枚举 enum

java的enum其实是一个类。编译器根据你enum的定义会为你生成一个 java.lang.Enum<E>的子类。java.lang.Enum<E>是一个抽象类，定义了很多final方法，也就是说你定义的enum类里不能覆盖它们，你唯一能覆盖的是Object.toString类。同时你定义的enum类是不能被继承的。

先来看看java.lang.Enum类的代码

注意，更具编码规范，枚举值应该全大写（按照常量编写规范）

/*
* %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() { }
}


如上所示，它本身提供了很多方法。
当然我们写的enum，将会直接从上面的父类继承下这些方法来。

例1

package test;

public enum Enum1 {
ONE, TWO, THREE
}

package test;

public class TestEnum1 {

/**
* 描述:
* @param args
* @author liyixing 2013-12-13 下午4:52:01
*/

public static void main(String[] args) {
for (Enum1 n: Enum1.values()) {
        System.out.println("ordinal: " + n.ordinal()); // enum's int value, start from 0 by default
        System.out.println(n.compareTo(Enum1.TWO));
        System.out.println(n == Enum1.THREE);
        System.out.println(n.name());
        System.out.println(n.getDeclaringClass()); // Number
    }
}

}


在使用枚举时，必须用类型名作前缀，比如Number.ONE，而不能直接用ONE。如果你需要这种便利的话，可以用import static Number.*;来达到目的。
如
import static test.Enum1.ONE;

System.out.println(Enum1.ONE == ONE); //

另外使用在switch语句中
switch (n) {
case ONE:
break;
case TWO:
break;
default:
break;
}

这里如果写case Enum1.ONE是不允许的。


我们可以往enum里添加属性和方法，构造函数：
需要注意的是，构造函数只能使用private或者不写（默认是private)修饰符

package test;

public enum Enum1 {
ONE, TWO, THREE;

private String name;
public Integer id;

// 默认构造
private Enum1() {

}

// 带参数构造
Enum1(String name) {

}

private void print() {
System.out.println(name);
}

public String say() {
print();
return name;
}
}