I've used following code in my project. When I evaluated it using sonar it shows  Unused private method violation for the private constructor defined in ObjectTypes inner class. If I removed ObjectTypes constructor it shows Hide Utility Class Constructor violation. Please help me to find the best possible way to overcome this issue.
public final class Constants
{
  private Constants()
  {
  }
  public static final String KEY_SEPARATOR = " ~ ";      
  public static final String COMMON_SEPARATOR = " : ";
  public final class ObjectTypes
  {
    private ObjectTypes()
    {
    }
    public static final String ACTION_CODES = "Action Codes";
    public static final String ALL_ACTION_CODES = "All Action Codes";
    //more lines
  }
}