Corak Builder dengan API Lancar

// Without builder — hard to read:
Pizza p = new Pizza("large", "thin", true, false, true, false, "mozzarella");

public final class Pizza {
    private final String size, crustType, cheese;
    private final boolean extraSauce, pepperoni;
    private Pizza(Builder b) {
        this.size = b.size; this.crustType = b.crustType;
        this.cheese = b.cheese; this.extraSauce = b.extraSauce;
        this.pepperoni = b.pepperoni;
    }
    public static class Builder {
        private final String size;
        private String crustType = "regular", cheese = "mozzarella";
        private boolean extraSauce, pepperoni;
        public Builder(String size) { this.size = size; }
        public Builder crustType(String c) { this.crustType = c; return this; }
        public Builder cheese(String c)    { this.cheese = c;    return this; }
        public Builder extraSauce()        { this.extraSauce = true; return this; }
        public Builder pepperoni()         { this.pepperoni = true; return this; }
        public Pizza build()               { return new Pizza(this); }
    }
}

Pizza p = new Pizza.Builder("large")
    .crustType("thin")
    .extraSauce()
    .pepperoni()
    .build();
System.out.println(p);

public Pizza build() {
    if (size == null || size.isBlank())
        throw new IllegalArgumentException("Size required");
    if (extraSauce && crustType.equals("stuffed"))
        throw new IllegalStateException("No extra sauce on stuffed crust");
    return new Pizza(this);
}

public final class Address {
    private final String street, city, country;
    private final String postalCode;
    private Address(Builder b) { ... } // all finals
    // no setters — immutable!
}

public abstract static class Builder<SELF extends Builder<SELF>> {
    String name;
    @SuppressWarnings("unchecked")
    public SELF name(String n) { this.name = n; return (SELF) this; }
    public abstract Vehicle build();
}

@Builder
public class Report {
    private final String title;
    @Builder.Default private final int pageCount = 1;
    @Singular private final List<String> authors;
}
// Usage:
Report r = Report.builder().title("Q4").author("Alice").author("Bob").build();

HttpRequest req = HttpRequest.newBuilder()
    .uri(URI.create("https://api.example.com/users"))
    .header("Accept", "application/json")
    .GET()
    .build();

User defaultUser() { return new User.Builder("test@example.com").name("Test User").build(); }
User adminUser()   { return new User.Builder("admin@example.com").name("Admin").role(ADMIN).build(); }