OOP Concepts for Interviews — The 4 Pillars With Real Examples

Every technical round touches OOP. The pillars are easy to list and hard to explain — interviews grade the explanation.

The four pillars with one running example

class Account {
  #balance = 0;                          // ENCAPSULATION — private state
  deposit(amt) {
    if (amt <= 0) throw new Error();     // controlled access
    this.#balance += amt;
  }
  get balance() { return this.#balance; }
}

class SavingsAccount extends Account {   // INHERITANCE — reuse + extend
  addInterest() { this.deposit(this.balance * 0.04); }
}

• Encapsulation: bundle data + methods, hide internals. "Change balance only through deposit()"
• Abstraction: expose WHAT, hide HOW. You call account.deposit() without knowing the ledger logic. (Follow-up trap: abstraction = hiding complexity at design level; encapsulation = hiding data at implementation level)
• Inheritance: is-a relationships. SavingsAccount IS An Account
• Polymorphism: same call, different behavior — account.calculateInterest() works differently on Savings vs Current. Method overriding = runtime polymorphism; overloading = compile-time (Java)

The follow-ups that filter

• "Abstract class vs interface?" — abstract: shared code + contract, single inheritance; interface: pure contract, multiple allowed
• "Composition vs inheritance?" — prefer composition ("has-a") for flexibility; inheritance couples tightly
• "Overriding vs overloading?" — same signature in child vs same name different params

Full practice: interview question bank.