Java tutorial
/* * Copyright (c) 2003, 2019, 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 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. * * @param <E> The enum type subclass * @serial exclude * @author Josh Bloch * @author Neal Gafter * @see Class#getEnumConstants() * @see java.util.EnumSet * @see java.util.EnumMap * @jls 8.9 Enum Types * @jls 8.9.3 Enum Members * @since 1.5 */ @SuppressWarnings("serial") // No serialVersionUID needed due to // special-casing of enum types. 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 {@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<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 {@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 {@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 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. */ @SuppressWarnings("deprecation") 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"); } /** * 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 {@code enum} class and name. * * @param constantType 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 constantType, String constantName) { super(ConstantDescs.BSM_ENUM_CONSTANT, requireNonNull(constantName), requireNonNull(constantType)); } /** * 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()); } } }