https://openjdk.java.net/ 
/*
 * Copyright (c) 2003, 2021, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * 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
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.lang;

import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectStreamException;
import java.io.Serializable;
import java.lang.constant.ClassDesc;
import java.lang.constant.Constable;
import java.lang.constant.ConstantDescs;
import java.lang.constant.DynamicConstantDesc;
import java.lang.invoke.MethodHandles;
import java.util.Optional;

import static java.util.Objects.requireNonNull;


This is the common base class of all Java language enumeration classes.
More information about enums, including descriptions of the
implicitly declared methods synthesized by the compiler, can be
found in section {@jls 8.9} of The Java Language
Specification. Enumeration classes are all serializable and receive special handling by the serialization mechanism. The serialized representation used for enum constants cannot be customized. Declarations of methods and fields that would otherwise interact with serialization are ignored, including serialVersionUID; see the Java
Object Serialization Specification for details.

 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 set and map implementations are available. 
Author: Josh Bloch,  Neal Gafter
Type parameters:
  • <E> – The type of the enum subclass
See Also:
@serialexclude
@jls8.9 Enum Classes
@jls8.9.3 Enum Members
Since:  1.5
/**
 * This is the common base class of all Java language enumeration classes.
 *
 * More information about enums, including descriptions of the
 * implicitly declared methods synthesized by the compiler, can be
 * found in section {@jls 8.9} of <cite>The Java Language
 * Specification</cite>.
 *
 * Enumeration classes are all serializable and receive special handling
 * by the serialization mechanism. The serialized representation used
 * for enum constants cannot be customized. Declarations of methods
 * and fields that would otherwise interact with serialization are
 * ignored, including {@code serialVersionUID}; see the <cite>Java
 * Object Serialization Specification</cite> for details.
 *
 * <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 type of the enum subclass
 * @serial exclude
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see     Class#getEnumConstants()
 * @see     java.util.EnumSet
 * @see     java.util.EnumMap
 * @jls 8.9 Enum Classes
 * @jls 8.9.3 Enum Members
 * @since   1.5
 */
@SuppressWarnings("serial") // No serialVersionUID needed due to
                            // special-casing of enum classes.
public abstract class Enum<E extends Enum<E>>
        implements Constable, Comparable<E>, Serializable {
    
The name of this enum constant, as declared in the enum declaration. Most programmers should use the toString method rather than accessing this field. 
/**
     * 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.
Most programmers should use the toString method in preference to this one, as the toString method may return a more user-friendly name.  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.

Returns:the name of this enum constant
/**
     * 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 EnumSet and EnumMap. 
/**
     * 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 EnumSet and EnumMap. 
Returns:the ordinal of this enumeration constant
/**
     * 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 class declarations.

Params:
  • name – - The name of this enum constant, which is the identifier
              used to declare it.
  • ordinal – - The ordinal of this enumeration constant (its position
        in the enum declaration, where the initial constant is assigned
        an ordinal of zero).
/**
     * Sole constructor.  Programmers cannot invoke this constructor.
     * It is for use by code emitted by the compiler in response to
     * enum class 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 class should override this
method when a more "programmer-friendly" string form exists.

Returns:the name of this enum constant
/**
     * 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 class 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.

Params:
  • other – the object to be compared for equality with this object.
Returns: true if the specified object is equal to this
         enum constant.
/**
     * 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.

Returns:a hash code for this enum constant.
/**
     * 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.

Returns:(never returns)
/**
     * 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.

/**
     * 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 = 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 Object.getClass method for enum constants with constant-specific class bodies.) 
Returns:the Class object corresponding to this enum constant's
    enum type
/**
     * 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;
    }

    
Returns an enum descriptor EnumDesc for this instance, if one can be constructed, or an empty Optional if one cannot be. 
Returns:An Optional containing the resulting nominal descriptor, or an empty Optional if one cannot be constructed.
Since:12
/**
     * Returns an enum descriptor {@code EnumDesc} for this instance, if one can be
     * constructed, or an empty {@link Optional} if one cannot be.
     *
     * @return An {@link Optional} containing the resulting nominal descriptor,
     * or an empty {@link Optional} if one cannot be constructed.
     * @since 12
     */
    @Override
    public final Optional<EnumDesc<E>> describeConstable() {
        return getDeclaringClass()
                .describeConstable()
                .map(c -> EnumDesc.of(c, name));
    }

    
Returns the enum constant of the specified enum class with the
specified name.  The name must match exactly an identifier used
to declare an enum constant in this class.  (Extraneous whitespace
characters are not permitted.)

Note that for a particular enum class T, the implicitly declared 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 class can be obtained by calling the implicit public static T[] values() method of that class. 
Params:
  • enumClass – the Class object of the enum class from which to return a constant
  • name – the name of the constant to return
Type parameters:
  • <T> – The enum class whose constant is to be returned
Throws:
Returns:the enum constant of the specified enum class with the
     specified name
Since:1.5
/**
     * Returns the enum constant of the specified enum class with the
     * specified name.  The name must match exactly an identifier used
     * to declare an enum constant in this class.  (Extraneous whitespace
     * characters are not permitted.)
     *
     * <p>Note that for a particular enum class {@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 class can be obtained by calling the
     * implicit {@code public static T[] values()} method of that
     * class.
     *
     * @param <T> The enum class whose constant is to be returned
     * @param enumClass the {@code Class} object of the enum class from which
     *      to return a constant
     * @param name the name of the constant to return
     * @return the enum constant of the specified enum class with the
     *      specified name
     * @throws IllegalArgumentException if the specified enum class has
     *         no constant with the specified name, or the specified
     *         class object does not represent an enum class
     * @throws NullPointerException if {@code enumClass} or {@code name}
     *         is null
     * @since 1.5
     */
    public static <T extends Enum<T>> T valueOf(Class<T> enumClass,
                                                String name) {
        T result = enumClass.enumConstantDirectory().get(name);
        if (result != null)
            return result;
        if (name == null)
            throw new NullPointerException("Name is null");
        throw new IllegalArgumentException(
            "No enum constant " + enumClass.getCanonicalName() + "." + name);
    }

    
enum classes cannot have finalize methods.

Deprecated:Finalization has been deprecated for removal. See Object.finalize for background information and details about migration options.
/**
     * enum classes cannot have finalize methods.
     *
     * @deprecated Finalization has been deprecated for removal.  See
     * {@link java.lang.Object#finalize} for background information and details
     * about migration options.
     */
    @Deprecated(since="18", forRemoval=true)
    @SuppressWarnings("removal")
    protected final void finalize() { }

    
prevent default deserialization

/**
     * prevent default deserialization
     */
    @java.io.Serial
    private void readObject(ObjectInputStream in) throws IOException,
        ClassNotFoundException {
        throw new InvalidObjectException("can't deserialize enum");
    }

    @java.io.Serial
    private void readObjectNoData() throws ObjectStreamException {
        throw new InvalidObjectException("can't deserialize enum");
    }

    
A nominal descriptor for an enum constant. 
Type parameters:
  • <E> – the type of the enum constant
Since:12
/**
     * A <a href="{@docRoot}/java.base/java/lang/constant/package-summary.html#nominal">nominal descriptor</a> for an
     * {@code enum} constant.
     *
     * @param <E> the type of the enum constant
     *
     * @since 12
     */
    public static final class EnumDesc<E extends Enum<E>>
            extends DynamicConstantDesc<E> {

        
Constructs a nominal descriptor for the specified enum class and name. 
Params:
  • constantClass – a ClassDesc describing the enum class
  • constantName – the unqualified name of the enum constant
Throws:
@jvms4.2.2 Unqualified Names
/**
         * Constructs a nominal descriptor for the specified {@code enum} class and name.
         *
         * @param constantClass a {@link ClassDesc} describing the {@code enum} class
         * @param constantName the unqualified name of the enum constant
         * @throws NullPointerException if any argument is null
         * @jvms 4.2.2 Unqualified Names
         */
        private EnumDesc(ClassDesc constantClass, String constantName) {
            super(ConstantDescs.BSM_ENUM_CONSTANT, requireNonNull(constantName), requireNonNull(constantClass));
        }

        
Returns a nominal descriptor for the specified enum class and name 
Params:
  • enumClass – a ClassDesc describing the enum class
  • constantName – the unqualified name of the enum constant
Type parameters:
  • <E> – the type of the enum constant
Throws:
Returns:the nominal descriptor
@jvms4.2.2 Unqualified Names
Since:12
/**
         * Returns a nominal descriptor for the specified {@code enum} class and name
         *
         * @param <E> the type of the enum constant
         * @param enumClass a {@link ClassDesc} describing the {@code enum} class
         * @param constantName the unqualified name of the enum constant
         * @return the nominal descriptor
         * @throws NullPointerException if any argument is null
         * @jvms 4.2.2 Unqualified Names
         * @since 12
         */
        public static<E extends Enum<E>> EnumDesc<E> of(ClassDesc enumClass,
                                                        String constantName) {
            return new EnumDesc<>(enumClass, constantName);
        }

        @Override
        @SuppressWarnings("unchecked")
        public E resolveConstantDesc(MethodHandles.Lookup lookup)
                throws ReflectiveOperationException {
            return Enum.valueOf((Class<E>) constantType().resolveConstantDesc(lookup), constantName());
        }

        @Override
        public String toString() {
            return String.format("EnumDesc[%s.%s]", constantType().displayName(), constantName());
        }
    }
}