Checkstyle 5.5

##### About

- Checkstyle
- Release Notes

##### Documentation

- Configuration
  - Property Types
- Running
  - Ant Task
  - Command Line
- Available Checks
- Standard Checks
  - Annotations
  - Block Checks
  - Class Design
  - Coding
  - Duplicate Code
  - Headers
  - Imports
  - Javadoc Comments
  - Metrics
  - Miscellaneous
  - Modifiers
  - Naming Conventions
  - Regexp
  - Size Violations
  - Whitespace
- Extending Checkstyle
  - Writing checks
  - Writing filters
  - Writing listeners

##### Developers

- Javadoc
- Project Page
- Contributing

##### Project Documentation

- Project Information
- Project Reports

## ArrayTrailingComma

### Description

Checks that array initialization contains a trailing comma.

    int[] a = new int[]
    {
        1,
        2,
        3,
    };

The check allows to not add a comma if both left and right curlys are on the same line.

    return new int[] { 0 };

Rationale: Putting this comma in makes it easier to change the order of the elements or add new elements on the end.

### Examples

To configure the check:

    <module name="ArrayTrailingComma"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## AvoidInlineConditionals

### Description

Detects inline conditionals. An example inline conditional is this:

    String a = getParameter("a");
    String b = (a==null || a.length<1) ? null : a.substring(1);

Rationale: Some developers find inline conditionals hard to read, so their company's coding standards forbids them.

### Examples

To configure the check:

    <module name="AvoidInlineConditionals"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## CovariantEquals

### Description

Checks that classes that define a covariant `equals()` method also override method `equals(java.lang.Object)`. Inspired by findbugs.

Rationale: Mistakenly defining a covariant `equals()` method without overriding method `equals(java.lang.Object)` can produce unexpected runtime behaviour.

### Example

To configure the check:

    <module name="CovariantEquals"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## DoubleCheckedLocking

### Description

The "double-checked locking" idiom (DCL) tries to avoid the runtime cost of synchronization. An example that uses the DCL idiom is this:

    public class MySingleton
    {
        private static theInstance = null;

        private MySingleton() {}

        public MySingleton getInstance() {
            if ( theInstance == null ) { // synchronize only if necessary
                synchronized( MySingleton.class ) {
                    if ( theInstance == null ) {
                        theInstance = new MySingleton();
                    }
                }
            }
        }
    }

The problem with the DCL idiom in Java is that it just does not work correctly. Using it introduces bugs that are extremely hard to track down and reproduce. The "Double-Checked Locking is Broken" Declaration has an in depth explanation of the exact problem which has to do with the semantics of the Java memory model.

The DoubleCheckedLocking check will find source code where a test is wrapped in a synchronized block that is wrapped in the same test, like in the example above.

### Examples

To configure the check:

    <module name="DoubleCheckedLocking"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## EmptyStatement

### Description

Detects empty statements (standalone ;).

### Examples

To configure the check:

    <module name="EmptyStatement"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## EqualsAvoidNull

### Description

Checks that any combination of String literals with optional assignment is on the left side of an `equals()` comparison. The check also processes `String.equalsIgnoreCase()` invocations (which can be suppressed).

Rationale: Calling the `equals()` method on String literals will avoid a potential NullPointerException. Also, it is pretty common to see null check right before equals comparisons which is not necessary in the below example.

For example:

    String nullString = null;
    nullString.equals("My_Sweet_String");

should be refactored to:

    String nullString = null;
    "My_Sweet_String".equals(nullString);

Limitations: If the equals method is overridden or a covariant equals method is defined and the implementation is incorrect (where `s.equals(t)` does not return the same result as `t.equals(s)`) then rearranging the called on object and parameter may have unexpected results

Java's Autoboxing feature has an affect on how this check is implemented. Pre Java 5 all IDENT + IDENT object concatenations would not cause a NullPointerException even if null. Those situations could have been included in this check. They would simply act as if they surrounded by `String.valueof()` which would concatenate the String null.

The following example will cause a NullPointerException as a result of what autoboxing does.

    Integer i = null, j = null;
    String number = "5"
    number.equals(i + j);

Since, it is difficult to determine what kind of Object is being concatenated all ident concatenation is considered unsafe.

### Properties

name | description | type | default value
--- | --- | --- | ---
ignoreEqualsIgnoreCase | whether to ignore `String.equalsIgnoreCase()` invocations | boolean | false

### Example

To configure the check:

    <module name="EqualsAvoidNull"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## EqualsHashCode

### Description

Checks that classes that override `equals()` also override `hashCode()`.

Rationale: The contract of `equals()` and `hashCode()` requires that equal objects have the same hashCode. Hence, whenever you override `equals()` you must override `hashCode()` to ensure that your class can be used in collections that are hash based.

### Example

To configure the check:

    <module name="EqualsHashCode"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## FinalLocalVariable

### Description

Checks that local variables that never have their values changed are declared final. The check can be configured to also check that unchanged parameters are declared final.

### Notes

When configured to check parameters, the check ignores parameters of interface methods and abstract methods.

### Properties

name | description | type | default value
--- | --- | --- | ---
tokens | tokens to check | subset of tokens PARAMETER_DEF, VARIABLE_DEF | VARIABLE_DEF

### Examples

To configure the check:

    <module name="FinalLocalVariable"/>

To configure the check so that it checks local variables and parameters:

    <module name="FinalLocalVariable">
        <property name="tokens" value="VARIABLE_DEF,PARAMETER_DEF"/>
    </module>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## HiddenField

### Description

Checks that a local variable or a parameter does not shadow a field that is defined in the same class.

### Properties

name | description | type | default value
--- | --- | --- | ---
tokens | tokens to check | subset of tokens PARAMETER_DEF, VARIABLE_DEF | PARAMETER_DEF, VARIABLE_DEF
ignoreFormat | pattern for names to ignore | regular expression | (not applied)
ignoreConstructorParameter | Controls whether to ignore constructor parameters. | Boolean | `false`
ignoreSetter | Controls whether to ignore the parameter of a property setter method, where the property setter method for field "xyz" has name "setXyz", one parameter named "xyz", and return type `void`. | Boolean | `false`
ignoreAbstractMethods | Controls whether to ignore parameters of abstract methods. | Boolean | `false`

### Examples

To configure the check:

    <module name="HiddenField"/>

To configure the check so that it checks local variables but not parameters:

    <module name="HiddenField">
        <property name="tokens" value="VARIABLE_DEF"/>
    </module>

To configure the check so that it ignores the name "rcsid":

    <module name="HiddenField">
        <property name="ignoreFormat" value="^rcsid$"/>
    </module>

To configure the check so that it ignores constructor parameters:

    <module name="HiddenField">
        <property name="ignoreConstructorParameter" value="true"/>
    </module>

To configure the check so that it ignores the parameter of setter methods:

    <module name="HiddenField">
        <property name="ignoreSetter" value="true"/>
    </module>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## IllegalInstantiation

### Description

Checks for illegal instantiations where a factory method is preferred.

Rationale: Depending on the project, for some classes it might be preferable to create instances through factory methods rather than calling the constructor.

A simple example is the `java.lang.Boolean` class. In order to save memory and CPU cycles, it is preferable to use the predefined constants `TRUE` and `FALSE`. Constructor invocations should be replaced by calls to `Boolean.valueOf()`.

Some extremely performance sensitive projects may require the use of factory methods for other classes as well, to enforce the usage of number caches or object pools.

### Notes

There is a limitation that it is currently not possible to specify array classes.

### Properties

name | description | type | default value
--- | --- | --- | ---
classes | classes that should not be instantiated | String Set | {}

### Example

To configure the check to find instantiations of java.lang.Boolean:

    <module name="IllegalInstantiation">
        <property name="classes" value="java.lang.Boolean"/>
    </module>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## IllegalToken

### Description

Checks for illegal tokens.

Rational: Certain language features often lead to hard to maintain code or are non-obvious to novice developers. Other features may be discouraged in certain frameworks, such as not having native methods in EJB components.

### Properties

name | description | type | default value
--- | --- | --- | ---
tokens | tokens to check | subset of TokenTypes, | LITERAL_SWITCH, POST_INC, POST_DEC

### Example

To configure the check to find token LITERAL_NATIVE:

    <module name="IllegalToken">
        <property name="tokens" value="LITERAL_NATIVE"/>
    </module>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## IllegalTokenText

### Description

Checks for illegal token text.

### Properties

name | description | type | default value
--- | --- | --- | ---
tokens | tokens to check | subset of TokenTypes | empty
format | illegal pattern | regular expression | `^$` (empty)
ignoreCase | Controls whether to ignore case when matching. | Boolean | `false`
message | Message which is used to notify about violations; if empty then the default message is used. | String | `""`(empty)

### Examples

To configure the check to forbid String literals containing `"a href"`:

    <module name="IllegalTokenText">
        <property name="tokens" value="STRING_LITERAL"/>
        <property name="format" value="a href"/>
    </module>

To configure the check to forbid leading zeros in an integer literal, other than zero and a hex literal:

    <module name="IllegalTokenText">
        <property name="tokens" value="NUM_INT,NUM_LONG"/>
        <property name="format" value="^0[^lx]"/>
        <property name="ignoreCase" value="true"/>
    </module>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## InnerAssignment

### Description

Checks for assignments in subexpressions, such as in `String s = Integer.toString(i = 2);`.

Rationale: With the exception of `for` iterators, all assignments should occur in their own toplevel statement to increase readability. With inner assignments like the above it is difficult to see all places where a variable is set.

### Properties

name | description | type | default value
--- | --- | --- | ---
tokens | assignments to check | subset of tokens ASSIGN, BAND_ASSIGN, BOR_ASSIGN, BSR_ASSIGN, BXOR_ASSIGN, DIV_ASSIGN, MINUS_ASSIGN, MOD_ASSIGN, PLUS_ASSIGN, SL_ASSIGN, SR_ASSIGN, STAR_ASSIGN | all tokens

### Examples

To configure the check:

    <module name="InnerAssignment"/>

To configure the check for only `=`, `+=`, and `-=` operators:

    <module name="InnerAssignment">
        <property name="tokens" value="ASSIGN,PLUS_ASSIGN,MINUS_ASSIGN"/>
    </module>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## MagicNumber

### Description

Checks that there are no "magic numbers", where a magic number is a numeric literal that is not defined as a constant. By default, -1, 0, 1, and 2 are not considered to be magic numbers.

### Properties

name | description | type | default value
--- | --- | --- | ---
tokens | tokens to check | subset of tokens NUM_DOUBLE, NUM_FLOAT, NUM_INT, NUM_LONG | all tokens
ignoreNumbers | non-magic numbers | list of numbers | -1, 0, 1, 2
ignoreHashCodeMethod | ignore magic numbers in hashCode methods | boolean | false
ignoreAnnotation | ignore magic numbers in annotation declarations. | boolean | false

### Examples

To configure the check:

    <module name="MagicNumber"/>

To configure the check so that it checks floating-point numbers that are neither 0, 0.5, nor 1:

    <module name="MagicNumber">
        <property name="tokens" value="NUM_DOUBLE, NUM_FLOAT"/>
        <property name="ignoreNumbers" value="0, 0.5, 1"/>
    </module>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## MissingSwitchDefault

### Description

Checks that switch statement has "default" clause.

Rationale: It's usually a good idea to introduce a default case in every switch statement. Even if the developer is sure that all currently possible cases are covered, this should be expressed in the default branch, e.g. by using an assertion. This way the code is protected aginst later changes, e.g. introduction of new types in an enumeration type.

### Examples

To configure the check:

    <module name="MissingSwitchDefault"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## ModifiedControlVariable

### Description

Check for ensuring that for loop control variables are not modified inside the for block. An example is:

    for (int i = 0; i < 1; i++) {
        i++;
    }

Rationale: If the control variable is modified inside the loop body, the program flow becomes more difficult to follow. An option is to replace the for loop with a while loop.

### Example

To configure the check:

    <module name="ModifiedControlVariable"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## RedundantThrows

### Description

Checks for redundant exceptions declared in throws clause such as duplicates, unchecked exceptions or subclasses of another declared exception.

### Properties

name | description | type | default value
--- | --- | --- | ---
allowUnchecked | whether unchecked exceptions in throws are allowed or not | boolean | `false`
allowSubclasses | whether subclass of another declared exception is allowed in throws clause | boolean | `false`

logLoadErrors

This check may need to load exception classes mentioned in the @throws tag to check whether they are RuntimeExceptions. If loading the class fails, this property allows to control checkstyle's error handling. If set to `false` a classpath configuration problem is assumed and the TreeWalker stops operating on the class completely. If set to `true` (the default), checkstyle assumes a typo or refactoring problem in the javadoc and logs the problem in the normal checkstyle report (potentially masking a configuration error).

boolean

`true`

suppressLoadErrors | When logLoadErrors is set to true, the TreeWalker completely processes a class and displays any problems with loading exceptions as checkstyle violations. When this property is set to true, the violations generated when logLoadErrors is set true are suppressed from being reported as violations in the checkstyle report. | boolean | `false`

### Examples

To configure the default check:

    <module name="RedundantThrows"/>

To configure the check to allow unchecked exception in throws clause

    <module name="RedundantThrows">
        <property name="allowUnchecked" value="true"/>
    </module>

### Note

The classpath should be configured to locate the class information. The classpath configuration is dependent on the mechanism used to invoke Checkstyle.

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## SimplifyBooleanExpression

### Description

Checks for overly complicated boolean expressions. Currently finds code like `if (b == true)`, `b || true`, `!false`, etc.

Rationale: Complex boolean logic makes code hard to understand and maintain.

### Example

To configure the check:

    <module name="SimplifyBooleanExpression"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## SimplifyBooleanReturn

### Description

Checks for overly complicated boolean return statements. For example the following code

    if (valid())
        return false;
    else
        return true;

could be written as

    return !valid();

The Idea for this Check has been shamelessly stolen from the equivalent PMD rule.

### Example

To configure the check:

    <module name="SimplifyBooleanReturn"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## StringLiteralEquality

### Description

Checks that string literals are not used with `==` or `!=`.

Rationale: Novice Java programmers often use code like:

    if (x == "something")

when they mean

    if ("something".equals(x))

### Example

To configure the check:

    <module name="StringLiteralEquality"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## NestedForDepth

### Description

Restricts nested `for` blocks to a specified depth (default = 1).

### Properties

name | description | type | default value
--- | --- | --- | ---
max | allowed nesting depth | Integer | 1

### Example

To configure the check:

    <module name="NestedForDepth"/>

To configure the check to allow nesting depth 3:

    <module name="NestedForDepth">
        <property name="max" value="3"/>
    </module>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## NestedIfDepth

### Description

Restricts nested if-else blocks to a specified depth (default = 1).

### Properties

name | description | type | default value
--- | --- | --- | ---
max | allowed nesting depth | Integer | `1`

### Example

To configure the check:

    <module name="NestedIfDepth"/>

To configure the check to allow nesting depth 3:

    <module name="NestedIfDepth">
        <property name="max" value="3"/>
    </module>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## NestedTryDepth

### Description

Restricts nested try blocks to a specified depth (default = 1).

### Properties

name | description | type | default value
--- | --- | --- | ---
max | allowed nesting depth | Integer | `1`

### Example

To configure the check:

    <module name="NestedTryDepth"/>

To configure the check to allow nesting depth 3:

    <module name="NestedTryDepth">
        <property name="max" value="3"/>
    </module>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## NoClone

### Description

Checks that the clone method is not overridden from the Object class.

Rationale: The clone method relies on strange/hard to follow rules that do not work it all situations. Consequently, it is difficult to override correctly. Below are some of the rules/reasons why the clone method should be avoided.

- Classes supporting the clone method should implement the Cloneable interface but the Cloneable interface does not include the clone method. As a result, it doesn't enforce the method override.
- The Cloneable interface forces the Object's clone method to work correctly. Without implementing it, the Object's clone method will throw a CloneNotSupportedException.
- Non-final classes must return the object returned from a call to super.clone().
- Final classes can use a constructor to create a clone which is different from non-final classes.
- If a super class implements the clone method incorrectly all subclasses calling super.clone() are doomed to failure.
- If a class has references to mutable objects then those object references must be replaced with copies in the clone method after calling super.clone().
- The clone method does not work correctly with final mutable object references because final references cannot be reassigned.
- If a super class overrides the clone method then all subclasses must provide a correct clone implementation.

Two alternatives to the clone method, in some cases, is a copy constructor or a static factory method to return copies of an object. Both of these approaches are simpler and do not conflict with final fields. The do not force the calling client to handle a CloneNotSuportException. They also are typed therefore no casting is necessary. Finally, they are more flexible since they can take interface types rather than concrete classes.

Sometimes a copy constructor or static factory is not an acceptable alternative to the clone method. The example below highlights the limitation of a copy constructor (or static factory). Assume Square is a subclass for Shape.

           Shape s1 = new Square();
           System.out.println(s1 instanceof Square); //true

...assume at this point the code knows nothing of s1 being a Square that's the beauty of polymorphism but the code wants to copy the Square which is declared as a Shape, its super type...

           Shape s2 = new Shape(s1); //using the copy constructor
           System.out.println(s2 instanceof Square); //false

The working solution (without knowing about all subclasses and doing many casts) is to do the following (assuming correct clone implementation).

           Shape s2 = s1.clone();
           System.out.println(s2 instanceof Square); //true

Just keep in mind if this type of polymorphic cloning is required then a properly implemented clone method may be the best choice.

Much of this information was taken from Effective Java: Programming Language Guide First Edition by Joshua Bloch pages 45-52. Give Bloch credit for writing an excellent book.

This check is almost exactly the same as the {@link NoFinalizerCheck}

### Example

To configure the check:

    <module name="NoClone"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## NoFinalizer

### Description

Verifies there are no `finalize()` methods defined in a class.

### Example

To configure the check:

    <module name="NoFinalizer"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## SuperClone

### Description

Checks that an overriding `clone()` method invokes `super.clone()`.

Reference: Object.clone().

### Examples

To configure the check:

    <module name="SuperClone"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## SuperFinalize

### Description

Checks that an overriding `finalize()` method invokes `super.finalize()`.

Reference: Cleaning Up Unused Objects.

### Examples

To configure the check:

    <module name="SuperFinalize"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## IllegalCatch

### Description

Catching java.lang.Exception, java.lang.Error or java.lang.RuntimeException is almost never acceptable.

Rationale: Junior developers often simply catch Exception in an attempt to handle multiple exception classes. This unfortunately leads to code that inadvertantly catchs NPE, OutOfMemoryErrors, etc.

### Properties

name | description | type | default value
--- | --- | --- | ---
illegalClassNames | exception class names to reject | list of strings | `"java.lang.Exception, java.lang.Throwable, java.lang.RuntimeException"`

### Examples

To configure the check:

    <module name="IllegalCatch"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## IllegalThrows

### Description

This check can be used to ensure that types are not declared to be thrown. Declaring to throw java.lang.Error or java.lang.RuntimeException is almost never acceptable.

### Properties

name | description | type | default value
--- | --- | --- | ---
illegalClassNames | throw class names to reject | list of strings | `"java.lang.Throwable, java.lang.Error, java.lang.RuntimeException"`
ignoredMethodNames | names of methods to ignore | list of strings | `finalize`

### Examples

To configure the check:

    <module name="IllegalThrows"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## PackageDeclaration

### Description

Ensure a class is has a package declaration, and (optionally) whether the package name matches the directory name for the source file.

Rationale: Classes that live in the null package cannot be imported. Many novice developers are not aware of this.

### Properties

name | description | type | default value
--- | --- | --- | ---
ignoreDirectoryName | whether to ignore checking that the package declaration matches the source directory name | boolean | `false`

### Examples

To configure the check:

    <module name="PackageDeclaration"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## JUnitTestCase

### Description

Ensures that the setUp(), tearDown()methods are named correctly, have no arguments, return void and are either public or protected.

Also ensures that suite() is named correctly, have no arguments, return junit.framewotk.Test, public and static.

Rationale: often times developers will misname one or more of these methods and not realise that the method is not being called.

### Examples

To configure the check:

    <module name="JUnitTestCase"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## ReturnCount

### Description

Restricts the number of return statements. Default = 2. Ignores specified methods (`equals()` by default).

Rationale: Too many return points can be indication that code is attempting to do too much or may be difficult to understand.

### Properties

name | description | type | default value
--- | --- | --- | ---
max | maximum allowed number of return statments | Integer | `2`
format | method names to ingone | regular expression | `^equals$` (empty)

### Examples

To configure the check so that it doesn't allow more than three return statements per method (`equals()` method ignored):

    <module name="ReturnCount">
        <property name="max" value="3"/>
    </module>

To configure the check so that it doesn't allow more than three return statements per method for all methods:

    <module name="ReturnCount">
        <property name="max" value="3"/>
        <property name="format" value="^$"/>
    </module>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## IllegalType

### Description

Checks that particular class are never used as types in variable declarations, return values or parameters. Includes a pattern check that by default disallows abstract classes.

Rationale: Helps reduce coupling on concrete classes. In addition abstract classes should be thought of a convenience base class implementations of interfaces and as such are not types themselves.

### Properties

name | description | type | default value
--- | --- | --- | ---
tokens | tokens to check | subset of tokens PARAMETER_DEF, VARIABLE_DEF METHOD_DEF | PARAMETER_DEF, VARIABLE_DEF METHOD_DEF
illegalClassNames | classes that should not be used as types in variable declarations, return values or parameters. | String Set | "java.util.GregorianCalendar, java.util.Hashtable, java.util.HashSet, java.util.HashMap, java.util.ArrayList, java.util.LinkedList, java.util.LinkedHashMap, java.util.LinkedHashSet, java.util.TreeSet, java.util.TreeMap, java.util.Vector"
legalAbstractClassNames | abstract classes that may be used as types. | String Set |
ignoredMethodNames | methods that should not be checked | String Set | "getInitialContext, getEnvironment"
format | pattern for illegal class name | regular expression | `^(.*[\\.])?Abstract.*$`

### Examples

To configure the check so that it ignore getInstance() method:

    <module name="IllegalType">
        <property name="ignoredMethodNames" value="getInstance"/>
    </module>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## DeclarationOrder

### Description

According to Code Conventions for the Java Programming Language , the parts of a class or interface declaration should appear in the following order:

1. Class (static) variables. First the public class variables, then the protected, then package level (no access modifier), and then the private.
2. Instance variables. First the public class variables, then the protected, then package level (no access modifier), and then the private.
3. Constructors
4. Methods

### Properties

name | description | type | default value
--- | --- | --- | ---
ignoreConstructors | whether to ignore constructors | Boolean | `false`
ignoreMethods | whether to ignore methods | Boolean | `false`
ignoreModifiers | whether to ignore modifiers | Boolean | `false`

### Examples

To configure the check:

    <module name="DeclarationOrder"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## ParameterAssignment

### Description

Disallow assignment of parameters.

Rationale: Parameter assignment is often considered poor programming practice. Forcing developers to declare parameters as final is often onerous. Having a check ensure that parameters are never assigned would give the best of both worlds.

### Examples

To configure the check:

    <module name="ParameterAssignment"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## ExplicitInitialization

### Description

Checks if any class or object member explicitly initialized to default for its type value (`null` for object references, zero for numeric types and `char` and `false` for `boolean`.

Rationale: each instance variable gets initialized twice, to the same value. Java initializes each instance variable to its default value (0 or null) before performing any initialization specified in the code. So in this case, x gets initialized to 0 twice, and bar gets initialized to null twice. So there is a minor inefficiency. This style of coding is a hold-over from C/C++ style coding, and it shows that the developer isn't really confident that Java really initializes instance variables to default values.

### Examples

To configure the check:

    <module name="ExplicitInitialization"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## DefaultComesLast

### Description

Check that the `default` is after all the `case`s in a `switch` statement.

Rationale: Java allows `default` anywhere within the `switch` statement. But it is more readable if it comes after the last `case`.

### Examples

To configure the check:

    <module name="DefaultComesLast"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## MissingCtor

### Description

Checks that classes (except abtract one) define a ctor and don't rely on the default one.

### Examples

To configure the check:

    <module name="MissingCtor"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## FallThrough

### Description

Checks for fall through in `switch` statements Finds locations where a `case` contains Java code - but lacks a `break`, `return`, `throw` or `continue` statement.

The check honores special comments to supress the warning. By default the text "fallthru", "fall through", "fallthrough", "falls through" and "fallsthrough" are recognized (case sensitive). The comment containing this words must be a one-liner and must be on the last none-empty line before the `case` triggering the warning or on the same line before the `case` (urgly, but possible).

    switch (i){
    case 0:
        i++; // fall through

    case 1:
        i++;
        // falls through
    case 2: {
        i++;
    }
    // fallthrough
    case 3:
        i++;
    /* fallthru */case 4:
        i++
        break;
    }

Note: the check works in assumption that there is no unreachable code in the `case`.

### Properties

name | description | type | default value
--- | --- | --- | ---
checkLastCaseGroup | Whether we need to check last case group or not. | Boolean | `false`
reliefPattern | Regulare expression to match the relief comment that supresses the warning about a fall through. | regular expression | `fallthru|falls? ?through`

### Examples

To configure the check:

    <module name="FallThrough"/>

or

    <module name="FallThrough">
        <property name="reliefPattern" value="continue in next case"/>
    <module name="FallThrough"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## MultipleStringLiterals

### Description

Checks for multiple occurrences of the same string literal within a single file.

Rationale: Code duplication makes maintenance more difficult, so it can be better to replace the multiple occurrences with a constant.

### Properties

name | description | type | default value
--- | --- | --- | ---
allowedDuplicates | The maximum number of occurences to allow without generating a warning | Integer | 1
ignoreStringsRegexp | regexp pattern for ignored strings (with quotation marks) | regular expression | `^""$` (ignore empty strings)
ignoreOccurrenceContext | Token type names where duplicate strings are ignored even if they don't match ignoredStringsRegexp. This allows you to exclude syntactical contexts like Annotations or static initializers from the check. | list of token type names | `ANNOTATION` (ignore strings inside the context of an annotation)

### Examples

To configure the check:

    <module name="MultipleStringLiterals"/>

To configure the check so that it allows two occurrences of each string:

    <module name="MultipleStringLiterals">
        <property name="allowedDuplicates" value="2"/>
    </module>

To configure the check so that it ignores ", " and empty strings:

    <module name="MultipleStringLiterals">
        <property name="ignoreStringsRegexp" value='^(("")|(", "))$'/>
    </module>

To configure the check so that it flags duplicate strings in all syntactical contexts, even in annotations like `@SuppressWarnings("unchecked")`:

    <module name="MultipleStringLiterals">
        <property name="ignoreOccurrenceContext" value=""/>
    </module>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## MultipleVariableDeclarations

### Description

Checks that each variable declaration is in its own statement and on its own line.

Rationale: the SUN Code conventions chapter 6.1 recommends that declarations should be one per line/statement.

### Examples

To configure the check:

    <module name="MultipleVariableDeclarations"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## RequireThis

### Description

Checks that code doesn't rely on the "this." default, i.e. references to instance variables and methods of the present object are explicitly of the form "this.varName" or "this.methodName(args)".

### Properties

name | description | type | default value
--- | --- | --- | ---
checkFields | whether we should check fields usage or not | boolean | `true`
checkMethods | whether we should check methods usage or not | boolean | `true`

### Examples

To configure the default check:

    <module name="RequireThis"/>

To configure to check `this` qualifier for fields only:

    <module name="RequireThis">
        <property name="checkMethods" value="false"/>
    </module>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## UnnecessaryParentheses

### Description

Checks for the use of unnecessary parentheses.

### Examples

To configure the check:

    <module name="UnnecessaryParentheses"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker

## OneStatementPerLine

### Description

Checks there is only one statement per line. The following line will be flagged as an error:

        x = 1; y = 2; // Two statments on a single line.

### Examples

To configure the check:

    <module name="OneStatementPerLine"/>

### Package

com.puppycrawl.tools.checkstyle.checks.coding

### Parent Module

TreeWalker