According to the Java Language Specification-8.9:

Nested enum types are implicitly static.

So there’s no need to declare them static explicitly.

Noncompliant Code Example

public class Flower {
  static enum Color { // Noncompliant; static is redundant here
    RED, YELLOW, BLUE, ORANGE
  }

  // ...
}

Compliant Solution

public class Flower {
  enum Color { // Compliant
    RED, YELLOW, BLUE, ORANGE
  }

  // ...
}