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() {}
}