# design-patterns

Practical design patterns reference for Java with production-ready code examples. Guides coding agents to implement Builder, Factory, or Strategy during refactoring.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 336 installs · ★ 760
- Author: decebals
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/decebals/claude-code-java/tree/HEAD/skills/design-patterns/SKILL.md
- Updated: 2026-09-06
- HTML: https://affpapa.org/en/nevibecode/skills/decebals-design-patterns

## What it does

The skill injects a Java design patterns reference directly into the agent context. It covers eight classic patterns: Builder, Factory, Strategy, Decorator, Observer, Singleton, Adapter, and Template Method. The model uses these blueprints whenever you ask it to structure classes or untangle rigid code.

Examples follow modern Java conventions. Alongside standard object-oriented boilerplate, the reference includes Lombok `@Builder` annotations and Spring `@Component` configurations for factories and singletons. Singleton usage comes with an explicit warning about hidden dependencies and testing hurdles.

The reference specifically targets Java and Spring workflows. Trying to apply these exact snippets to other runtimes will fail because the idioms depend on JVM features.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add decebals/claude-code-java --skill design-patterns -a claude-code
```

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

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add decebals/claude-code-java --skill design-patterns -a codex
```

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

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add decebals/claude-code-java --skill design-patterns -a cursor
```

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

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install decebals/claude-code-java design-patterns
```

2. **Manually: put the files in place**
- File: `.github/skills/design-patterns/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add decebals/claude-code-java --skill design-patterns -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/decebals/claude-code-java --path skills/design-patterns
```

### Windsurf (Devin Desktop)

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

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add decebals/claude-code-java --skill design-patterns -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/decebals/claude-code-java/tree/HEAD/skills/design-patterns/SKILL.md

## When to use it
- replace telescoping constructors with the Builder pattern
- decouple object creation using Factory Method
- swap runtime algorithms dynamically via Strategy
- avoid brittle global state when configuring singletons

## Example prompts
- Refactor the User class to use a Builder pattern for optional fields.
- Extract the payment logic into a Strategy pattern to allow runtime switching.
- Create a NotificationFactory to produce email and SMS senders based on type.

## 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: a7e538747ae66043.

BEGIN THIRD-PARTY FILE a7e538747ae66043
````text
---
name: design-patterns
description: Common design patterns with Java examples (Factory, Builder, Strategy, Observer, Decorator, etc.). Use when user asks "implement pattern", "use factory", "strategy pattern", or when designing extensible components.
license: MIT
---

# Design Patterns Skill

Practical design patterns reference for Java with modern examples.

## When to Use
- User asks to implement a specific pattern
- Designing extensible/flexible components
- Refactoring rigid code structures
- Code review suggests pattern usage

---

## Quick Reference: When to Use What

| Problem | Pattern |
|---------|---------|
| Complex object construction | **Builder** |
| Create objects without specifying class | **Factory** |
| Multiple algorithms, swap at runtime | **Strategy** |
| Add behavior without changing class | **Decorator** |
| Notify multiple objects of changes | **Observer** |
| Ensure single instance | **Singleton** |
| Convert incompatible interfaces | **Adapter** |
| Define algorithm skeleton | **Template Method** |

---

## Creational Patterns

### Builder

**Use when:** Object has many parameters, some optional.

```java
// ❌ Telescoping constructor antipattern
public class User {
    public User(String name) { }
    public User(String name, String email) { }
    public User(String name, String email, int age) { }
    public User(String name, String email, int age, String phone) { }
    // ... explosion of constructors
}

// ✅ Builder pattern
public class User {
    private final String name;      // required
    private final String email;     // required
    private final int age;          // optional
    private final String phone;     // optional
    private final String address;   // optional

    private User(Builder builder) {
        this.name = builder.name;
        this.email = builder.email;
        this.age = builder.age;
        this.phone = builder.phone;
        this.address = builder.address;
    }

    public static Builder builder(String name, String email) {
        return new Builder(name, email);
    }

    public static class Builder {
        // Required
        private final String name;
        private final String email;
        // Optional with defaults
        private int age = 0;
        private String phone = "";
        private String address = "";

        private Builder(String name, String email) {
            this.name = name;
            this.email = email;
        }

        public Builder age(int age) {
            this.age = age;
            return this;
        }

        public Builder phone(String phone) {
            this.phone = phone;
            return this;
        }

        public Builder address(String address) {
            this.address = address;
            return this;
        }

        public User build() {
            return new User(this);
        }
    }
}

// Usage
````
END THIRD-PARTY FILE a7e538747ae66043

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