core-engineering

Object-Oriented Design Skill

This skill defines Core Engineering: Object-Oriented Programming (OOP), SOLID, clean-code principles, and relationship types with concrete examples so an AI agent or developer can apply them consistently.

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1. Four Pillars of OOP

1.1 Abstraction

What it is: Focus on what an object does, not how. Hide complex details and expose only essential behavior.

How it looks: Public methods define the contract; internals are private or hidden.

// Abstraction: caller uses start() and stop() without knowing engine details
class Car {
  private engine: Engine;

  start(): void {
    this.engine.ignite();
  }
  stop(): void {
    this.engine.shutOff();
  }
}

Types of abstraction:

• Data abstraction: Hide how data is stored (e.g. use getters instead of raw fields).
• Control abstraction: Hide how a process is implemented (e.g. save() instead of open/write/close).

// Data abstraction: storage details hidden
class UserRepo {
  private store: Map<string, User>;
  getById(id: string): User | undefined {
    return this.store.get(id);
  }
}

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1.2 Encapsulation

What it is: Bundle data and methods that act on that data in one unit (class). Restrict direct access to internal state to avoid accidental misuse.

How it looks: Private fields, public accessors or methods; no direct property access from outside.

// Encapsulation: balance is protected; changes only through deposit/withdraw
class BankAccount {
  private balance: number = 0;

  deposit(amount: number): void {
    if (amount > 0) this.balance += amount;
  }
  withdraw(amount: number): boolean {
    if (amount > 0 && amount <= this.balance) {
      this.balance -= amount;
      return true;
    }
    return false;
  }
  getBalance(): number {
    return this.balance;
  }
}

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1.3 Inheritance

What it is: A child class gets properties and behavior from a parent class to reuse code and model "is-a" relationships.