JVM中的枚举

【问题】

　　写代码过程中，定义了一组枚举WEEKDAYS[MONDAY,TUESDAY,......]。此时，如果输入参数为一个WEEKDAYS的参数，我们对这个参数进行值判定的时候，到底用eaquals还是==去判断呢？在JVM中，枚举到底是如何存放的？<Effective Java>中说枚举是单例模式的最佳实现方式，为什么？

【查阅资料】

　　使用javac将代码进行编译，并用javap命令查看编译之后的字节码文件：

　　可以看到，枚举类经过编译器的处理，拥有以下特点：

1. 构造函数私有；
2. 类为final类，继承了java.lang.Enum<E>
3. 定义了values()方法返回所有的枚举实例；
4. valueOf可以根据枚举的字符串形式转为枚举类实例；

　　这里，我们可以再看看java.lang.Enum<E>的实现：

public abstract class Enum<E extends Enum<E>>
        implements Comparable<E>, Serializable {

    private final String name;
    public final String name() {
        return name;
    }

    private final int ordinal;
    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;
    }

    public String toString() {
        return name;
    }
    public final boolean equals(Object other) {
        return this==other;
    }

    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();
    }

    public final int compareTo(E o) {
        Enum<?> other = (Enum<?>)o;
        Enum<E> 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
     */
    @SuppressWarnings("unchecked")
    public final Class<E> getDeclaringClass() {
        Class<?> clazz = getClass();
        Class<?> zuper = clazz.getSuperclass();
        return (zuper == Enum.class) ? (Class<E>)clazz : (Class<E>)zuper;
    }

    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");
    }
}

　　对于枚举类型来说，equals与==是一样的。而name()和toString()也是一样的，都是返回枚举内部的name字段。为了保护这种单例形态，枚举不能进行clone()。所以，在使用枚举时，拿到枚举实例再进行A.toString().equals(B.toString())的比较是完全没有必要的，直接A==B来判断就好，因为枚举的定义就是static final的，只有一份且值也无法被修改。

【参考】

https://www.cnblogs.com/alter888/p/9163612.html