枚举及其高级用法

Posted 2022-01-26 yangyongjie

Enum源码：

/*
 * Copyright (c) 2003, 2009, 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.
 *
 * More information about enums, including descriptions of the
 * implicitly declared methods synthesized by the compiler, can be
 * found in section 8.9 of
 * <cite>The Java&trade; Language Specification</cite>.
 *
 * <p> Note that when using an enumeration type as the type of a set
 * or as the type of the keys in a map, specialized and efficient
 * @linkplain java.util.EnumSet set and @linkplain
 * java.util.EnumMap map implementations are available.
 *
 * @param <E> The enum type subclass
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see     Class#getEnumConstants()
 * @see     java.util.EnumSet
 * @see     java.util.EnumMap
 * @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.)
     *
     * <p>Note that for a particular enum type @code T, the
     * implicitly declared @code public static T valueOf(String)
     * method on that enum may be used instead of this method to map
     * from a name to the corresponding enum constant.  All the
     * constants of an enum type can be obtained by calling the
     * implicit @code public static T[] values() method of that
     * type.
     *
     * @param <T> The enum type whose constant is to be returned
     * @param enumType the @code Class 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 @code enumType or @code name
     *         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 constant " + enumType.getCanonicalName() + "." + name);
    

    /**
     * enum classes cannot have finalize methods.
     */
    protected final void finalize()  

    /**
     * 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");
    

我们自定义一个枚举类：

public enum Week 
    MONDAY,
    TUESDAY,
    WEDNESDAY,
    THURSDAY,
    FRIDAY

测试结果：

 System.out.println(Week.MONDAY.name());
 System.out.println(Week.MONDAY.ordinal());

MONDAY
0

可以看到，我们不用创建枚举类的对象（也不允许创建实例），直接用枚举类名即可访问其枚举元素，且Java自动给按照枚举值出现的顺序，从0开始分配了序号，

通过name()可以获得枚举值的名称，通过ordinal可以获得枚举值的编号。