this.optionalData.invalidate();
        }

        @Override
        public CompoundTag serializeNBT() {
            return this.backend.serializeNBT();
        }

        @Override
        public void deserializeNBT(CompoundTag nbt) {
            this.backend.deserializeNBT(nbt);
        }
    }

    public static void attach(final AttachCapabilitiesEvent<Entity> event) {
        final MyCapabilityProvider provider = new MyCapabilityProvider();

        event.addCapability(MyCapabilityProvider.IDENTIFIER, provider);
    }

    private MyCapabilityAttacher() {
    }
}

MyCapabilityImplementation.java

public class MyCapabilityImplementation implements MyCapabilityInterface {

    private static final String NBT_KEY_DAMAGE_DEALT = "damageDealt";

    private String myValue = "";

    @Override
    public String getValue() {
        return this.myValue;
    }

    @Override
    public void setMyValue(String myValue) {
        this.myValue = myValue;
    }

    @Override
    public CompoundTag serializeNBT() {
        final CompoundTag tag = new CompoundTag();
        tag.putString(NBT_KEY_DAMAGE_DEALT, this.myValue);
        return tag;
    }

    @Override
    public void deserializeNBT(CompoundTag nbt) {
        this.myValue = nbt.getString(NBT_KEY_DAMAGE_DEALT);
    }
}

MyCapabilityInterface.java

public interface MyCapabilityInterface extends INBTSerializable<CompoundTag> {

    String getValue();

    void setMyValue(String myValue);
}