The Law of Demeter, or Principle of Least Knowledge, is a design heuristic from Ian Holland and Karl Lieberherr's Demeter Project at Northeastern University (1987): a method should only talk to its immediate friends. It may call methods on itself, its parameters, objects it creates and its direct components, but not on objects those calls return. Avoiding such train-wreck chains limits coupling and keeps an object's internal structure encapsulated.

Law of Demeter

Details
Full Name

Law of Demeter (LoD)

Also known as

Principle of Least Knowledge, "Don’t talk to strangers"

Core Concepts:

Only talk to your immediate friends

A method may call methods on: itself, its parameters, objects it creates, and its direct component objects — but not on objects returned by those calls

Train-wreck calls

Chains like a.getB().getC().doSomething() reach through intermediaries and couple the caller to a deep object graph — the classic LoD violation

Tell, don’t ask

Push behaviour to the object that owns the data instead of pulling data out and operating on it externally

Encapsulation boundary

LoD limits how much of an object’s internal structure leaks to its collaborators, so internal changes don’t ripple outward

Pragmatic limits

It is a heuristic, not an absolute; fluent builders and some data-pipeline/query DSLs chain deliberately and are reasonable exceptions

Key Proponents

Ian Holland & Karl Lieberherr (Northeastern University, Demeter Project, 1987); popularized by "The Pragmatic Programmer" (Hunt & Thomas)

When to Use:

  • Reviewing code for hidden coupling and fragile call chains

  • Designing object APIs that hide their internal structure

  • Teaching encapsulation and "tell, don’t ask"

  • Guiding refactorings away from train-wreck expressions

When NOT to Use:

  • For fluent interfaces / builders where chaining is the intended design

  • For immutable value objects and data-query DSLs where traversal is the point

  • SOLID Principles — complementary coupling/cohesion guidance

  • GRASP — Low Coupling and Information Expert overlap with LoD

  • Cohesion Criteria — the cohesion side of the same design concern