M4 — State, Identity, and Object Creation

SENG 365 — Software Engineering

Neil Ernst

University of Victoria

2026-07-24

Learning Objectives

Reading: Robillard, 3rd ed. — Ch. 4, Object State (pp. 67–95).

Concrete state: the object at runtime.
Abstract state: an abstraction of the potential state space (e.g. “positive account balance”).
Choosing the right abstract states helps the concrete implementation.

Capture the state of a system simple enough to manage cruise control. Try modelling a course at UVic.

States reflect the world/system at a point in time — usually nouns.
Transitions are the verbs.
A transition may be modelled as a state if it has domain importance (a question of abstraction level).
Keep the distinction between states of the world and states of the system capturing that world.

debugger and physical locations

Identity: typically the memory location on the heap.
Java uses references: two objects are == if the references point to the same id.
Two Aces are not == — different addresses.

We want to say two “Ace of Clubs” are the same card → override equals() and hashCode().
Equality is programmer-defined.
Default Object.equals() compares ids (==) — often not what our design needs.
A good hashCode() combines Rank + Suit.

Unique: design so two identical cards are impossible (no cheating!). Hard to guarantee — Flyweight and Singleton attempt it.
Immutable ≠ unique: a unique Car can still be changed (e.g. add an engine).

We don’t always want new objects. For invariant, immutable state, value objects are far more efficient than thousands of objects.
The Flyweight factory checks whether an object with this property already exists.
Problem: avoid large numbers of objects. Solution: an immutable Flyweight managing invariant state.
Examples: characters in word processing, primitive sprites in graphics.

Prevent new instances of a class (as opposed to unique instances).
Manage access to shared global state — e.g. the state of a game of chess or solitaire.
A single instance of a class, vs. unique instances of a class (Flyweight).
Video: implementing Singleton.

Anonymous classes/functions (lambdas) have a strange scope: they copy the current state into a new object.
We like anonymous classes because (for small model problems) they reduce clutter and aid encapsulation.
A closure is a method definition together with references to its local environment variables.
“The value in the enclosing scope is remembered even when the variable goes out of scope.”