# unit-test-utility-methods

Writes unit tests for utility classes and static methods, covering nulls, empty input, boundaries and floating-point precision.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 3700 installs · ★ 363
- Author: giuseppe-trisciuoglio
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/giuseppe-trisciuoglio/developer-kit/tree/HEAD/plugins/developer-kit-java/skills/unit-test-utility-methods/SKILL.md
- Updated: 2026-09-10
- HTML: https://affpapa.org/en/nevibecode/skills/unit-test-utility-methods

## What it does

The skill lays out a sequence for utility code: name the test class after the utility, cover the happy path, then edge cases — null, empty string, whitespace, a single element — and boundaries such as max values and precision. Test names read like `shouldCapitalizeFirstLetter` rather than `test1`. Assertions use AssertJ, and repeated inputs go through `@ParameterizedTest`.

Worked examples cover string helpers, math and percentage calculations, validators for email, URL and card numbers, and collection helpers. Pure functions need no mocks. The exception is a dependency like `Clock`, shown with `MockitoExtension` and `@Mock`.

The constraints are spelled out. Static methods are not mocked, and reflection is a last resort. Exact equality fails on floating point, so the skill calls for `isCloseTo` with a delta. Stateful utilities are harder to test, which is why the guidance leans on immutability.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add giuseppe-trisciuoglio/developer-kit --skill unit-test-utility-methods -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/unit-test-utility-methods/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/unit-test-utility-methods/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add giuseppe-trisciuoglio/developer-kit --skill unit-test-utility-methods -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/unit-test-utility-methods/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add giuseppe-trisciuoglio/developer-kit --skill unit-test-utility-methods -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/unit-test-utility-methods/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install giuseppe-trisciuoglio/developer-kit unit-test-utility-methods
```

2. **Manually: put the files in place**
- File: `.github/skills/unit-test-utility-methods/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add giuseppe-trisciuoglio/developer-kit --skill unit-test-utility-methods -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/giuseppe-trisciuoglio/developer-kit --path plugins/developer-kit-java/skills/unit-test-utility-methods
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/unit-test-utility-methods/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add giuseppe-trisciuoglio/developer-kit --skill unit-test-utility-methods -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/giuseppe-trisciuoglio/developer-kit/tree/HEAD/plugins/developer-kit-java/skills/unit-test-utility-methods/SKILL.md

## When to use it
- write tests for static methods on a utility class
- check how a helper behaves on null and empty input
- cover boundary values and numeric precision
- build a parameterized test over similar inputs
- test email, URL and card validators

## Example prompts
- Write tests for `StringUtils` covering `capitalize` and `isEmpty`
- Cover `MathUtils.percentage` and `MathUtils.round`, including division by zero
- Create a `@ParameterizedTest` for `ValidatorUtils.isValidEmail` over a set of addresses

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 04bdc3583c576244.

BEGIN THIRD-PARTY FILE 04bdc3583c576244
````text
---
name: unit-test-utility-methods
description: Provides patterns for testing utility classes, static methods, and helper functions. Validates pure functions, null handling, edge cases, and boundary conditions. Generates AssertJ assertions and @ParameterizedTest for string utils, math utils, validators, and collection helpers. Use when testing utils, test helpers, helper functions, static methods, or verifying utility code correctness.
allowed-tools: Read, Write, Bash, Glob, Grep
---

# Unit Testing Utility Classes and Static Methods

## Overview

This skill generates tests for utility classes with static helper methods and pure functions. It provides patterns for testing null handling, edge cases, boundary conditions, and common utilities like string manipulation, calculations, data validation, and collections. Pure functions require no mocking.

## When to Use

Use this skill when:
- Writing tests for utility/helper classes with static methods
- Testing pure functions with no state or side effects
- Testing string manipulation, formatting, or transformation utilities
- Testing calculation, conversion, or math helper functions
- Testing data validation and formatter utilities
- Verifying null/empty input handling in utility code
- Testing collections or array helper methods

## Instructions

1. **Create test class**: Name it after the utility (e.g., `StringUtilsTest`)
2. **Test happy path**: Valid inputs with expected outputs
3. **Test edge cases**: null, empty, whitespace, single elements
4. **Test boundary conditions**: max/min values, large numbers, precision
5. **Use descriptive names**: `shouldCapitalizeFirstLetter` instead of `test1`
6. **Use AssertJ**: For readable, chainable assertions
7. **Use `@ParameterizedTest`**: For multiple similar inputs (see `references/parameterized-tests.md`)
8. **Avoid mocking**: Pure utilities need no mocks

## Examples

### Basic Static Utility Test

```java
import org.junit.jupiter.api.Test;
import static org.assertj.core.api.Assertions.*;

class StringUtilsTest {

    @Test
    void shouldCapitalizeFirstLetter() {
        assertThat(StringUtils.capitalize("hello")).isEqualTo("Hello");
    }

    @Test
    void shouldReturnNullForNullInput() {
        assertThat(StringUtils.capitalize(null)).isNull();
    }

    @Test
    void shouldHandleEmptyString() {
        assertThat(StringUtils.capitalize("")).isEmpty();
    }

    @Test
    void shouldHandleSingleCharacter() {
````
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