Why is this an issue?

Whenever there are portions of code that are duplicated and do not depend on the state of their container class, they can be centralized inside a "utility class". A utility class is a class that only has static members, hence it should not be instantiated.

Exceptions

When a class contains public static void main(String[] args) method it is not considered as a utility class and will be ignored by this rule.

How to fix it

To prevent the class from being instantiated, you should define a non-public constructor. This will prevent the compiler from implicitly generating a public parameterless constructor.

Code examples

Noncompliant code example

class StringUtils { // Noncompliant

  public static String concatenate(String s1, String s2) {
    return s1 + s2;
  }

}

Compliant solution

class StringUtils { // Compliant

  private StringUtils() {
    throw new IllegalStateException("Utility class");
  }

  public static String concatenate(String s1, String s2) {
    return s1 + s2;
  }

}