Package com.puppycrawl.tools.checkstyle.checks.design

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

    • Field Detail

      • MSG_KEY

        public static final java.lang.String MSG_KEY
        A key is pointing to the warning message text in "messages.properties" file.
        See Also:
        Constant Field Values

    • Constructor Detail

      • HideUtilityClassConstructorCheck

        public HideUtilityClassConstructorCheck()

    • Method Detail

      • getDefaultTokens

        public int[] getDefaultTokens()
        Description copied from class: AbstractCheck
        Returns the default token a check is interested in. Only used if the configuration for a check does not define the tokens.
        Specified by:
        getDefaultTokens in class AbstractCheck
        Returns:
        the default tokens
        See Also:
        TokenTypes

      • 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

      • getRequiredTokens

        public int[] getRequiredTokens()
        Description copied from class: AbstractCheck
        The tokens that this check must be registered for.
        Specified by:
        getRequiredTokens in class AbstractCheck
        Returns:
        the token set this must be registered for.
        See Also:
        TokenTypes