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|
<?xml version="1.0" encoding="UTF-8"?>
<document xmlns="http://maven.apache.org/XDOC/2.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/XDOC/2.0 http://maven.apache.org/xsd/xdoc-2.0.xsd">
<head>
<title>Class Design</title>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.3/jquery.min.js"/>
<script type="text/javascript" src="js/anchors.js"/>
<script type="text/javascript" src="js/google-analytics.js"/>
<link rel="icon" href="images/favicon.png" type="image/x-icon" />
<link rel="shortcut icon" href="images/favicon.ico" type="image/ico" />
</head>
<body>
<section name="Content">
<macro name="toc">
<param name="fromDepth" value="1"/>
<param name="toDepth" value="1"/>
</macro>
</section>
<section name="DesignForExtension">
<subsection name="Description">
<p>
The Check finds classes that are designed for extension (subclass creation).
</p>
<p>
Problem is described at "Effective Java, 2nd Edition by Josh Bloch" book, chapter "Item 17: Design and document for inheritance or else prohibit it".
</p>
<p>
Some quotes from book:
</p>
<blockquote>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.”
</blockquote>
<blockquote>The best solution to this problem is to prohibit subclassing in classes that
are not designed and documented to be safely subclassed.
</blockquote>
<blockquote>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.
</blockquote>
<p>
The exact rule is that non-private, non-static methods of classes that
can be subclassed must
</p>
<ul>
<li>be abstract or</li>
<li>be final or</li>
<li>have an empty implementation.</li>
</ul>
<p>
Rationale: This API 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.
</p>
<p>
More specifically,
it enforces a programming style where superclasses provide empty
"hooks" that can be implemented by subclasses.
</p>
<p>
Example of code that cause violation as it is designed for extension:
</p>
<source>
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();
}
}
</source>
<p>
Example of code without violation:
</p>
<source>
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();
}
</source>
</subsection>
<subsection name="Examples">
<p>
To configure the check:
</p>
<source>
<module name="DesignForExtension"/>
</source>
</subsection>
<subsection name="Example of Usage">
<ul>
<li>
<a href="https://github.com/search?q=path%3Asrc%2Fmain%2Fresources+filename%3Asun_checks.xml+repo%3Acheckstyle%2Fcheckstyle+DesignForExtension">
Sun Style</a>
</li>
<li>
<a href="https://github.com/search?q=path%3Aconfig+filename%3Acheckstyle_checks.xml+repo%3Acheckstyle%2Fcheckstyle+DesignForExtension">
Checkstyle Style</a>
</li>
</ul>
</subsection>
<subsection name="Package">
<p>
com.puppycrawl.tools.checkstyle.checks.design
</p>
</subsection>
<subsection name="Parent Module">
<p>
<a href="config.html#TreeWalker">TreeWalker</a>
</p>
</subsection>
</section>
<section name="FinalClass">
<subsection name="Description">
<p>
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 <code>final</code> there.
</p>
</subsection>
<subsection name="Examples">
<p>
To configure the check:
</p>
<source>
<module name="FinalClass"/>
</source>
</subsection>
<subsection name="Example of Usage">
<ul>
<li>
<a href="https://github.com/search?q=path%3Asrc%2Fmain%2Fresources+filename%3Asun_checks.xml+repo%3Acheckstyle%2Fcheckstyle+FinalClass">
Sun Style</a>
</li>
<li>
<a href="https://github.com/search?q=path%3Aconfig+filename%3Acheckstyle_checks.xml+repo%3Acheckstyle%2Fcheckstyle+FinalClass">
Checkstyle Style</a>
</li>
</ul>
</subsection>
<subsection name="Package">
<p>
com.puppycrawl.tools.checkstyle.checks.design
</p>
</subsection>
<subsection name="Parent Module">
<p>
<a href="config.html#TreeWalker">TreeWalker</a>
</p>
</subsection>
</section>
<section name="HideUtilityClassConstructor">
<subsection name="Description">
<p>
Makes sure that utility classes (classes that contain only static
methods or fields in their API) do not have a public constructor.
</p>
<p>
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.
</p>
<p>
If you make the constructor protected you may want to consider the
following constructor implementation technique to disallow
instantiating subclasses:
</p>
<source>
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) {
// ...
}
}
</source>
</subsection>
<subsection name="Examples">
<p>
To configure the check:
</p>
<source>
<module name="HideUtilityClassConstructor"/>
</source>
</subsection>
<subsection name="Example of Usage">
<ul>
<li>
<a href="https://github.com/search?q=path%3Asrc%2Fmain%2Fresources+filename%3Asun_checks.xml+repo%3Acheckstyle%2Fcheckstyle+HideUtilityClassConstructor">
Sun Style</a>
</li>
<li>
<a href="https://github.com/search?q=path%3Aconfig+filename%3Acheckstyle_checks.xml+repo%3Acheckstyle%2Fcheckstyle+HideUtilityClassConstructor">
Checkstyle Style</a>
</li>
</ul>
</subsection>
<subsection name="Package">
<p>
com.puppycrawl.tools.checkstyle.checks.design
</p>
</subsection>
<subsection name="Parent Module">
<p>
<a href="config.html#TreeWalker">TreeWalker</a>
</p>
</subsection>
</section>
<section name="InnerTypeLast">
<subsection name="Description">
<p>
Check nested (inner) classes/interfaces are declared at the
bottom of the class after all method and field declarations.
</p>
</subsection>
<subsection name="Examples">
<p>
To configure the check:
</p>
<source>
<module name="InnerTypeLast"/>
</source>
</subsection>
<subsection name="Example of Usage">
<ul>
<li>
<a href="https://github.com/search?q=path%3Aconfig+filename%3Acheckstyle_checks.xml+repo%3Acheckstyle%2Fcheckstyle+InnerTypeLast">
Checkstyle Style</a>
</li>
</ul>
</subsection>
<subsection name="Package">
<p>
com.puppycrawl.tools.checkstyle.checks.design
</p>
</subsection>
<subsection name="Parent Module">
<p>
<a href="config.html#TreeWalker">TreeWalker</a>
</p>
</subsection>
</section>
<section name="InterfaceIsType">
<subsection name="Description">
<p>
Implements Joshua Bloch, Effective Java, Item 17 - Use Interfaces only to
define types.
</p>
<p>
According to Bloch, an interface should describe a <em>type</em>.
It is therefore inappropriate to define an interface that does not
contain any methods but only constants. The Standard class <a
href="http://docs.oracle.com/javase/8/docs/api/javax/swing/SwingConstants.html">javax.swing.SwingConstants</a>
is an example of a class that would be flagged by this check.
</p>
<p>
The check can be configured to also disallow marker interfaces like
<code>java.io.Serializable</code>, that do not contain methods or
constants at all.
</p>
</subsection>
<subsection name="Properties">
<table>
<tr>
<th>name</th>
<th>description</th>
<th>type</th>
<th>default value</th>
</tr>
<tr>
<td>allowMarkerInterfaces</td>
<td>
Controls whether marker interfaces like Serializable are
allowed.
</td>
<td><a href="property_types.html#boolean">Boolean</a></td>
<td><code>true</code></td>
</tr>
</table>
</subsection>
<subsection name="Examples">
<p>
To configure the check:
</p>
<source>
<module name="InterfaceIsType"/>
</source>
</subsection>
<subsection name="Example of Usage">
<ul>
<li>
<a href="https://github.com/search?q=path%3Asrc%2Fmain%2Fresources+filename%3Asun_checks.xml+repo%3Acheckstyle%2Fcheckstyle+InterfaceIsType">
Sun Style</a>
</li>
<li>
<a href="https://github.com/search?q=path%3Aconfig+filename%3Acheckstyle_checks.xml+repo%3Acheckstyle%2Fcheckstyle+InterfaceIsType">
Checkstyle Style</a>
</li>
</ul>
</subsection>
<subsection name="Package">
<p>
com.puppycrawl.tools.checkstyle.checks.design
</p>
</subsection>
<subsection name="Parent Module">
<p>
<a href="config.html#TreeWalker">TreeWalker</a>
</p>
</subsection>
</section>
<section name="MutableException">
<subsection name="Description">
<p>
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.
</p>
<p>
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 <code>setStackTrace</code>
which changes the exception's stack trace). But, at least, all information
provided by this exception type is unchangeable.
</p>
<p>
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.
</p>
</subsection>
<subsection name="Properties">
<table>
<tr>
<th>name</th>
<th>description</th>
<th>type</th>
<th>default value</th>
</tr>
<tr>
<td>format</td>
<td>pattern for exception class names</td>
<td><a href="property_types.html#regexp">regular expression</a></td>
<td><code>^.*Exception$|^.*Error$|^.*Throwable$</code></td>
</tr>
<tr>
<td>extendedClassNameFormat</td>
<td>pattern for extended class names</td>
<td><a href="property_types.html#regexp">regular expression</a></td>
<td><code>^.*Exception$|^.*Error$|^.*Throwable$</code></td>
</tr>
</table>
</subsection>
<subsection name="Examples">
<p>
To configure the check:
</p>
<source>
<module name="MutableException"/>
</source>
</subsection>
<subsection name="Example of Usage">
<ul>
<li>
<a href="https://github.com/search?q=path%3Aconfig+filename%3Acheckstyle_checks.xml+repo%3Acheckstyle%2Fcheckstyle+MutableException">
Checkstyle Style</a>
</li>
</ul>
</subsection>
<subsection name="Package">
<p>
com.puppycrawl.tools.checkstyle.checks.design
</p>
</subsection>
<subsection name="Parent Module">
<p>
<a href="config.html#TreeWalker">TreeWalker</a>
</p>
</subsection>
</section>
<section name="OneTopLevelClass">
<subsection name="Description">
<p>
Checks that each top-level class, interface or
enum resides in a source file of its own.
Official description of a 'top-level' term:<a
href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-7.html#jls-7.6">7.6. Top Level Type Declarations</a>.
If file doesn't contains public class, enum or interface,
top-level type is the first type in file.
</p>
</subsection>
<subsection name="Examples">
<p>
An example of check's configuration:
</p>
<source>
<module name="OneTopLevelClass"/>
</source>
<p>
<b>ATTENTION:</b> This Check does not support customization of validated tokens, so do not use the "tokens" property.
</p>
<p>
An example of code with violations:
</p>
<source>
public class Foo{
//methods
}
class Foo2{
//methods
}
</source>
<p>
An example of code without public top-level type:
</p>
<source>
class Foo{ // top-level class
//methods
}
class Foo2{
//methods
}
</source>
<p>
An example of code without violations:
</p>
<source>
public class Foo{
//methods
}
</source>
</subsection>
<subsection name="Example of Usage">
<ul>
<li>
<a href="https://github.com/search?q=path%3Asrc%2Fmain%2Fresources+filename%3Agoogle_checks.xml+repo%3Acheckstyle%2Fcheckstyle+OneTopLevelClass">
Google Style</a>
</li>
<li>
<a href="https://github.com/search?q=path%3Aconfig+filename%3Acheckstyle_checks.xml+repo%3Acheckstyle%2Fcheckstyle+OneTopLevelClass">
Checkstyle Style</a>
</li>
</ul>
</subsection>
<subsection name="Package">
<p>
com.puppycrawl.tools.checkstyle.checks.design
</p>
</subsection>
<subsection name="Parent Module">
<p>
<a href="config.html#TreeWalker">TreeWalker</a>
</p>
</subsection>
</section>
<section name="ThrowsCount">
<subsection name="Description">
<p>
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.
</p>
<p>
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 <code>catch(Exception ex)</code>.
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.
</p>
<p>
<b>ignorePrivateMethods</b> - allows to skip private methods as they do
not cause problems for other classes.
</p>
</subsection>
<subsection name="Properties">
<table>
<tr>
<th>name</th>
<th>description</th>
<th>type</th>
<th>default value</th>
</tr>
<tr>
<td>max</td>
<td>maximum allowed number of throws statements</td>
<td><a href="property_types.html#integer">Integer</a></td>
<td><code>4</code></td>
</tr>
<tr>
<td>ignorePrivateMethods</td>
<td>whether private methods must be ignored</td>
<td><a href="property_types.html#boolean">Boolean</a></td>
<td><code>true</code></td>
</tr>
</table>
</subsection>
<subsection name="Examples">
<p>
To configure the check so that it doesn't allow more than two throws
per method:
</p>
<source>
<module name="ThrowsCount">
<property name="max" value="2"/>
</module>
</source>
<p>
To configure the check so that it doesn't skip private methods:
</p>
<source>
<module name="ThrowsCount">
<property name="ignorePrivateMethods" value="false"/>
</module>
</source>
</subsection>
<subsection name="Example of Usage">
<ul>
<li>
<a href="https://github.com/search?q=path%3Aconfig+filename%3Acheckstyle_checks.xml+repo%3Acheckstyle%2Fcheckstyle+ThrowsCount">
Checkstyle Style</a>
</li>
</ul>
</subsection>
<subsection name="Package">
<p>
com.puppycrawl.tools.checkstyle.checks.design
</p>
</subsection>
<subsection name="Parent Module">
<p>
<a href="config.html#TreeWalker">TreeWalker</a>
</p>
</subsection>
</section>
<section name="VisibilityModifier">
<subsection name="Description">
<p>
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 <code>protectedAllowed</code> or <code>packageAllowed</code> is set.
</p>
<p>
Public members are not flagged if the name matches the public
member regular expression (contains <code>"^serialVersionUID$"</code> by default).
</p>
<p>Note that
Checkstyle 2 used to include <code>"^f[A-Z][a-zA-Z0-9]*$"</code> 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.
</p>
<p>
Rationale: Enforce encapsulation.
</p>
<p>
Check also has options making it less strict:
</p>
<p>
<b>ignoreAnnotationCanonicalNames</b> - 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
</p>
<p>
<b>allowPublicImmutableFields</b> - which allows immutable fields be declared as
public if defined in final class. Default value is <b>true</b>
</p>
<p>
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)
</p>
<p>
Classes known to be immutable are listed in <b>immutableClassCanonicalNames</b> by their
<b>canonical</b> names.
</p>
<p>
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.
</p>
<p>
<b>Restriction</b>: 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
<b>if current field is immutable and defined in final class</b>
</p>
<p>
Star imports are out of scope of this Check. So if one of type imported via
<b>star import</b> collides with user specified one by its short name -
there won't be Check's violation.
</p>
</subsection>
<subsection name="Properties">
<table>
<tr>
<th>name</th>
<th>description</th>
<th>type</th>
<th>default value</th>
</tr>
<tr>
<td>packageAllowed</td>
<td>whether package visible members are allowed</td>
<td><a href="property_types.html#boolean">boolean</a></td>
<td><code>false</code></td>
</tr>
<tr>
<td>protectedAllowed</td>
<td>whether protected members are allowed</td>
<td><a href="property_types.html#boolean">boolean</a></td>
<td><code>false</code></td>
</tr>
<tr>
<td>publicMemberPattern</td>
<td>pattern for public members that should be ignored</td>
<td><a href="property_types.html#regexp">regular expression</a></td>
<td><code>^serialVersionUID$</code></td>
</tr>
<tr>
<td>allowPublicImmutableFields</td>
<td>allows immutable fields be declared as public if defined in final class</td>
<td><a href="property_types.html#boolean">boolean</a></td>
<td><code>true</code></td>
</tr>
<tr>
<td>immutableClassCanonicalNames</td>
<td>immutable classes canonical names</td>
<td><a href="property_types.html#stringSet">String Set</a></td>
<td>java.lang.String, java.lang.Integer, java.lang.Byte, java.lang.Character,
java.lang.Short, java.lang.Boolean, java.lang.Long, java.lang.Double, java.lang.Float,
java.lang.StackTraceElement, java.math.BigInteger, java.math.BigDecimal, java.io.File,
java.util.Locale, java.util.UUID, java.net.URL, java.net.URI,
java.net.Inet4Address, java.net.Inet6Address, java.net.InetSocketAddress,</td>
</tr>
<tr>
<td>ignoreAnnotationCanonicalNames</td>
<td>ignore annotations canonical names</td>
<td><a href="property_types.html#stringSet">String Set</a></td>
<td>org.junit.Rule, com.google.common.annotations.VisibleForTesting</td>
</tr>
</table>
</subsection>
<subsection name="Examples">
<p>
To configure the check:
</p>
<source>
<module name="VisibilityModifier"/>
</source>
<p>
To configure the check so that it allows package visible members:
</p>
<source>
<module name="VisibilityModifier">
<property name="packageAllowed" value="true"/>
</module>
</source>
<p>
To configure the check so that it allows no public members:
</p>
<source>
<module name="VisibilityModifier">
<property name="publicMemberPattern" value="^$"/>
</module>
</source>
<p>
To configure the Check so that it allows public immutable fields
(mostly for immutable classes):
</p>
<source>
<module name="VisibilityModifier"/>
</source>
<p>
Example of allowed public immutable fields:
</p>
<source>
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;
}
}
</source>
<p>
To configure the Check which allows user specified immutable class names:
</p>
<source>
<module name="VisibilityModifier">
<property name="immutableClassCanonicalNames" value="
com.google.common.collect.ImmutableSet"/>
</module>
</source>
<p>
Example of allowed public immutable fields:
</p>
<source>
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;
}
}
</source>
<p>
To configure the Check passing fields annotated with @com.annotation.CustomAnnotation:
</p>
<source>
<module name="VisibilityModifier">
<property name="ignoreAnnotationCanonicalNames" value=
"com.annotation.CustomAnnotation"/>
</module>
</source>
<p>
Example of allowed field:
</p>
<source>
class SomeClass
{
@com.annotation.CustomAnnotation
String annotatedString; // no warning
@CustomAnnotation
String shortCustomAnnotated; // no warning
}
</source>
<p>
To configure the Check passing fields annotated with @org.junit.Rule and
@com.google.common.annotations.VisibleForTesting annotations:
</p>
<source>
<module name="VisibilityModifier"/>
</source>
<p>
Example of allowed fields:
</p>
<source>
class SomeClass
{
@org.junit.Rule
public TemporaryFolder publicJUnitRule = new TemporaryFolder(); // no warning
@com.google.common.annotations.VisibleForTesting
public String testString = ""; // no warning
}
</source>
<p>
To configure the Check passing fields annotated with short annotation name:
</p>
<source>
<module name="VisibilityModifier">
<property name="ignoreAnnotationCanonicalNames"
value="CustomAnnotation"/>
</module>
</source>
<p>
Example of allowed fields:
</p>
<source>
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
}
</source>
</subsection>
<subsection name="Example of Usage">
<ul>
<li>
<a href="https://github.com/search?q=path%3Asrc%2Fmain%2Fresources+filename%3Asun_checks.xml+repo%3Acheckstyle%2Fcheckstyle+VisibilityModifier">
Sun Style</a>
</li>
<li>
<a href="https://github.com/search?q=path%3Aconfig+filename%3Acheckstyle_checks.xml+repo%3Acheckstyle%2Fcheckstyle+VisibilityModifier">
Checkstyle Style</a>
</li>
</ul>
</subsection>
<subsection name="Package">
<p>
com.puppycrawl.tools.checkstyle.checks.design
</p>
</subsection>
<subsection name="Parent Module">
<p>
<a href="config.html#TreeWalker">TreeWalker</a>
</p>
</subsection>
</section>
</body>
</document>
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