- Equals for java enums
- Constructor Summary
- Method Summary
- Methods declared in class java.lang.Object
- Constructor Detail
- Enum
- Method Detail
- name
- ordinal
- toString
- equals
- hashCode
- clone
- compareTo
- getDeclaringClass
- valueOf
- finalize
- Как лучше сравнивать перечисляемые типы в Java
- Они же оба работают, верно?
- Какие есть преимущества у оператора ==?
- Ого, как серьёзно все подошли к вопросу! Ну теперь точно буду использовать оператор равенства.
- Equals for java enums
- Constructor Summary
- Method Summary
- Methods declared in class java.lang.Object
- Constructor Detail
- Enum
- Method Detail
- name
- ordinal
- toString
- equals
- hashCode
- clone
- compareTo
- getDeclaringClass
- valueOf
- finalize
Equals for java enums
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 The Java™ Language Specification . 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.
Constructor Summary
Method Summary
Returns the ordinal of this enumeration constant (its position in its enum declaration, where the initial constant is assigned an ordinal of zero).
Methods declared in class java.lang.Object
Constructor Detail
Enum
Sole constructor. Programmers cannot invoke this constructor. It is for use by code emitted by the compiler in response to enum type declarations.
Method Detail
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.
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 .
toString
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.
equals
hashCode
public final int hashCode()
clone
protected final Object clone() throws CloneNotSupportedException
Throws CloneNotSupportedException. This guarantees that enums are never cloned, which is necessary to preserve their «singleton» status.
compareTo
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.
getDeclaringClass
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.)
valueOf
public static Enum> T valueOf(Class enumType, String 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.) Note that for a particular enum type 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 type can be obtained by calling the implicit public static T[] values() method of that type.
finalize
protected final void finalize()
Report a bug or suggest an enhancement
For further API reference and developer documentation see the Java SE Documentation, which contains more detailed, developer-targeted descriptions with conceptual overviews, definitions of terms, workarounds, and working code examples.
Java is a trademark or registered trademark of Oracle and/or its affiliates in the US and other countries.
Copyright © 1993, 2023, Oracle and/or its affiliates, 500 Oracle Parkway, Redwood Shores, CA 94065 USA.
All rights reserved. Use is subject to license terms and the documentation redistribution policy.
Как лучше сравнивать перечисляемые типы в Java
Недавно на Stack Overflow я наткнулся на, казалось бы, простой вопрос:
Что лучше использовать для сравнения enum’ов — == или equals() ?
Вы, конечно, можете сходу ответить, что никакой разницы нет, но будете неправы — в ответах к этому вопросу было приведено много интересных аргументов в пользу обоих вариантов. Я решил, что будет интересно перевести этот спор на русский язык.
Они же оба работают, верно?
Да. Как написано в документации, «допустимо использовать оператор == вместо метода equals , если доподлинно известно, что хотя бы один из них ссылается на перечислимый тип» («it is permissible to use the == operator in place of the equals method when comparing two object references if it is known that at least one of them refers to an enum constant»). Причина этого очень простая — каждый из объектов enum’а создаётся только единожды, и поэтому, если вы создадите десять переменных равных SomeEnum.RED , они все будут ссылаться на один и тот же объект (а оператор == как раз это и проверяет).
Какие есть преимущества у оператора ==?
enum Color < BLACK, WHITE >; Color nothing = null; if (nothing == Color.BLACK); // Всё отлично if (nothing.equals(Color.BLACK)); // выбрасывает NullPointerException
enum Color < BLACK, WHITE >; enum Chiral < LEFT, RIGHT >; if (Color.BLACK.equals(Chiral.LEFT)); // Компилируется if (Color.BLACK == Chiral.LEFT); // НЕ КОМПИЛИРУЕТСЯ. Несовместимые типы!
Ого, как серьёзно все подошли к вопросу! Ну теперь точно буду использовать оператор равенства.
Не спешите так. Не все единогласно за == . Вот что пишут сторонники equals() :
- Нет ни единого случая, когда переменная перечисляемого типа должна быть равна null — если вы так описываете какое-то особое состояние, то его можно просто заменить на ещё одно допустимое состояние enum’а. А значит, NPE не надо скрывать, так как это, вероятнее всего, ошибка, и чем раньше о ней станет известно, тем лучше.
- Аргумент про скорость == весьма сомнителен. Современные компиляторы, скорее всего умеют заменять equals на == самостоятельно. Если это не так (как доказали позже, это действительно не так), то это проблема Java, над которой нужно работать.
- Какой Java-программист не знает, что делает equals ? Такой вариант наоборот более читаем, так как для объектов мы привыкли использовать именно метод equals . Значит, и для enum’ов нужно поступать так же, чтобы не возникало путаницы.
Equals for java enums
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 The Java™ Language Specification . 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.
Constructor Summary
Method Summary
Returns the ordinal of this enumeration constant (its position in its enum declaration, where the initial constant is assigned an ordinal of zero).
Methods declared in class java.lang.Object
Constructor Detail
Enum
Sole constructor. Programmers cannot invoke this constructor. It is for use by code emitted by the compiler in response to enum type declarations.
Method Detail
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.
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 .
toString
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.
equals
hashCode
public final int hashCode()
clone
protected final Object clone() throws CloneNotSupportedException
Throws CloneNotSupportedException. This guarantees that enums are never cloned, which is necessary to preserve their «singleton» status.
compareTo
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.
getDeclaringClass
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.)
valueOf
public static Enum> T valueOf(Class enumType, String 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.) Note that for a particular enum type 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 type can be obtained by calling the implicit public static T[] values() method of that type.
finalize
protected final void finalize()
Report a bug or suggest an enhancement
For further API reference and developer documentation see the Java SE Documentation, which contains more detailed, developer-targeted descriptions with conceptual overviews, definitions of terms, workarounds, and working code examples.
Java is a trademark or registered trademark of Oracle and/or its affiliates in the US and other countries.
Copyright © 1993, 2023, Oracle and/or its affiliates, 500 Oracle Parkway, Redwood Shores, CA 94065 USA.
All rights reserved. Use is subject to license terms and the documentation redistribution policy.