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java.lang.Object org.apache.commons.lang.enum.Enum
org.apache.commons.lang.enums.Enum
and will be removed in version 3.0. All classes in this package are deprecated and repackaged to
org.apache.commons.lang.enums
since enum
is a Java 1.5 keyword.
public abstract class Enum
Abstract superclass for type-safe enums.
One feature of the C programming language lacking in Java is enumerations. The C implementation based on ints was poor and open to abuse. The original Java recommendation and most of the JDK also uses int constants. It has been recognised however that a more robust type-safe class-based solution can be designed. This class follows the basic Java type-safe enumeration pattern.
NOTE:Due to the way in which Java ClassLoaders work, comparing
Enum objects should always be done using equals()
, not ==
.
The equals() method will try == first so in most cases the effect is the same.
Of course, if you actually want (or don't mind) Enums in different class
loaders being non-equal, then you can use ==
.
To use this class, it must be subclassed. For example:
public final class ColorEnum extends Enum { public static final ColorEnum RED = new ColorEnum("Red"); public static final ColorEnum GREEN = new ColorEnum("Green"); public static final ColorEnum BLUE = new ColorEnum("Blue"); private ColorEnum(String color) { super(color); } public static ColorEnum getEnum(String color) { return (ColorEnum) getEnum(ColorEnum.class, color); } public static Map getEnumMap() { return getEnumMap(ColorEnum.class); } public static List getEnumList() { return getEnumList(ColorEnum.class); } public static Iterator iterator() { return iterator(ColorEnum.class); } }
As shown, each enum has a name. This can be accessed using getName
.
The getEnum
and iterator
methods are recommended.
Unfortunately, Java restrictions require these to be coded as shown in each subclass.
An alternative choice is to use the EnumUtils
class.
A hierarchy of Enum classes can be built. In this case, the superclass is unaffected by the addition of subclasses (as per normal Java). The subclasses may add additional Enum constants of the type of the superclass. The query methods on the subclass will return all of the Enum constants from the superclass and subclass.
public final class ExtraColorEnum extends ColorEnum { // NOTE: Color enum declared above is final, change that to get this // example to compile. public static final ColorEnum YELLOW = new ExtraColorEnum("Yellow"); private ExtraColorEnum(String color) { super(color); } public static ColorEnum getEnum(String color) { return (ColorEnum) getEnum(ExtraColorEnum.class, color); } public static Map getEnumMap() { return getEnumMap(ExtraColorEnum.class); } public static List getEnumList() { return getEnumList(ExtraColorEnum.class); } public static Iterator iterator() { return iterator(ExtraColorEnum.class); } }
This example will return RED, GREEN, BLUE, YELLOW from the List and iterator methods in that order. The RED, GREEN and BLUE instances will be the same (==) as those from the superclass ColorEnum. Note that YELLOW is declared as a ColorEnum and not an ExtraColorEnum.
The enums can have functionality by defining subclasses and
overriding the getEnumClass()
method:
public static final OperationEnum PLUS = new PlusOperation(); private static final class PlusOperation extends OperationEnum { private PlusOperation() { super("Plus"); } public int eval(int a, int b) { return a + b; } } public static final OperationEnum MINUS = new MinusOperation(); private static final class MinusOperation extends OperationEnum { private MinusOperation() { super("Minus"); } public int eval(int a, int b) { return a - b; } } private OperationEnum(String color) { super(color); } public final Class getEnumClass() { // NOTE: new method! return OperationEnum.class; } public abstract double eval(double a, double b); public static OperationEnum getEnum(String name) { return (OperationEnum) getEnum(OperationEnum.class, name); } public static Map getEnumMap() { return getEnumMap(OperationEnum.class); } public static List getEnumList() { return getEnumList(OperationEnum.class); } public static Iterator iterator() { return iterator(OperationEnum.class); } }
The code above will work on JDK 1.2. If JDK1.3 and later is used, the subclasses may be defined as anonymous.
Care must be taken with class loading when defining a static nested class for enums. The static nested class can be loaded without the surrounding outer class being loaded. This can result in an empty list/map/iterator being returned. One solution is to define a static block that references the outer class where the constants are defined. For example:
public final class Outer { public static final BWEnum BLACK = new BWEnum("Black"); public static final BWEnum WHITE = new BWEnum("White"); // static nested enum class public static final class BWEnum extends Enum { static { // explicitly reference BWEnum class to force constants to load Object obj = Outer.BLACK; } // ... other methods omitted } }
Although the above solves the problem, it is not recommended. The best solution is to define the constants in the enum class, and hold references in the outer class:
public final class Outer { public static final BWEnum BLACK = BWEnum.BLACK; public static final BWEnum WHITE = BWEnum.WHITE; // static nested enum class public static final class BWEnum extends Enum { // only define constants in enum classes - private if desired private static final BWEnum BLACK = new BWEnum("Black"); private static final BWEnum WHITE = new BWEnum("White"); // ... other methods omitted } }
For more details, see the 'Nested' test cases.
Enum
,
Serialized FormField Summary | |
---|---|
protected String |
iToString
Deprecated. The toString representation of the Enum. |
Constructor Summary | |
---|---|
protected |
Enum(String name)
Deprecated. Constructor to add a new named item to the enumeration. |
Method Summary | |
---|---|
int |
compareTo(Object other)
Deprecated. Tests for order. |
boolean |
equals(Object other)
Deprecated. Tests for equality. |
protected static Enum |
getEnum(Class enumClass,
String name)
Deprecated. Gets an Enum object by class and name. |
Class |
getEnumClass()
Deprecated. Retrieves the Class of this Enum item, set in the constructor. |
protected static List |
getEnumList(Class enumClass)
Deprecated. Gets the List of Enum objects using the
Enum class. |
protected static Map |
getEnumMap(Class enumClass)
Deprecated. Gets the Map of Enum objects by
name using the Enum class. |
String |
getName()
Deprecated. Retrieve the name of this Enum item, set in the constructor. |
int |
hashCode()
Deprecated. Returns a suitable hashCode for the enumeration. |
protected static Iterator |
iterator(Class enumClass)
Deprecated. Gets an Iterator over the Enum objects in
an Enum class. |
protected Object |
readResolve()
Deprecated. Handle the deserialization of the class to ensure that multiple copies are not wastefully created, or illegal enum types created. |
String |
toString()
Deprecated. Human readable description of this Enum item. |
Methods inherited from class java.lang.Object |
---|
clone, finalize, getClass, notify, notifyAll, wait, wait, wait |
Field Detail |
---|
protected transient String iToString
Constructor Detail |
---|
protected Enum(String name)
Constructor to add a new named item to the enumeration.
name
- the name of the enum object,
must not be empty or null
IllegalArgumentException
- if the name is null
or an empty string
IllegalArgumentException
- if the getEnumClass() method returns
a null or invalid ClassMethod Detail |
---|
protected Object readResolve()
Handle the deserialization of the class to ensure that multiple copies are not wastefully created, or illegal enum types created.
protected static Enum getEnum(Class enumClass, String name)
Gets an Enum
object by class and name.
enumClass
- the class of the Enum to get, must not
be null
name
- the name of the Enum
to get,
may be null
null
if the enum does not exist
IllegalArgumentException
- if the enum class
is null
protected static Map getEnumMap(Class enumClass)
Gets the Map
of Enum
objects by
name using the Enum
class.
If the requested class has no enum objects an empty
Map
is returned.
enumClass
- the class of the Enum
to get,
must not be null
IllegalArgumentException
- if the enum class is null
IllegalArgumentException
- if the enum class is not a subclass of Enumprotected static List getEnumList(Class enumClass)
Gets the List
of Enum
objects using the
Enum
class.
The list is in the order that the objects were created (source code order).
If the requested class has no enum objects an empty List
is
returned.
enumClass
- the class of the Enum
to get,
must not be null
IllegalArgumentException
- if the enum class is null
IllegalArgumentException
- if the enum class is not a subclass of Enumprotected static Iterator iterator(Class enumClass)
Gets an Iterator
over the Enum
objects in
an Enum
class.
The Iterator
is in the order that the objects were
created (source code order). If the requested class has no enum
objects an empty Iterator
is returned.
enumClass
- the class of the Enum
to get,
must not be null
IllegalArgumentException
- if the enum class is null
IllegalArgumentException
- if the enum class is not a subclass of Enumpublic final String getName()
Retrieve the name of this Enum item, set in the constructor.
String
name of this Enum itempublic Class getEnumClass()
Retrieves the Class of this Enum item, set in the constructor.
This is normally the same as getClass()
, but for
advanced Enums may be different. If overridden, it must return a
constant value.
Class
of the enumpublic final boolean equals(Object other)
Tests for equality.
Two Enum objects are considered equal if they have the same class names and the same names. Identity is tested for first, so this method usually runs fast.
If the parameter is in a different class loader than this instance, reflection is used to compare the names.
equals
in class Object
other
- the other object to compare for equality
true
if the Enums are equalpublic final int hashCode()
Returns a suitable hashCode for the enumeration.
hashCode
in class Object
public int compareTo(Object other)
Tests for order.
The default ordering is alphabetic by name, but this can be overridden by subclasses.
If the parameter is in a different class loader than this instance, reflection is used to compare the names.
compareTo
in interface Comparable
other
- the other object to compare to
0
of equal
ClassCastException
- if other is not an Enum
NullPointerException
- if other is null
Comparable.compareTo(Object)
public String toString()
Human readable description of this Enum item.
toString
in class Object
type[name]
, for example:
Color[Red]
. Note that the package name is stripped from
the type name.
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