LoD

Law of Demeter

“Talk only to your immediate friends.”

The Principle of Least Knowledge. A module should not know about the innards of the objects it manipulates.

When to use

When you see a chain of dot accessors (e.g., a.getB().getC().doSomething()).

Why it matters

Decoupling: Classes are less dependent on the internal structure of strange classes.
Maintainability: Changing Class C doesn’t break Class A if they aren’t directly talking.

Signs of Violation

“Train Wrecks”: getObj().getChild().getParam().getValue().
Excessive number of imports for types that are only used to “get to” other types.

Explanation

Problem

If A talks to B to get C to get D, then A is now coupled to B, C, and D. Any change in the relationship hierarchy breaks A.

Solution

Ask B to do the work, or ask B to give you what you need directly. Don’t reach into B’s pockets.

Real world analogy

When you buy something at a store, you give the cashier your money. You don’t hand your wallet to the cashier and let them dig through it to find the money.

Pros and Cons

Pros	Cons
Low Coupling Robustness	Can lead to many “Wrapper” methods in the middle classes

Comparison

Information Expert: LoD enforces Information Expert (asking the object that has the info).

LoD