class DataKeyBase<T> implements MutableDataValueSetter<T> {
    final private @NotNull String name;
    final private @NotNull DataValueFactory<T> factory;
    final private T defaultValue;

    /**
     * Creates a NullableDataKey with a computed default value and a provided default value when data holder is null.
     * <p>
     * Use this constructor to ensure that factory is never called with null data holder value
     *
     * @param name         See {@link #getName()}.
     * @param defaultValue default to use when data holder is null
     * @param factory      data value factory for creating a new default value for the key for a non-null data holder
     */
    public DataKeyBase(@NotNull String name, T defaultValue, @NotNull DataValueFactory<T> factory) {
        this.name = name;
        this.defaultValue = defaultValue;
        this.factory = factory;
    }

    /**
     * Creates a NullableDataKey with a dynamic default value taken from a value of another key
     * <p>
     * does not cache the returned default value but will always delegate to another key until this key
     * gets its own value set.
     *
     * @param name       See {@link #getName()}.
     * @param defaultKey The NullableDataKey to take the default value from at time of construction.
     */
    public DataKeyBase(@NotNull String name, @NotNull DataKeyBase<T> defaultKey) {
        this(name, defaultKey.defaultValue, defaultKey::get);
    }

    public DataKeyBase(@NotNull String name, T defaultValue) {
        this(name, defaultValue, options -> defaultValue);
    }

    @NotNull
    public String getName() {
        return name;
    }

    @NotNull
    public DataValueFactory<T> getFactory() {
        return factory;
    }

    public T getDefaultValue() {
        return defaultValue;
    }

    public T getDefaultValue(@NotNull DataHolder holder) {
        return factory.apply(holder);
    }

    public T get(@Nullable DataHolder holder) {
        //noinspection unchecked
        return holder == null ? defaultValue : (T) holder.getOrCompute(this, this::getDefaultValue);
    }

    /**
     * @param holder data holder
     * @return return default value if holder is null, current value in holder or compute a new value
     * @deprecated use get
     */
    @Deprecated
    final public T getFrom(@Nullable DataHolder holder) {
        return get(holder);
    }

    @Override
    public String toString() {<FILL_FUNCTION_BODY>}

    /**
     * Compare only by address. Every key instance is unique
     *
     * @param o other
     * @return true if equal
     */
    @Override
    final public boolean equals(Object o) {
        return this == o;
    }

    @Override
    final public int hashCode() {
        return super.hashCode();
    }
}

        if (defaultValue != null) {
            return "NullableDataKey<" + defaultValue.getClass().getSimpleName() + "> " + name;
        } else {
            return "NullableDataKey<unknown> " + name;
        }
    
<no_super_class>