Skip to main content

Number of Implemented Interfaces (NOII)

v1.4.0
short name: NOII
effort: 10m
default threshold: 3

Number of implemented interfaces is a quantitative metric that measures the number of interfaces a class/enum/mixin implements (including direct and indirect interfaces).

High number of interfaces usually indicates classes with too many responsibilities that are hard to understand and maintain.

Why Be Cautious​

High number of implemented interfaces can lead to:

  • Reduced Readability: Classes with too many implemented interfaces are harder to read and understand.
  • Maintainability Challenges: High interface implementation can reduce cohesion and increase coupling, making the codebase more fragile and harder to modify.
  • Increased Complexity: A high number of implemented interfaces increases class complexity, making it harder to manage and maintain.

How Can You Improve High Number of Implemented Interfaces?​

First, understand the responsibility of the class and each implemented interface. Depending on that, consider splitting the class into smaller classes or reevaluating applicability of each interface.

Config Example​

analysis_options.yaml
dcm:
  metrics:
    number-of-implemented-interfaces:
      threshold: 5

To set multiple threshold or other common config options, refer to the Configuring Metrics page.

Example​

info

To view what contributes to the metric value, generate an HTML report.

❌ Bad: High Number of Implemented Interfaces

abstract class Printable {
  void print();
}

abstract class Savable {
  void save();
}

abstract class Shareable {
  void share();
}

abstract class Editable {
  void edit();
}

// NOII: 4.
class Document implements Printable, Savable, Shareable, Editable {
  @override
  void print() {}

  @override
  void save() {}

  @override
  void share() {}

  @override
  void edit() {}
}

✅ Good: Low Number of Implemented Interfaces

abstract class Printable {
  void print();
}

abstract class Savable {
  void save();
}

abstract class Shareable {
  void share();
}

// NOII: 3
class Document implements Printable, Savable, Shareable {
  @override
  void print() {}

  @override
  void save() {}

  @override
  void share() {}
}

// Another class can handle editing responsibilities separately.
class EditableDocument extends Document {
  void edit() {}
}
Last updated on