Class HideUtilityClassConstructorCheck

  • All Implemented Interfaces:
    Configurable, Contextualizable

    public class HideUtilityClassConstructorCheck
    extends AbstractCheck

    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
    Since:
    3.1
    • Method Detail

      • getAcceptableTokens

        public int[] getAcceptableTokens()
        Description copied from class: AbstractCheck
        The configurable token set. Used to protect Checks against malicious users who specify an unacceptable token set in the configuration file. The default implementation returns the check's default tokens.
        Specified by:
        getAcceptableTokens in class AbstractCheck
        Returns:
        the token set this check is designed for.
        See Also:
        TokenTypes
      • isAbstract

        private static boolean isAbstract​(DetailAST ast)
        Returns true if given class is abstract or false.
        Parameters:
        ast - class definition for check.
        Returns:
        true if a given class declared as abstract.
      • isStatic

        private static boolean isStatic​(DetailAST ast)
        Returns true if given class is static or false.
        Parameters:
        ast - class definition for check.
        Returns:
        true if a given class declared as static.