Enum.java

/*
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package java.lang;

import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectStreamException;
import java.io.Serializable;

/**
 * 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
 */
/*
 * 可以看作是所有enum的包装类，不过Enum类是禁止直接继承的
 *
 * 示例：
 * public enum Color {
 *     WHITE, BLACK
 * }
 *
 * 等价于：
 * public final class Color extends Enum<Color> {
 *   public static final Color WHITE = new Color("WHITE", 0);
 *   public static final Color BLACK = new Color("BLACK", 1);
 *
 *   public Color(String name, int ordinal) {
 *       super(name, ordinal);
 *   }
 *
 *   // ......
 * }
 */
@SuppressWarnings("serial")
public abstract class Enum<E extends Enum<E>> implements Comparable<E>, Serializable {
    /* No serialVersionUID needed due to special-casing of enum types. */
    
    /**
     * 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;  // 枚举实例名称
    
    /**
     * 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;  // 枚举实例的值
    
    
    /**
     * 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, 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
     */
    // 返回枚举实例名称
    public final String name() {
        return name;
    }
    
    /**
     * 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
     */
    // 返回枚举实例的值
    public final int ordinal() {
        return 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
     */
    // 返回枚举类类对象
    @SuppressWarnings("unchecked")
    public final Class<E> getDeclaringClass() {
        Class<?> clazz = getClass();
        Class<?> zuper = clazz.getSuperclass();
        return (zuper == Enum.class) ? (Class<E>) clazz : (Class<E>) zuper;
    }
    
    /**
     * 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.
     */
    // 比较枚举实例的值；声明靠前的枚举，其"值"较小
    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 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
     */
    /*
     * 获取枚举类enumType中名称为name的枚举实例，其他实例不受影响
     * Color c = Enum.valueOf(Color.class, "WHITE");
     */
    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);
    }
    
    
    
    /*▼ 继承自Object ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns a hash code for this enum constant.
     *
     * @return a hash code for this enum constant.
     */
    public final int hashCode() {
        return super.hashCode();
    }
    
    /**
     * 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 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;
    }
    
    /**
     * 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();
    }
    
    /**
     * enum classes cannot have finalize methods.
     */
    @SuppressWarnings("deprecation")
    protected final void finalize() {
    }
    
    /*▲ 继承自Object ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /**
     * 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");
    }
}