
Java Fundamentals
- 569 installs
- 40 repo stars
- Updated January 5, 2026
- pluginagentmarketplace/custom-plugin-java
java-fundamentals is a Claude Code skill that generates clean, idiomatic Java code for backend services, APIs, and agent components using modern Java 8–21 patterns for developers who need production-quality core language
About
java-fundamentals is a Java & JVM skill at version 3.0.0 that teaches agents to write production-quality core Java for backend services, APIs, and agent components. It parameterizes java_version across 8, 11, 17, and 21 and topic across six areas: syntax, OOP, collections, streams, exceptions, and generics. Allowed tools include Read, Write, Bash, Glob, and Grep so agents can scaffold and edit real project files. Developers reach for java-fundamentals when bootstrapping Spring-style services, CLI utilities, or JVM agent modules and want idiomatic patterns instead of outdated Java 7-era snippets. The skill bonds to agent 01-java-fundamentals under SASMP 1.3.0 for marketplace plugin workflows.
- Covers syntax, OOP, collections, streams, and exception handling for Java 8–21
- Enforces SOLID principles and production-quality patterns
- Supports Records, sealed classes, pattern matching and functional streams
- Includes 6 focused learning and generation topics: syntax, oop, collections, streams, exceptions, generics
- Bonded agent with Read, Write, Bash, Glob, Grep tools for direct codebase work
Java Fundamentals by the numbers
- 569 all-time installs (skills.sh)
- +8 installs in the week ending Aug 4, 2026 (Skillselion tracking)
- Ranked #715 of 4,347 Backend & APIs skills by installs in the Skillselion catalog
- Data as of Aug 4, 2026 (Skillselion catalog sync)
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| Installs | 569 |
|---|---|
| repo stars | ★ 40 |
| Last updated | January 5, 2026 |
| Repository | pluginagentmarketplace/custom-plugin-java ↗ |
How do you write idiomatic modern Java backend code?
Generate clean, idiomatic Java code for backend services, APIs, and agent components using modern Java 8–21 patterns.
Who is it for?
Developers scaffolding JVM backend services or agent components who need version-aware Java 8 through 21 fundamentals in agent-assisted coding.
Skip if: Teams seeking Spring Boot-specific wiring, Android UI patterns, or advanced concurrency frameworks beyond core language fundamentals.
When should I use this skill?
A user asks for Java syntax, OOP, collections, streams, generics, or exception handling in a backend or API module.
What you get
Production-ready Java source files with typed collections, streams, generics, and exception handling
- Java source files
- Backend service scaffolds
- Idiomatic API handler examples
By the numbers
- Covers 6 topic areas: syntax, OOP, collections, streams, exceptions, and generics
- Supports 4 Java versions: 8, 11, 17, and 21
- Skill version 3.0.0 with SASMP 1.3.0 bonding
Files
Java Fundamentals Skill
Master core Java programming with production-quality patterns.
Overview
This skill covers Java fundamentals including syntax, OOP, collections, streams API, and exception handling for Java 8-21.
When to Use This Skill
Use when you need to:
- Write clean, idiomatic Java code
- Design classes following OOP principles
- Choose appropriate collection types
- Implement functional programming patterns
- Handle exceptions properly
Topics Covered
Core Syntax (Java 8-21)
- Variables, data types, operators
- Control flow, methods, classes
- Records (Java 16+), sealed classes (Java 17+)
- Pattern matching (Java 21)
Object-Oriented Programming
- Classes, inheritance, polymorphism
- Interfaces and abstract classes
- SOLID principles
Collections Framework
- List: ArrayList, LinkedList
- Set: HashSet, TreeSet
- Map: HashMap, ConcurrentHashMap
- Queue: ArrayDeque, PriorityQueue
Streams API
- filter, map, flatMap, reduce, collect
- Optional handling
- Parallel streams
Exception Handling
- Checked vs unchecked exceptions
- Try-with-resources
- Custom exceptions
Quick Reference
// Record (Java 16+)
public record User(String name, String email) {}
// Pattern matching (Java 21)
String format(Object obj) {
return switch (obj) {
case Integer i -> "Int: %d".formatted(i);
case String s -> "String: %s".formatted(s);
default -> obj.toString();
};
}
// Stream operations
List<String> names = users.stream()
.filter(User::isActive)
.map(User::getName)
.sorted()
.toList();
// Optional handling
String name = Optional.ofNullable(user)
.map(User::getName)
.orElse("Unknown");Collection Selection
| Need | Use | Reason |
|---|---|---|
| Indexed access | ArrayList | O(1) random access |
| Unique elements | HashSet | O(1) contains |
| Sorted unique | TreeSet | O(log n) sorted |
| Key-value pairs | HashMap | O(1) get/put |
Troubleshooting
| Problem | Cause | Solution |
|---|---|---|
| NullPointerException | Null reference | Use Optional |
| ConcurrentModificationException | Modify during iteration | Iterator.remove() |
| ClassCastException | Wrong type | Use generics |
Usage
Skill("java-fundamentals")# java-fundamentals Configuration
# Category: general
# Generated: 2025-12-30
skill:
name: java-fundamentals
version: "1.0.0"
category: general
settings:
# Default settings for java-fundamentals
enabled: true
log_level: info
# Category-specific defaults
validation:
strict_mode: false
auto_fix: false
output:
format: markdown
include_examples: true
# Environment-specific overrides
environments:
development:
log_level: debug
validation:
strict_mode: false
production:
log_level: warn
validation:
strict_mode: true
# Integration settings
integrations:
# Enable/disable integrations
git: true
linter: true
formatter: true
{
"$schema": "http://json-schema.org/draft-07/schema#",
"title": "java-fundamentals Configuration Schema",
"type": "object",
"properties": {
"skill": {
"type": "object",
"properties": {
"name": {
"type": "string"
},
"version": {
"type": "string",
"pattern": "^\\d+\\.\\d+\\.\\d+$"
},
"category": {
"type": "string",
"enum": [
"api",
"testing",
"devops",
"security",
"database",
"frontend",
"algorithms",
"machine-learning",
"cloud",
"containers",
"general"
]
}
},
"required": [
"name",
"version"
]
},
"settings": {
"type": "object",
"properties": {
"enabled": {
"type": "boolean",
"default": true
},
"log_level": {
"type": "string",
"enum": [
"debug",
"info",
"warn",
"error"
]
}
}
}
},
"required": [
"skill"
]
}Java Fundamentals Guide
Overview
This guide provides comprehensive documentation for the java-fundamentals skill in the custom-plugin-java plugin.
Category: General
Quick Start
Prerequisites
- Familiarity with general concepts
- Development environment set up
- Plugin installed and configured
Basic Usage
# Invoke the skill
claude "java-fundamentals - [your task description]"
# Example
claude "java-fundamentals - analyze the current implementation"Core Concepts
Key Principles
1. Consistency - Follow established patterns 2. Clarity - Write readable, maintainable code 3. Quality - Validate before deployment
Best Practices
- Always validate input data
- Handle edge cases explicitly
- Document your decisions
- Write tests for critical paths
Common Tasks
Task 1: Basic Implementation
# Example implementation pattern
def implement_java_fundamentals(input_data):
"""
Implement java-fundamentals functionality.
Args:
input_data: Input to process
Returns:
Processed result
"""
# Validate input
if not input_data:
raise ValueError("Input required")
# Process
result = process(input_data)
# Return
return resultTask 2: Advanced Usage
For advanced scenarios, consider:
- Configuration customization via
assets/config.yaml - Validation using
scripts/validate.py - Integration with other skills
Troubleshooting
Common Issues
| Issue | Cause | Solution |
|---|---|---|
| Skill not found | Not installed | Run plugin sync |
| Validation fails | Invalid config | Check config.yaml |
| Unexpected output | Missing context | Provide more details |
Related Resources
- SKILL.md - Skill specification
- config.yaml - Configuration options
- validate.py - Validation script
---
Last updated: 2025-12-30
Java Fundamentals Patterns
Design Patterns
Pattern 1: Input Validation
Always validate input before processing:
def validate_input(data):
if data is None:
raise ValueError("Data cannot be None")
if not isinstance(data, dict):
raise TypeError("Data must be a dictionary")
return TruePattern 2: Error Handling
Use consistent error handling:
try:
result = risky_operation()
except SpecificError as e:
logger.error(f"Operation failed: {e}")
handle_error(e)
except Exception as e:
logger.exception("Unexpected error")
raisePattern 3: Configuration Loading
Load and validate configuration:
import yaml
def load_config(config_path):
with open(config_path) as f:
config = yaml.safe_load(f)
validate_config(config)
return configAnti-Patterns to Avoid
❌ Don't: Swallow Exceptions
# BAD
try:
do_something()
except:
pass✅ Do: Handle Explicitly
# GOOD
try:
do_something()
except SpecificError as e:
logger.warning(f"Expected error: {e}")
return default_valueCategory-Specific Patterns: General
Recommended Approach
1. Start with the simplest implementation 2. Add complexity only when needed 3. Test each addition 4. Document decisions
Common Integration Points
- Configuration:
assets/config.yaml - Validation:
scripts/validate.py - Documentation:
references/GUIDE.md
---
Pattern library for java-fundamentals skill
#!/usr/bin/env python3
"""
Validation script for java-fundamentals skill.
Category: general
"""
import os
import sys
import yaml
import json
from pathlib import Path
def validate_config(config_path: str) -> dict:
"""
Validate skill configuration file.
Args:
config_path: Path to config.yaml
Returns:
dict: Validation result with 'valid' and 'errors' keys
"""
errors = []
if not os.path.exists(config_path):
return {"valid": False, "errors": ["Config file not found"]}
try:
with open(config_path, 'r') as f:
config = yaml.safe_load(f)
except yaml.YAMLError as e:
return {"valid": False, "errors": [f"YAML parse error: {e}"]}
# Validate required fields
if 'skill' not in config:
errors.append("Missing 'skill' section")
else:
if 'name' not in config['skill']:
errors.append("Missing skill.name")
if 'version' not in config['skill']:
errors.append("Missing skill.version")
# Validate settings
if 'settings' in config:
settings = config['settings']
if 'log_level' in settings:
valid_levels = ['debug', 'info', 'warn', 'error']
if settings['log_level'] not in valid_levels:
errors.append(f"Invalid log_level: {settings['log_level']}")
return {
"valid": len(errors) == 0,
"errors": errors,
"config": config if not errors else None
}
def validate_skill_structure(skill_path: str) -> dict:
"""
Validate skill directory structure.
Args:
skill_path: Path to skill directory
Returns:
dict: Structure validation result
"""
required_dirs = ['assets', 'scripts', 'references']
required_files = ['SKILL.md']
errors = []
# Check required files
for file in required_files:
if not os.path.exists(os.path.join(skill_path, file)):
errors.append(f"Missing required file: {file}")
# Check required directories
for dir in required_dirs:
dir_path = os.path.join(skill_path, dir)
if not os.path.isdir(dir_path):
errors.append(f"Missing required directory: {dir}/")
else:
# Check for real content (not just .gitkeep)
files = [f for f in os.listdir(dir_path) if f != '.gitkeep']
if not files:
errors.append(f"Directory {dir}/ has no real content")
return {
"valid": len(errors) == 0,
"errors": errors,
"skill_name": os.path.basename(skill_path)
}
def main():
"""Main validation entry point."""
skill_path = Path(__file__).parent.parent
print(f"Validating java-fundamentals skill...")
print(f"Path: {skill_path}")
# Validate structure
structure_result = validate_skill_structure(str(skill_path))
print(f"\nStructure validation: {'PASS' if structure_result['valid'] else 'FAIL'}")
if structure_result['errors']:
for error in structure_result['errors']:
print(f" - {error}")
# Validate config
config_path = skill_path / 'assets' / 'config.yaml'
if config_path.exists():
config_result = validate_config(str(config_path))
print(f"\nConfig validation: {'PASS' if config_result['valid'] else 'FAIL'}")
if config_result['errors']:
for error in config_result['errors']:
print(f" - {error}")
else:
print("\nConfig validation: SKIPPED (no config.yaml)")
# Summary
all_valid = structure_result['valid']
print(f"\n==================================================")
print(f"Overall: {'VALID' if all_valid else 'INVALID'}")
return 0 if all_valid else 1
if __name__ == "__main__":
sys.exit(main())
Related skills
FAQ
Which Java versions does java-fundamentals support?
java-fundamentals accepts java_version values 8, 11, 17, and 21 through its parameter enum. Agents generate idiomatic code aligned with the selected release rather than mixing deprecated APIs.
What topics does java-fundamentals cover?
java-fundamentals spans six topics: syntax, OOP, collections, streams, exceptions, and generics. The skill focuses on core language patterns for backend services, APIs, and agent components.