Inappropriate Intimacy — Specification¶

This chapter turns the smell into numbers. Reviewers say "these two classes know too much about each other" — but how much is too much? Coupling metrics give us a quantitative baseline, an early warning, and a way to compare two refactoring options objectively. Below are the metrics that matter for Inappropriate Intimacy, exact definitions, how to compute them, and thresholds that have held up in real Java codebases.

1. CBO — Coupling Between Objects¶

Definition. For a class C, CBO is the number of other classes to which C is coupled. Two classes are coupled when one uses a method or field of the other, or one is the parameter/return type/local variable type of a method in the other. Inheritance and use of language types (String, int, JDK types in java.lang) are typically excluded.

Formal form.

CBO(C) = |{ D : D != C and C uses D or D uses C }|

Many tools (PMD, CK, JaCoCo plugins, NDepend-style ports) compute an outgoing CBO (only C -> D edges) and an incoming CBO (D -> C). Sum is the symmetric CBO.

Why it matters for intimacy. A class with CBO of 25 is touching a quarter of the system. The probability that at least one of those 25 partners is leaking field-level details into C rises sharply with CBO.

Thresholds (industry-typical).

For domain entities the cap is stricter: anything above 8 is suspicious.

2. MPC — Message Passing Coupling¶

Definition. MPC counts the number of method calls from class C to methods of other classes. Unlike CBO (which counts partners), MPC counts messages. Two classes can have CBO = 1 but MPC = 200 — that single partner is being hammered.

Formal form.

MPC(C) = sum over methods m in C of |{ call sites in m to methods of any D != C }|

Why it matters for intimacy. High MPC with low CBO means deep dependence on one partner: the textbook Inappropriate Intimacy signature. The two classes are not exchanging information through a small protocol, they are continuously poking each other.

Thresholds.

A useful derived metric is MPC / CBO — average messages per partner. Values above 8 indicate concentrated dependency on a small set of classes.

3. Bidirectional edge count¶

Definition. Build the directed coupling graph G where an edge A -> B means A uses B. The bidirectional edge count is the number of unordered pairs {A, B} with both A -> B and B -> A present.

BiEdges(G) = |{ {A, B} : A -> B in G and B -> A in G }|

Why it matters. Inappropriate Intimacy is, by definition, mutual. A unidirectional dependency is normal; two-way coupling is where the smell lives. Bidirectional edges also block independent deployment, independent testing, and clean module extraction.

Thresholds.

For a single pair, any bidirectional edge between entities in the domain model (as opposed to associations between Aggregate Root and its parts) is a defect.

4. Fan-in and Fan-out¶

Definitions.

• Fan-out(C) = number of distinct classes C depends on (outgoing degree in the coupling graph). This equals outgoing CBO.
• Fan-in(C) = number of distinct classes that depend on C (incoming degree).

FanOut(C) = |{ D : C -> D }|
FanIn(C)  = |{ D : D -> C }|

Why it matters.

• High fan-out with low fan-in → the class is a "needy" client; often a God Class in disguise reaching into everything.
• High fan-in with low fan-out → the class is a stable abstraction (good — utilities, value objects, ports). Not intimate per se, but changes ripple widely.
• High fan-in AND high fan-out → an architectural hub; almost always involved in intimacy because it lives at the centre of every cycle.

Thresholds for application classes.

Derived ratio: Instability I = FanOut / (FanIn + FanOut) (Martin's instability metric). Values near 0 are stable, near 1 are unstable. Domain entities should sit at I < 0.3; orchestrators at 0.5–0.8.

5. RFC — Response For a Class (supporting metric)¶

Definition. RFC = number of methods of C plus the number of distinct methods invoked by methods of C.

RFC(C) = |methods(C)| + |{ external methods reachable in one step from methods(C) }|

Tracks the total "surface" a single instance of C responds to. High RFC with high MPC is the strongest signal of Inappropriate Intimacy — the class is doing many things, and most of them are someone else's.

Thresholds. RFC > 100 in a non-controller class is a strong refactor signal.

6. LCOM* and intimacy¶

LCOM measures cohesion. It is not a coupling metric, but low cohesion paired with high CBO is the classic intimacy footprint — the class has multiple unrelated responsibilities and reaches out to a different partner for each. When triaging intimacy hot spots, sort classes by CBO * (1 + LCOM) and review the top of the list.

7. Tooling that produces these metrics¶

8. Putting thresholds on a dashboard¶

A practical "intimacy gate" for CI:

FAIL build if any non-controller class has
    CBO > 20
 OR MPC per method > 15 (average across methods)
 OR FanOut > 16
 OR participates in a bidirectional edge with a domain entity
 OR is part of any package-level cycle

Track CBO, MPC, FanOut as time series. A 25% rise quarter-over-quarter is the early warning that intimacy is being introduced before any single threshold is breached.

9. Worked example — reading the numbers together¶

Suppose OrderService reports:

Diagnosis: CBO is acceptable, but average MPC per method (17.7) is in the refactor band; one of the nine partners is being hit constantly. The bidirectional edge with Order confirms it: OrderService reaches into Order, and Order calls back into OrderService. Refactor — push the intimate behaviour onto Order (or extract a new collaborator) and the MPC will halve and the bidirectional edge will disappear.

Memorize this¶

• CBO counts partners; MPC counts messages; both matter.
• Bidirectional edges between domain classes are intimacy by definition.
• Fan-out > 15 in a domain class is almost always a refactor signal.
• Watch MPC / CBO (messages per partner) — high values mean concentrated, intimate dependence.
• Track metrics over time, not just at a point: a doubling in three months is the real alarm.
• Make at least one threshold a build-failing gate, or the numbers will only ever be looked at after the damage is done.