---
title: "NPA — Number of Public Attributes"
description: "Number of public attributes declared on classes and interfaces."
keywords: ["npa", "public attributes", "encapsulation"]
---

**NPA** (Number of Public Attributes) counts public fields/properties exposed by classes and interfaces
in a file. A high NPA usually signals weak encapsulation — internal state leaking outside the type.

## What mehen emits

| Key | Type | Description |
|---|---|---|
| `npa` | int | Total public attributes across classes and interfaces. |
| `npa.classes` | int | NPA contributed by classes. |
| `npa.interfaces` | int | NPA contributed by interfaces. |
| `npa.class_attributes` | int | Same as `npa.classes` (alias). |
| `npa.interface_attributes` | int | Same as `npa.interfaces` (alias). |
| `npa.classes_average` | float | Mean attributes per class. |
| `npa.interfaces_average` | float | Mean attributes per interface. |
| `npa.total_attributes` | int | Total NPA in the file (alias of `npa`). |
| `npa.average` | float | Mean attributes per declaration (class or interface). |

## What counts as "public"

The exact rule is language-specific:

| Language | Public attribute is… |
|---|---|
| Java / Kotlin | A field declared `public` (or with no modifier in Kotlin classes). |
| TypeScript / JS | A class field without `private` / `protected` / `#` prefix. |
| Python | A class attribute that does not start with `_`. |
| Ruby | An `attr_accessor` / `attr_reader` / `attr_writer` declaration (Ruby fields are private by
default). |
| C# | A field, constant, or event declared `public`. A member with no access modifier is `private`, and `internal` is assembly-scoped, so neither counts. `enum` members are always public. |

## How to read it

A "few" public attributes is usually fine. Many can mean:

- Data classes / records: `NPA = number of fields` is expected and benign.
- Property bags or "god objects": NPA grows past 10 — refactor candidate.

Read NPA alongside [NPM](/metrics/code/npm). A class with NPA much greater than NPM is a struct in
disguise; a class with NPM much greater than NPA is encapsulating state behind methods.

## References

- Lorenz, M. & Kidd, J. (1994). *Object-Oriented Software Metrics: A Practical Guide.* Prentice Hall.
- Chidamber, S. R. & Kemerer, C. F. (1994). *A Metrics Suite for Object Oriented Design.* IEEE TSE
  20(6): 476–493. [DOI](https://doi.org/10.1109/32.295895).
- Briand, L. C., Daly, J. W. & Wüst, J. K. (1998). *A Unified Framework for Coupling Measurement in
  Object-Oriented Systems.* IEEE TSE 25(1): 91–121.
  [DOI](https://doi.org/10.1109/32.748920).

## See also

- [NPM](/metrics/code/npm) — number of public methods.
- [NOM](/metrics/code/nom) — number of methods (public + private).
