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

Class DesignForExtensionCheck

  • All Implemented Interfaces:
    Configurable, Contextualizable
    public class DesignForExtensionCheck
extends AbstractCheck

    Checks that classes are designed for extension (subclass creation).

    Nothing wrong could be with founded classes. This check makes sense only for library projects (not application projects) which care of ideal OOP-design to make sure that class works in all cases even misusage. Even in library projects this check most likely will find classes that are designed for extension by somebody. User needs to use suppressions extensively to got a benefit from this check, and keep in suppressions all confirmed/known classes that are deigned for inheritance intentionally to let the check catch only new classes, and bring this to team/user attention.

    ATTENTION: Only user can decide whether a class is designed for extension or not. The check just shows all classes which are possibly designed for extension. If smth inappropriate is found please use suppression.

    ATTENTION: If the method which can be overridden in a subclass has a javadoc comment (a good practice is to explain its self-use of overridable methods) the check will not rise a violation. The violation can also be skipped if the method which can be overridden in a subclass has one or more annotations that are specified in ignoredAnnotations option. Note, that by default @Override annotation is not included in the ignoredAnnotations set as in a subclass the method which has the annotation can also be overridden in its subclass.

    Problem is described at "Effective Java, 2nd Edition by Joshua Bloch" book, chapter "Item 17: Design and document for inheritance or else prohibit it".

    Some quotes from book:

    The class must document its self-use of overridable methods. By convention, a method that invokes overridable methods contains a description of these invocations at the end of its documentation comment. The description begins with the phrase “This implementation.”
    The best solution to this problem is to prohibit subclassing in classes that are not designed and documented to be safely subclassed.
    If a concrete class does not implement a standard interface, then you may inconvenience some programmers by prohibiting inheritance. If you feel that you must allow inheritance from such a class, one reasonable approach is to ensure that the class never invokes any of its overridable methods and to document this fact. In other words, eliminate the class’s self-use of overridable methods entirely. In doing so, you’ll create a class that is reasonably safe to subclass. Overriding a method will never affect the behavior of any other method.

    The check finds classes that have overridable methods (public or protected methods that are non-static, not-final, non-abstract) and have non-empty implementation.

    Rationale: This library design style protects superclasses against being broken by subclasses. The downside is that subclasses are limited in their flexibility, in particular they cannot prevent execution of code in the superclass, but that also means that subclasses cannot corrupt the state of the superclass by forgetting to call the superclass's method.

    More specifically, it enforces a programming style where superclasses provide empty "hooks" that can be implemented by subclasses.

    Example of code that cause violation as it is designed for extension: 

     public abstract class Plant {
   private String roots;
   private String trunk;

   protected void validate() {
     if (roots == null) throw new IllegalArgumentException("No roots!");
     if (trunk == null) throw new IllegalArgumentException("No trunk!");
   }

   public abstract void grow();
 }

 public class Tree extends Plant {
   private List leaves;

   @Overrides
   protected void validate() {
     super.validate();
     if (leaves == null) throw new IllegalArgumentException("No leaves!");
   }

   public void grow() {
     validate();
   }
 }

    Example of code without violation: 

     public abstract class Plant {
   private String roots;
   private String trunk;

   private void validate() {
     if (roots == null) throw new IllegalArgumentException("No roots!");
     if (trunk == null) throw new IllegalArgumentException("No trunk!");
     validateEx();
   }

   protected void validateEx() { }

   public abstract void grow();
 }
    • Property ignoredAnnotations - Specify annotations which allow the check to skip the method from validation. Type is java.lang.String[]. Default value is After, AfterClass, Before, BeforeClass, Test.

    To configure the check: 

     <module name="DesignForExtension"/>

    To configure the check to allow methods which have @Override and @Test annotations to be designed for extension. 

     <module name="DesignForExtension">
   <property name="ignoredAnnotations" value="Override, Test"/>
 </module>
 
     public class A extends B {
   @Override
   public int foo() {
     return 2;
   }

   public int foo2() {return 8;} // violation
 }

 public class B {
   /**
    * This implementation ...
      @return some int value.
    */
   public int foo() {
     return 1;
   }

   public int foo3() {return 3;} // violation
 }

 public class FooTest {
   @Test
   public void testFoo() {
     final B b = new A();
     assertEquals(2, b.foo());
   }

   public int foo4() {return 4;} // violation
 }

    Parent is com.puppycrawl.tools.checkstyle.TreeWalker

    Violation Message Keys:

    • design.forExtension
    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

      • DesignForExtensionCheck

        public DesignForExtensionCheck()

    • Method Detail

      • setIgnoredAnnotations

        public void setIgnoredAnnotations​(java.lang.String... ignoredAnnotations)
        Setter to specify annotations which allow the check to skip the method from validation.
        Parameters:
        ignoredAnnotations - method annotations.

      • 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

      • isCommentNodesRequired

        public boolean isCommentNodesRequired()
        Description copied from class: AbstractCheck
        Whether comment nodes are required or not.
        Overrides:
        isCommentNodesRequired in class AbstractCheck
        Returns:
        false as a default value.