Java 枚举：理解枚举本质
C 语言中可以这样来定义枚举
我们可以使用普通类来模拟枚举，下面定义一个 Color 类。
/* * %W% %E% * * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */package java.lang;import java.io.Serializable;import java.io.IOException;import java.io.InvalidObjectException;import java.io.ObjectInputStream;import java.io.ObjectStreamException;/** * This is the common base class of all Java language enumeration types. * * @author Josh Bloch * @author Neal Gafter * @version %I%, %G% * @since 1.5 */public abstract class Enum<E extends Enum<E>> implements Comparable<E>, Serializable { /** * The name of this enum constant, as declared in the enum declaration. * Most programmers should use the {@link #toString} method rather than * accessing this field. */ private final String name; /** * Returns the name of this enum constant, exactly as declared in its * enum declaration. * * <b>Most programmers should use the {@link #toString} method in * preference to this one, as the toString method may return * a more user-friendly name.</b> This method is designed primarily for * use in specialized situations where correctness depends on getting the * exact name, which will not vary from release to release. * * @return the name of this enum constant */ public final String name() {return name; } /** * The ordinal of this enumeration constant (its position * in the enum declaration, where the initial constant is assigned * an ordinal of zero). * * Most programmers will have no use for this field. It is designed * for use by sophisticated enum-based data structures, such as * {@link java.util.EnumSet} and {@link java.util.EnumMap}. */ private final int ordinal; /** * Returns the ordinal of this enumeration constant (its position * in its enum declaration, where the initial constant is assigned * an ordinal of zero). * * Most programmers will have no use for this method. It is * designed for use by sophisticated enum-based data structures, such * as {@link java.util.EnumSet} and {@link java.util.EnumMap}. * * @return the ordinal of this enumeration constant */ public final int ordinal() {return ordinal; } /** * Sole constructor. Programmers cannot invoke this constructor. * It is for use by code emitted by the compiler in response to * enum type declarations. * * @param name - The name of this enum constant, which is the identifier * used to declare it. * @param ordinal - The ordinal of this enumeration constant (its position * in the enum declaration, where the initial constant is assigned * an ordinal of zero). */ protected Enum(String name, int ordinal) {this.name = name;this.ordinal = ordinal; } /** * Returns the name of this enum constant, as contained in the * declaration. This method may be overridden, though it typically * isn't necessary or desirable. An enum type should override this * method when a more "programmer-friendly" string form exists. * * @return the name of this enum constant */ public String toString() {return name; } /** * Returns true if the specified object is equal to this * enum constant. * * @param other the object to be compared for equality with this object. * @return true if the specified object is equal to this * enum constant. */ public final boolean equals(Object other) { return this==other; } /** * Returns a hash code for this enum constant. * * @return a hash code for this enum constant. */ public final int hashCode() { return super.hashCode(); } /** * Throws CloneNotSupportedException. This guarantees that enums * are never cloned, which is necessary to preserve their "singleton" * status. * * @return (never returns) */ protected final Object clone() throws CloneNotSupportedException {throw new CloneNotSupportedException(); } /** * Compares this enum with the specified object for order. Returns a * negative integer, zero, or a positive integer as this object is less * than, equal to, or greater than the specified object. * * Enum constants are only comparable to other enum constants of the * same enum type. The natural order implemented by this * method is the order in which the constants are declared. */ public final int compareTo(E o) {Enum other = (Enum)o;Enum self = this;if (self.getClass() != other.getClass() && // optimization self.getDeclaringClass() != other.getDeclaringClass()) throw new ClassCastException();return self.ordinal - other.ordinal; } /** * Returns the Class object corresponding to this enum constant's * enum type. Two enum constants e1 and e2 are of the * same enum type if and only if * e1.getDeclaringClass() == e2.getDeclaringClass(). * (The value returned by this method may differ from the one returned * by the {@link Object#getClass} method for enum constants with * constant-specific class bodies.) * * @return the Class object corresponding to this enum constant's * enum type */ public final Class<E> getDeclaringClass() {Class clazz = getClass();Class zuper = clazz.getSuperclass();return (zuper == Enum.class) ? clazz : zuper; } /** * Returns the enum constant of the specified enum type with the * specified name. The name must match exactly an identifier used * to declare an enum constant in this type. (Extraneous whitespace * characters are not permitted.) * * @param enumType the <tt>Class</tt> object of the enum type from which * to return a constant * @param name the name of the constant to return * @return the enum constant of the specified enum type with the * specified name * @throws IllegalArgumentException if the specified enum type has * no constant with the specified name, or the specified * class object does not represent an enum type * @throws NullPointerException if <tt>enumType</tt> or <tt>name</tt> * is null * @since 1.5 */ public static <T extends Enum<T>> T valueOf(Class<T> enumType, String name) { T result = enumType.enumConstantDirectory().get(name); if (result != null) return result; if (name == null) throw new NullPointerException("Name is null"); throw new IllegalArgumentException( "No enum const " + enumType +"." + name); } /** * prevent default deserialization */ private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException { throw new InvalidObjectException("can't deserialize enum"); } private void readObjectNoData() throws ObjectStreamException { throw new InvalidObjectException("can't deserialize enum"); } /** * enum classes cannot have finalize methods. */ protected final void finalize() { }}
