PHASE 2Beginner ~7 min· topic 3 of 22Creational

Creational 3 — Builder

In one line

Construct complex objects step-by-step with readable, fluent calls — especially when there are many optional or immutable fields.

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Think of it like this

Ordering a Subway sandwich. You pick bread, then cheese, then veggies, then sauce, one step at a time, and only at the end do you get your finished sandwich.

Key ideas

01
Solves: telescoping constructors (5 overloads) and configurable immutable objects.
02
A fluent builder returns this from each setter, then a build() that validates and freezes.
03
Records + builders pair well in Java 17+ for immutable DTOs.

Java / Spring map

→
Lombok @Builder; Spring's UriComponentsBuilder, ObjectMapper for JSON.

Code & diagrams

TicketBuilder.javajava

Parking ticket as immutable object with fluent construction.

public class ParkingTicket {
  private final String id;
  private final String plate;
  private final long enteredAt;
  private final int floor;
  private final int spot;

  private ParkingTicket(Builder b) {
    this.id = b.id; this.plate = b.plate; this.enteredAt = b.enteredAt;
    this.floor = b.floor; this.spot = b.spot;
  }

  public static Builder builder() { return new Builder(); }

  public static class Builder {
    private String id = "T-" + System.nanoTime(); // sensible defaults
    private String plate = "";
    private long enteredAt = System.currentTimeMillis();
    private int floor = 1, spot = 0;

    public Builder id(String v)   { this.id = v; return this; }
    public Builder plate(String v){ this.plate = v; return this; }
    public Builder enteredAt(long v){ this.enteredAt = v; return this; }
    public Builder floor(int v)   { this.floor = v; return this; }
    public Builder spot(int v)    { this.spot = v; return this; }

    public ParkingTicket build() {
      if (plate.isBlank()) throw new IllegalStateException("plate required");
      return new ParkingTicket(this);
    }
  }
}
// Usage: ParkingTicket.builder().plate("MH12AB1234").floor(2).spot(14).build();

Explain without notes

Where does validation live in a builder?

Practice

Build an immutable 'RideRequest' with 6 optional fields for the Ride Booking problem.

Trade-offs

↔
Builder boilerplate is real; use records + compact constructors where possible.

Completion checklist

I can write a builder whose build() validates cross-field invariants and returns an immutable object.

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