Class HideUtilityClassConstructorCheck

Makes sure that utility classes (classes that contain only static methods or fields in their API) do not have a public constructor.

Rationale: Instantiating utility classes does not make sense. Hence the constructors should either be private or (if you want to allow subclassing) protected. A common mistake is forgetting to hide the default constructor.

If you make the constructor protected you may want to consider the following constructor implementation technique to disallow instantiating subclasses:

 public class StringUtils // not final to allow subclassing
 {
   protected StringUtils() {
     // prevents calls from subclass
     throw new UnsupportedOperationException();
   }

   public static int count(char c, String s) {
     // ...
   }
 }
 

To configure the check:

 <module name="HideUtilityClassConstructor"/>
 

Example:

 class Test { // violation, class only has a static method and a constructor

   public Test() {
   }

   public static void fun() {
   }
 }

 class Foo { // OK

   private Foo() {
   }

   static int n;
 }

 class Bar { // OK

   protected Bar() {
     // prevents calls from subclass
     throw new UnsupportedOperationException();
   }
 }

 class UtilityClass { // violation, class only has a static field

   static float f;
 }
 

Parent is com.puppycrawl.tools.checkstyle.TreeWalker

Violation Message Keys:

• hide.utility.class