Law of Demeter

Law of Demeter

Stumpled across this from PMD at 2AM at office 😓, and what did I do?? Ping chat about the topic, fix my code and sleep

What is the Law of Demeter?

• 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 friend

In simple terms

LoD tells us that it is a bad idea for single functions to know the entire navigation structure of the system.

class A {

  public void method1(O obj) {
    // Class A has unnecessary access to objects X, Y, Z
    // Don't talk to strangers.
    obj.getX()
          .getY()
            .getZ()
              .doSomething();
  }

  public void method2(O obj) {
    // Have maybe a utility method which does this
    obj.doSomething();
  }

}

Why use the Law of Demeter?

Law of Demeter stems from Seperation of Concerns. Following it reduces the coupling between modules and it increases maintainability.

An example of why coupling will become a problem?

Currently Class A directly depends on Class O.

After a few months the code might become something like this.

class A {

  public void method1(O obj) {
    X = obj.getX();

    X.doSomethingElse();

    X.getY()
      .getZ()
      .doSomething();
  }

}

Now Class A is directly depends on both Class O and Class X. Let’s say owner of Class O want to deprecate the use of Class X as he is moving to a new model.

He will not be able to as he needs to deal with the dependency in Class A.

This also affects the testability of Class A.

References

• https://hackernoon.com/object-oriented-tricks-2-law-of-demeter-4ecc9becad85
• https://pmd.github.io/pmd-6.0.1/pmd_rules_java_design.html#lawofdemeter