Explain the four pillars of object-oriented programming
Reported in SumUp European engineering loops. Core OOP concepts asked in almost every fresher technical screen.
Interview scenario
Often asked in SumUp loops at European offices (London, Berlin, Amsterdam, Paris, Stockholm, Dublin, and remote EU). Prepare a clear spoken answer plus key trade-offs.
Model answer
Try answering aloud first
Cover trade-offs, structure, and a concrete example before revealing the baseline response.
How to frame this at SumUp: Connect your answer to measurable impact, clarity of thought, and trade-offs the team cares about. Below is a strong baseline response you can adapt with your own project examples.
Encapsulation bundles data and behavior inside a class while hiding internal state behind controlled access (private fields, public methods). This preserves invariants—a BankAccount should not allow negative balance through a public field.
Abstraction exposes only essential behavior through interfaces or abstract classes, hiding implementation details. Callers depend on PaymentGateway.charge(), not whether it uses Stripe or Razorpay internally.
Inheritance lets a child class reuse and extend parent behavior via extends. Use it for true is-a relationships; prefer composition when behavior can be mixed in without a rigid hierarchy.
Polymorphism allows one interface with many implementations—method overriding and interface-based dispatch let code work with List without caring if the concrete type is ArrayList or LinkedList.
In interviews, give one concise example per pillar and mention that composition over inheritance reduces tight coupling in large codebases.
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