Class Enum

  • All Implemented Interfaces:
    java.io.Serializable, java.lang.Comparable
    Direct Known Subclasses:
    ValuedEnum

    public abstract class Enum
    extends java.lang.Object
    implements java.lang.Comparable, java.io.Serializable

    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 ==.

    Simple Enums

    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.

    Subclassed Enums

    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.

    Functional Enums

    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.

    Nested class Enums

    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.

    Lang Enums and Java 5.0 Enums

    Enums were added to Java in Java 5.0. The main differences between Lang's implementation and the new official JDK implementation are:

    • The standard Enum is a not just a superclass, but is a type baked into the language.
    • The standard Enum does not support extension, so the standard methods that are provided in the Lang enum are not available.
    • Lang mandates a String name, whereas the standard Enum uses the class name as its name. getName() changes to name().

    Generally people should use the standard Enum. Migrating from the Lang enum to the standard Enum is not as easy as it might be due to the lack of class inheritence in standard Enums. This means that it's not possible to provide a 'super-enum' which could provide the same utility methods that the Lang enum does. The following utility class is a Java 5.0 version of our EnumUtils class and provides those utility methods.

     import java.util.*;
     
     public class EnumUtils {
     
       public static Enum getEnum(Class enumClass, String token) {
         return Enum.valueOf(enumClass, token);
       }
     
       public static Map getEnumMap(Class enumClass) {
         HashMap map = new HashMap();
         Iterator itr = EnumUtils.iterator(enumClass);
         while(itr.hasNext()) {
           Enum enm = (Enum) itr.next();
           map.put( enm.name(), enm );
         }
         return map;
       }
     
       public static List getEnumList(Class enumClass) {
         return new ArrayList( EnumSet.allOf(enumClass) );
       }
     
       public static Iterator iterator(Class enumClass) {
         return EnumUtils.getEnumList(enumClass).iterator();
       }
     }
     
    Since:
    2.1 (class existed in enum package from v1.0)
    Version:
    $Id: Enum.java 912394 2010-02-21 20:16:22Z niallp $
    See Also:
    Serialized Form
    • Nested Class Summary

      Nested Classes 
      Modifier and Type Class Description
      private static class  Enum.Entry
      Enable the iterator to retain the source code order.
    • Field Summary

      Fields 
      Modifier and Type Field Description
      private static java.util.Map cEnumClasses
      Map, key of class name, value of Entry.
      private static java.util.Map EMPTY_MAP
      An empty Map, as JDK1.2 didn't have an empty map.
      private int iHashCode
      The hashcode representation of the Enum.
      private java.lang.String iName
      The string representation of the Enum.
      protected java.lang.String iToString
      The toString representation of the Enum.
      private static long serialVersionUID
      Required for serialization support.
    • Constructor Summary

      Constructors 
      Modifier Constructor Description
      protected Enum​(java.lang.String name)
      Constructor to add a new named item to the enumeration.
    • Method Summary

      All Methods Static Methods Instance Methods Concrete Methods 
      Modifier and Type Method Description
      int compareTo​(java.lang.Object other)
      Tests for order.
      private static Enum.Entry createEntry​(java.lang.Class enumClass)
      Creates an Entry for storing the Enums.
      boolean equals​(java.lang.Object other)
      Tests for equality.
      private static Enum.Entry getEntry​(java.lang.Class enumClass)
      Gets an Entry from the map of Enums.
      protected static Enum getEnum​(java.lang.Class enumClass, java.lang.String name)
      Gets an Enum object by class and name.
      java.lang.Class getEnumClass()
      Retrieves the Class of this Enum item, set in the constructor.
      protected static java.util.List getEnumList​(java.lang.Class enumClass)
      Gets the List of Enum objects using the Enum class.
      protected static java.util.Map getEnumMap​(java.lang.Class enumClass)
      Gets the Map of Enum objects by name using the Enum class.
      java.lang.String getName()
      Retrieve the name of this Enum item, set in the constructor.
      private java.lang.String getNameInOtherClassLoader​(java.lang.Object other)
      Use reflection to return an objects class name.
      int hashCode()
      Returns a suitable hashCode for the enumeration.
      private void init​(java.lang.String name)
      Initializes the enumeration.
      protected static java.util.Iterator iterator​(java.lang.Class enumClass)
      Gets an Iterator over the Enum objects in an Enum class.
      protected java.lang.Object readResolve()
      Handle the deserialization of the class to ensure that multiple copies are not wastefully created, or illegal enum types created.
      java.lang.String toString()
      Human readable description of this Enum item.
      • Methods inherited from class java.lang.Object

        clone, finalize, getClass, notify, notifyAll, wait, wait, wait
    • Field Detail

      • serialVersionUID

        private static final long serialVersionUID
        Required for serialization support.
        See Also:
        Serializable, Constant Field Values
      • EMPTY_MAP

        private static final java.util.Map EMPTY_MAP
        An empty Map, as JDK1.2 didn't have an empty map.
      • cEnumClasses

        private static java.util.Map cEnumClasses
        Map, key of class name, value of Entry.
      • iName

        private final java.lang.String iName
        The string representation of the Enum.
      • iHashCode

        private final transient int iHashCode
        The hashcode representation of the Enum.
      • iToString

        protected transient java.lang.String iToString
        The toString representation of the Enum.
        Since:
        2.0
    • Constructor Detail

      • Enum

        protected Enum​(java.lang.String name)

        Constructor to add a new named item to the enumeration.

        Parameters:
        name - the name of the enum object, must not be empty or null
        Throws:
        java.lang.IllegalArgumentException - if the name is null or an empty string
        java.lang.IllegalArgumentException - if the getEnumClass() method returns a null or invalid Class
    • Method Detail

      • init

        private void init​(java.lang.String name)
        Initializes the enumeration.
        Parameters:
        name - the enum name
        Throws:
        java.lang.IllegalArgumentException - if the name is null or empty or duplicate
        java.lang.IllegalArgumentException - if the enumClass is null or invalid
      • readResolve

        protected java.lang.Object readResolve()

        Handle the deserialization of the class to ensure that multiple copies are not wastefully created, or illegal enum types created.

        Returns:
        the resolved object
      • getEnum

        protected static Enum getEnum​(java.lang.Class enumClass,
                                      java.lang.String name)

        Gets an Enum object by class and name.

        Parameters:
        enumClass - the class of the Enum to get, must not be null
        name - the name of the Enum to get, may be null
        Returns:
        the enum object, or null if the enum does not exist
        Throws:
        java.lang.IllegalArgumentException - if the enum class is null
      • getEnumMap

        protected static java.util.Map getEnumMap​(java.lang.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.

        Parameters:
        enumClass - the class of the Enum to get, must not be null
        Returns:
        the enum object Map
        Throws:
        java.lang.IllegalArgumentException - if the enum class is null
        java.lang.IllegalArgumentException - if the enum class is not a subclass of Enum
      • getEnumList

        protected static java.util.List getEnumList​(java.lang.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.

        Parameters:
        enumClass - the class of the Enum to get, must not be null
        Returns:
        the enum object Map
        Throws:
        java.lang.IllegalArgumentException - if the enum class is null
        java.lang.IllegalArgumentException - if the enum class is not a subclass of Enum
      • iterator

        protected static java.util.Iterator iterator​(java.lang.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.

        Parameters:
        enumClass - the class of the Enum to get, must not be null
        Returns:
        an iterator of the Enum objects
        Throws:
        java.lang.IllegalArgumentException - if the enum class is null
        java.lang.IllegalArgumentException - if the enum class is not a subclass of Enum
      • getEntry

        private static Enum.Entry getEntry​(java.lang.Class enumClass)

        Gets an Entry from the map of Enums.

        Parameters:
        enumClass - the class of the Enum to get
        Returns:
        the enum entry
      • createEntry

        private static Enum.Entry createEntry​(java.lang.Class enumClass)

        Creates an Entry for storing the Enums.

        This accounts for subclassed Enums.

        Parameters:
        enumClass - the class of the Enum to get
        Returns:
        the enum entry
      • getName

        public final java.lang.String getName()

        Retrieve the name of this Enum item, set in the constructor.

        Returns:
        the String name of this Enum item
      • getEnumClass

        public java.lang.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.

        Returns:
        the Class of the enum
        Since:
        2.0
      • equals

        public final boolean equals​(java.lang.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.

        Overrides:
        equals in class java.lang.Object
        Parameters:
        other - the other object to compare for equality
        Returns:
        true if the Enums are equal
      • hashCode

        public final int hashCode()

        Returns a suitable hashCode for the enumeration.

        Overrides:
        hashCode in class java.lang.Object
        Returns:
        a hashcode based on the name
      • compareTo

        public int compareTo​(java.lang.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.

        Specified by:
        compareTo in interface java.lang.Comparable
        Parameters:
        other - the other object to compare to
        Returns:
        -ve if this is less than the other object, +ve if greater than, 0 of equal
        Throws:
        java.lang.ClassCastException - if other is not an Enum
        java.lang.NullPointerException - if other is null
        See Also:
        Comparable.compareTo(Object)
      • getNameInOtherClassLoader

        private java.lang.String getNameInOtherClassLoader​(java.lang.Object other)

        Use reflection to return an objects class name.

        Parameters:
        other - The object to determine the class name for
        Returns:
        The class name
      • toString

        public java.lang.String toString()

        Human readable description of this Enum item.

        Overrides:
        toString in class java.lang.Object
        Returns:
        String in the form type[name], for example: Color[Red]. Note that the package name is stripped from the type name.