java-fundamentals

SKILL.md

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")