Objects, Classes & OOP Design

Object Creation & Initialization

Constructors initialize objects, but they are not the only way to get one — static factory methods and builders often serve callers better. Know the initialization order, and reach for a builder when parameters pile up.

No constructor written → compiler provides a no-arg constructor; write any → it disappears
Static factories beat constructors: they have names, can cache, and can return subtypes (EJ 1)
Builder pattern for many/optional parameters (EJ 2)
Initialization order: field initializers & init blocks in source order, then the constructor body
this(...) chains to another constructor of the same class

Consider static factory methods instead of constructors (EJ Item 1)

Named creation reads better (BigInteger.probablePrime(...)), instances can be cached (Integer.valueOf, Boolean.valueOf), the return type can be an interface or subtype (List.of, EnumSet.of picks RegularEnumSet vs JumboEnumSet), and the type can vary per call. Convention names: of, from, valueOf, getInstance, newInstance, create.

Java

LocalDate date = LocalDate.of(2026, 7, 7);
List<String> sizes = List.of("S", "M", "L");
Optional<User> u = Optional.empty();

Use a builder for many constructor parameters (EJ Item 2)

Telescoping constructors (1, 2, 3, 4 params…) become unreadable and error-prone; JavaBeans-style setters leave the object inconsistent mid-construction. A builder gives named, optional parameters and an immutable result.

Java

NutritionFacts cola = new NutritionFacts.Builder(240, 8)
        .calories(100)
        .sodium(35)
        .carbohydrate(27)
        .build();

Initialization order

When new runs: fields get default values → field initializers and instance initializer blocks execute in source order → the constructor body runs. Static fields/blocks run once, when the class is first loaded (see Class Loading). Overloaded constructors can delegate with this(...) as the first statement — one master constructor avoids duplication.

Constructor chaining

public class Session {
    private final String id;
    private final Duration timeout;

    public Session(String id, Duration timeout) {
        this.id = Objects.requireNonNull(id);
        this.timeout = timeout;
    }

    public Session(String id) {
        this(id, Duration.ofMinutes(30));   // delegate to the master constructor
    }
}