原来枚举是可以相互比较的~~~~~~~~~~

本文已参与「新人创作礼」活动，一起开启掘金创作之路。

用了这么久的枚举，居然才发现枚举是可以比较的。在日志框架里面，我们控制日志输出级别的时候，所有的日志实现都是用定义的日志级别枚举进行比较，来控制可输出的日志级别的。今天就好好的重新认识下枚举这个类。

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.
 * 这是所有Java语言枚举类型的公共基类。
 */
public abstract class Enum<E extends Enum<E>>
        implements Comparable<E>, Serializable {
    /**
     * 该变量的值即我们定义的枚举名称
     */
    private final String name;

    /**
     * 更多的人会使用toString方法
     * @return the name of this enum constant
     */
    public final String name() {
        return name;
    }

    /**
     * 表示定义枚举的顺序
     *
     * 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;

    /**
     *
     * @return the ordinal of this enumeration constant
     */
    public final int ordinal() {
        return ordinal;
    }

    /**
     * 唯一的构造函数，是哪个汇编程序使用的。
     *
     */
    protected Enum(String name, int ordinal) {
        this.name = name;
        this.ordinal = ordinal;
    }

    /**
     *
     * @return the name of this enum constant
     */
    public String toString() {
        return name;
    }

    /**
     *
     *
     */
    public final boolean equals(Object other) {
        return this==other;
    }

    /**
     *
     * @return a hash code for this enum constant.
     */
    public final int hashCode() {
        return super.hashCode();
    }

    /**
     * 枚举类是不支持clone方法的
     *
     * @return (never returns)
     */
    protected final Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException();
    }

    /**
     * 枚举的比较就是把定义序号进行比较
     */
    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;
    }

    /**
     *
     */
    @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");
    }
}

所以，以后可以通过枚举来对数据进行排序了。