Javac语法糖之Enum类

枚举类在Javac中是被当作类来看待的。

An enum type is implicitly final unless it contains at least one enum constant that has a class body.

举例如下：

enum EnumTest {
    MON(1), TUE(2), WED(3), THU(4), FRI(5),
    SAT(6) {
        @Override
        public boolean isRest() {
            return true;
        }
    },
    SUN(0) {
        @Override
        public boolean isRest() {
            return true;
        }
    };

    private int value;

    private EnumTest(int value) {
        this.value = value;
    }

    public int getValue() {
        return value;
    }

    public boolean isRest() {
        return false;
    }
}

下面写一个枚举类：

enum Color {
    RED, GREEN, BLUE
}

可调用的方法如下：

其中有一个静态方法valueOf(Class<T> enumType,String name)是继承自Enum类，而另外的valueOf(String arg0)与values()方法则是由javac编译器为我们生成的。

The direct superclass of an enum type named E is Enum<E>。并且生成了一些代码，包括values()与valueOf(String s)方法。

在执行完所有的visitClassDef()方法后，类如下：

public enum Color extends Enum<Color> {
    /*public static final*/ RED /* = new Color("RED", 0) */,
    /*public static final*/ GREEN /* = new Color("GREEN", 1) */,
    /*public static final*/ BLUE /* = new Color("BLUE", 2) */;
    /*synthetic*/ private static final Color[] $VALUES =
    		new Color[]{Color.RED, Color.GREEN, Color.BLUE};

    public static Color[] values() {
        return (Color[])$VALUES.clone();
    }

    public static Color valueOf(String name) {
        return (Color)Enum.valueOf(Color.class, name);
    }

    private Color(/*synthetic*/ String $enum$name, /*synthetic*/ int $enum$ordinal) {
        super($enum$name, $enum$ordinal);
    }
}

如果Object中没有clone()方法时，生成的values()方法如下：

public static Color[] values() {
    /*synthetic*/ final Color[] $result = new Color[$VALUES.length];
    System.arraycopy($VALUES, 0, $result, 0, $VALUES.length);
    return $result;
}

再来看一个实例，如下：

import java.net.URL;
import java.util.List;

public enum PropertyType {
	INT("int") {
		@Override
		protected boolean validateString(String obj) throws Exception {
			Integer.parseInt(obj);
			return true;
		}
	},

	COMPONENT_LIST("ComponentList", List.class),
	STRING("String", String.class),

	RESOURCE("Resource") {
		@Override
		public boolean validateString(String obj) throws Exception {
			if (obj.toLowerCase().startsWith("resource:/"))
				return true;
			if (obj.indexOf(':') == -1)
				obj = "file:" + obj;
			new URL(obj);
			return true;
		}
	},

	STRING_LIST("StringList", List.class);

	private final String displayName;
	private Class<?> checkClass;

	private PropertyType(String displayName) {
		this.displayName = displayName;
	}

	private PropertyType(String displayName, Class<?> checkClass) {
		this.displayName = displayName;
		this.checkClass = checkClass;
	}

	public String toString() {
		return displayName;
	}

	protected boolean validateString(String obj) throws Exception {
		return true; // default implementation
	}
}

最后的结果如下：

public enum PropertyType {
    /*public static final*/ INT /*enum*/ ("int") {

        PropertyType(.java.lang.String x0) {
            super(x0);
        }

        @Override()
        protected boolean validateString(String obj) throws Exception {
            Integer.parseInt(obj);
            return true;
        }
    },
    /*public static final*/ COMPONENT_LIST /*enum*/ ("ComponentList",List.class),
    /*public static final*/ STRING /*enum*/ ("String",String.class),
    /*public static final*/ RESOURCE /*enum*/ ("Resource") {

        PropertyType(.java.lang.String x0) {
            super(x0);
        }

        @Override()
        public boolean validateString(String obj) throws Exception {
            if (obj.toLowerCase().startsWith("resource:/")) return true;
            if (obj.indexOf(':') == -1) obj = "file:" + obj;
            new URL(obj);
            return true;
        }
    },
    /*public static final*/ STRING_LIST /*enum*/ ("StringList",List.class);
    private final String displayName;
    private Class checkClass;

    private PropertyType(String displayName) {
        super();
        this.displayName = displayName;
    }

    private PropertyType(String displayName, Class checkClass) {
        super();
        this.displayName = displayName;
        this.checkClass = checkClass;
    }

    public String toString() {
        return displayName;
    }

    protected boolean validateString(String obj) throws Exception {
        return true;
    }
}

枚举类之泛型<E extends Enum<E>>

就是一个Enum只接受一个Enum或者他的子类作为参数。相当于把一个子类或者自己当成参数，传入到自身

查看Enum类的定义，如下：

/**
 * 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™ 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.
 *
 */
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");
    }
}

1、关于clone()、readObject()与readObjectNoData() 方法：

The final clone method in Enum ensures that enum constants can never be cloned, and the special treatment by the serialization mechanism ensures that duplicate instances are never created as a result of deserialization. Reflective instantiation of enum types is prohibited. Together, these four things ensure that no instances of an enum type exist beyond those defined by the enum constants.　　

2、关于equals()方法：

Because there is only one instance of each enum constant, it is permissible to use the == operator in place of the equalsmethod when comparing two object references if it is known that at least one of them refers to an enum constant.

The equals method in Enum is a final method that merely invokes super.equals on its argument and returns the result, thus performing an identity comparison.

可以阅读：https://www.javaworld.com/article/2074292/core-java/use-----or-----to-compare-java-enums.html

参考：

（1）通过反编译字节码来理解 Java 枚举

（2）透析Java本质-开关选择表达式switch

（3）JLS参考地址：https://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.9