Since Checkstyle 3.1
The check finds classes that 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 Josh 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();
}
| name | description | type | default value | since |
|---|---|---|---|---|
| ignoredAnnotations | Annotations which allow the check to skip the method from validation. | String Set | Before, BeforeClass, Test, After, AfterClass | 7.2 |
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
}
All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.
Since Checkstyle 3.1
Checks that a class which has only private constructors is declared as final. Doesn't check for classes nested in interfaces or annotations, as they are always final there.
All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.
Since Checkstyle 3.1
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) {
// ...
}
}
All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.
Since Checkstyle 5.2
Check nested (inner) classes/interfaces are declared at the bottom of the class after all method and field declarations.
All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.
Since Checkstyle 3.1
Implements Joshua Bloch, Effective Java, Item 17 - Use Interfaces only to define types.
According to Bloch, an interface should describe a type. It is therefore inappropriate to define an interface that does not contain any methods but only constants. The Standard class javax.swing.SwingConstants is an example of a class that would be flagged by this check.
The check can be configured to also disallow marker interfaces like java.io.Serializable, that do not contain methods or constants at all.
| name | description | type | default value | since |
|---|---|---|---|---|
| allowMarkerInterfaces | Controls whether marker interfaces like Serializable are allowed. | Boolean | true | 3.1 |
All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.
Since Checkstyle 3.2
Ensures that exception classes (classes with names conforming to some regular expression and explicitly extending classes with names conforming to other regular expression) are immutable, that is, that they have only final fields.
The current algorithm is very simple: it checks that all members of exception are final. The user can still mutate an exception's instance (e.g. Throwable has a method called setStackTrace which changes the exception's stack trace). But, at least, all information provided by this exception type is unchangeable.
Rationale: Exception instances should represent an error condition. Having non final fields not only allows the state to be modified by accident and therefore mask the original condition but also allows developers to accidentally forget to set the initial state. In both cases, code catching the exception could draw incorrect conclusions based on the state.
| name | description | type | default value | since |
|---|---|---|---|---|
| format | pattern for exception class names | Regular Expression | "^.*Exception$|^.*Error$|^.*Throwable$" | 3.2 |
| extendedClassNameFormat | pattern for extended class names | Regular Expression | "^.*Exception$|^.*Error$|^.*Throwable$" | 6.2 |
All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.
Since Checkstyle 5.8
Checks that each top-level class, interface or enum resides in a source file of its own. Official description of a 'top-level' term:7.6. Top Level Type Declarations. If file doesn't contains public class, enum or interface, top-level type is the first type in file.
An example of check's configuration:
<module name="OneTopLevelClass"/>
ATTENTION: This Check does not support customization of validated tokens, so do not use the "tokens" property.
An example of code with violations:
public class Foo{
//methods
}
class Foo2{
//methods
}
An example of code without public top-level type:
class Foo{ // top-level class
//methods
}
class Foo2{
//methods
}
An example of code without violations:
public class Foo{
//methods
}
All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.
Since Checkstyle 3.2
Restricts throws statements to a specified count (4 by default). Methods with "Override" or "java.lang.Override" annotation are skipped from validation as current class cannot change signature of these methods.
Rationale: Exceptions form part of a method's interface. Declaring a method to throw too many differently rooted exceptions makes exception handling onerous and leads to poor programming practices such as writing code like catch(Exception ex). 4 is the empirical value which is based on reports that we had for the ThrowsCountCheck over big projects such as OpenJDK. This check also forces developers to put exceptions into a hierarchy such that in the simplest case, only one type of exception need be checked for by a caller but any subclasses can be caught specifically if necessary.For more information on rules for the exceptions and their issues, see Effective Java: Programming Language Guide Second Edition by Joshua Bloch pages 264-273.
ignorePrivateMethods - allows to skip private methods as they do not cause problems for other classes.
| name | description | type | default value | since |
|---|---|---|---|---|
| max | maximum allowed number of throws statements | Integer | 4 | 3.2 |
| ignorePrivateMethods | whether private methods must be ignored | Boolean | true | 6.7 |
To configure the check so that it doesn't allow more than two throws per method:
<module name="ThrowsCount">
<property name="max" value="2"/>
</module>
To configure the check so that it doesn't skip private methods:
<module name="ThrowsCount">
<property name="ignorePrivateMethods" value="false"/>
</module>
All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.
Since Checkstyle 3.0
Checks visibility of class members. Only static final, immutable or annotated by specified annotation members may be public; other class members must be private unless the property protectedAllowed or packageAllowed is set.
Public members are not flagged if the name matches the public member regular expression (contains "^serialVersionUID$" by default).
Note that Checkstyle 2 used to include "^f[A-Z][a-zA-Z0-9]*$" in the default pattern to allow names used in container-managed persistence for Enterprise JavaBeans (EJB) 1.1 with the default settings. With EJB 2.0 it is no longer necessary to have public access for persistent fields, so the default has been changed.
Rationale: Enforce encapsulation.
Check also has options making it less strict:
ignoreAnnotationCanonicalNames - the list of annotations which ignore variables in consideration. If user will provide short annotation name that type will match to any named the same type without consideration of package
allowPublicFinalFields - which allows public final fields. Default value is false
allowPublicImmutableFields - which allows immutable fields to be declared as public if defined in final class. Default value is false
Field is known to be immutable if: - It's declared as final - Has either a primitive type or instance of class user defined to be immutable (such as String, ImmutableCollection from Guava and etc)
Classes known to be immutable are listed in immutableClassCanonicalNames by their canonical names.
Rationale: Forcing all fields of class to have private modified by default is good in most cases, but in some cases it drawbacks in too much boilerplate get/set code. One of such cases are immutable classes.
Restriction: Check doesn't check if class is immutable, there's no checking if accessory methods are missing and all fields are immutable, we only check if current field is immutable or final. Under the flag allowPublicImmutableFields, the enclosing class must also be final, to encourage immutability. Under the flag allowPublicFinalFields, the final modifier on the enclosing class is optional.
Star imports are out of scope of this Check. So if one of type imported via star import collides with user specified one by its short name - there won't be Check's violation.
| name | description | type | default value | since |
|---|---|---|---|---|
| packageAllowed | whether package visible members are allowed | Boolean | false | 3.0 |
| protectedAllowed | whether protected members are allowed | Boolean | false | 3.0 |
| publicMemberPattern | pattern for public members that should be ignored | Regular Expression | "^serialVersionUID$" | 3.0 |
| allowPublicFinalFields | allows public final fields | Boolean | false | 7.0 |
| allowPublicImmutableFields | allows immutable fields to be declared as public if defined in final class | Boolean | false | 6.4 |
| immutableClassCanonicalNames | immutable classes canonical names | String Set | java.lang.String, java.lang.Integer, java.lang.Byte, java.lang.Character, java.lang.Short, java.lang.Boolean, java.lang.Long, java.lang.Double, java.lang.Float, java.lang.StackTraceElement, java.math.BigInteger, java.math.BigDecimal, java.io.File, java.util.Locale, java.util.UUID, java.net.URL, java.net.URI, java.net.Inet4Address, java.net.Inet6Address, java.net.InetSocketAddress | 6.4.1 |
| ignoreAnnotationCanonicalNames | ignore annotations canonical names | String Set | org.junit.Rule, org.junit.ClassRule, com.google.common.annotations.VisibleForTesting | 6.5 |
To configure the check:
<module name="VisibilityModifier"/>
To configure the check so that it allows package visible members:
<module name="VisibilityModifier">
<property name="packageAllowed" value="true"/>
</module>
To configure the check so that it allows no public members:
<module name="VisibilityModifier">
<property name="publicMemberPattern" value="^$"/>
</module>
To configure the Check so that it allows public immutable fields (mostly for immutable classes):
<module name="VisibilityModifier">
<property name="allowPublicImmutableFields" value="true"/>
</module>
Example of allowed public immutable fields:
public class ImmutableClass
{
public final ImmutableSet<String> includes; // No warning
public final ImmutableSet<String> excludes; // No warning
public final java.lang.String notes; // No warning
public final BigDecimal value; // No warning
public ImmutableClass(Collection<String> includes, Collection<String> excludes,
BigDecimal value, String notes)
{
this.includes = ImmutableSet.copyOf(includes);
this.excludes = ImmutableSet.copyOf(excludes);
this.value = value;
this.notes = notes;
}
}
To configure the Check in order to allow user specified immutable class names:
<module name="VisibilityModifier">
<property name="allowPublicImmutableFields" value="true"/>
<property name="immutableClassCanonicalNames" value="
com.google.common.collect.ImmutableSet"/>
</module>
Example of allowed public immutable fields:
public class ImmutableClass
{
public final ImmutableSet<String> includes; // No warning
public final ImmutableSet<String> excludes; // No warning
public final java.lang.String notes; // Warning here because
//'java.lang.String' wasn't specified as allowed class
public final int someValue; // No warning
public ImmutableClass(Collection<String> includes, Collection<String> excludes,
String notes, int someValue)
{
this.includes = ImmutableSet.copyOf(includes);
this.excludes = ImmutableSet.copyOf(excludes);
this.value = value;
this.notes = notes;
this.someValue = someValue;
}
}
Note, if allowPublicImmutableFields is set to true, the check will also check whether generic type parameters are immutable. If at least one generic type parameter is mutable, there will be a violation.
<module name="VisibilityModifier">
<property name="allowPublicImmutableFields" value="true"/>
<property name="immutableClassCanonicalNames"
value="com.google.common.collect.ImmutableSet, com.google.common.collect.ImmutableMap,java.lang.String"/>
</module>
Example of how the check works:
public final class Test {
public final String s;
public final ImmutableSet<String> names;
public final ImmutableSet<Object> objects; // violation (Object class is mutable)
public final ImmutableMap<String, Object> links; // violation (Object class is mutable)
public Test() {
s = "Hello!";
names = ImmutableSet.of();
objects = ImmutableSet.of();
links = ImmutableMap.of();
}
}
To configure the Check passing fields annotated with @com.annotation.CustomAnnotation:
<module name="VisibilityModifier">
<property name="ignoreAnnotationCanonicalNames" value=
"com.annotation.CustomAnnotation"/>
</module>
Example of allowed field:
class SomeClass
{
@com.annotation.CustomAnnotation
String annotatedString; // no warning
@CustomAnnotation
String shortCustomAnnotated; // no warning
}
To configure the Check passing fields annotated with @org.junit.Rule, @org.junit.ClassRule and @com.google.common.annotations.VisibleForTesting annotations:
<module name="VisibilityModifier"/>
Example of allowed fields:
class SomeClass
{
@org.junit.Rule
public TemporaryFolder publicJUnitRule = new TemporaryFolder(); // no warning
@org.junit.ClassRule
public static TemporaryFolder publicJUnitClassRule = new TemporaryFolder(); // no warning
@com.google.common.annotations.VisibleForTesting
public String testString = ""; // no warning
}
To configure the Check passing fields annotated with short annotation name:
<module name="VisibilityModifier">
<property name="ignoreAnnotationCanonicalNames"
value="CustomAnnotation"/>
</module>
Example of allowed fields:
class SomeClass
{
@CustomAnnotation
String customAnnotated; // no warning
@com.annotation.CustomAnnotation
String customAnnotated1; // no warning
@mypackage.annotation.CustomAnnotation
String customAnnotatedAnotherPackage; // another package but short name matches
// so no violation
}
To understand the difference between allowPublicImmutableFields and allowPublicFinalFields options, please, study the following examples.
1) To configure the check to use only 'allowPublicImmutableFields' option:
<module name="VisibilityModifier">
<property name="allowPublicImmutableFields" value="true"/>
</module>
Code example:
public class InputPublicImmutable {
public final int someIntValue; // violation
public final ImmutableSet<String> includes; // violation
public final java.lang.String notes; // violation
public final BigDecimal value; // violation
public final List list; // violation
public InputPublicImmutable(Collection<String> includes,
BigDecimal value, String notes, int someValue, List l) {
this.includes = ImmutableSet.copyOf(includes);
this.value = value;
this.notes = notes;
this.someIntValue = someValue;
this.list = l;
}
}
2) To configure the check to use only 'allowPublicFinalFields' option:
<module name="VisibilityModifier">
<property name="allowPublicFinalFields" value="true"/>
</module>
Code example:
public class InputPublicImmutable {
public final int someIntValue;
public final ImmutableSet<String> includes;
public final java.lang.String notes;
public final BigDecimal value;
public final List list;
public InputPublicImmutable(Collection<String> includes,
BigDecimal value, String notes, int someValue, List l) {
this.includes = ImmutableSet.copyOf(includes);
this.value = value;
this.notes = notes;
this.someIntValue = someValue;
this.list = l;
}
}
All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.