Demeter : Java Glossary

aka The Law of Demeter aka Principle of Least Knowledge is a design guideline for developing software, particularly object-oriented programs. The Law of Demeter is a specific case of loose coupling. The guideline was invented at Northeastern University in 1987 and can be summarized in one of the following ways:

• Each unit should have only limited knowledge about other units: only units closely related to the current unit.
• Each unit should only talk to its friends; don’t talk to strangers.
• Only talk to your immediate friends.

The fundamental notion is that a given object should assume as little as possible about the structure or properties of anything else (including its subcomponents).