Java-枚举
Java 枚举使用笔记
EnumUtils
org.apache.commons.lang3.EnumUtils
getEnum() 根据枚举name获取枚举实例
public static <E extends Enum<E>> E getEnum(final Class<E> enumClass, final String enumName) {
return getEnum(enumClass, enumName, null);
}
Enum 枚举
每个枚举都是抽象类 java.lang.Enum 的子类，都可以访问Enum类提供的方法，比如hashCode、name、valueOf等，其中valueOf方法会把一个String类型的名称转变成枚举项，也就是在枚举项中查找字面值和该参数相等的枚举项。
实际上在使用关键字enum创建枚举类型并编译后，编译器会为我们生成一个相关的类，这个类继承了Java API中的java.lang.Enum类，也就是说通过关键字enum创建枚举类型在编译后事实上也是一个类类型而且该类继承自java.lang.Enum类。
枚举的构造方法都是private的
枚举的构造方法都是默认 private 的，如果改为 public 会报错，如果显式写出 private IDE 会提示 Modifier ‘private’ is redundant for enum constructors
Enum.valueOf(Class, String)
valueOf 静态方法会把一个 String 类型的名称转变成枚举项，也就是在枚举项中查找字面值和该参数相等的枚举项。
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);
}
参数
enumType – 这是枚举类型，返回一个常量的类的对象。
name – 这是常量，要返回的名称。
返回值
此方法返回具有指定名称的枚举类型的枚举常量。
异常
IllegalArgumentException – 如果指定的枚举类型没有常量指定名称，或指定的类对象不表示枚举类型。
NullPointerException – 如果 enumType 或 name 为 null.
valudOf方法通过反射从枚举类的常量声明中查找，若找到就直接返回，若找不到则抛出无效参数异常。valueOf的本意是保护编码的枚举安全性，使其不产生空枚举对象，简化枚举操作，但是却又引入了一个我们无法避免的IllegalArgumentException异常。
注意：
the name must match exactly an identifier used to declare an enum constant in this type
即 name 参数必须和枚举name完全匹配，大小写都不能错。
例如：Color myColor = Enum.valueOf(Color.class, "RED");
Color.valueOf(String)
其实每个具体的枚举类都有一个 valueOf(String) 方法，直接用这个方法就行，没必要使用 Enum 类的静态方法：Color color = Color.valueOf("RED");
例如
public enum Color {
RED("红色"),
BLUE("蓝色");
private String desc;
Color(String desc) {
this.desc = desc;
}
public String getDesc() {
return desc;
}
public static void main(String[] args) {
// 大小写不匹配，错误
try {
Color.valueOf("red");
} catch (IllegalArgumentException e) {
System.out.println(e.toString());
}
// Enum 类的静态方法
Color myColor = Enum.valueOf(Color.class, "RED");
System.out.println("myColor: " + myColor);
// 枚举类自己的静态方法
Color myColor2 = Color.valueOf("BLUE");
System.out.println("myColor2: " + myColor2);
}
}
结果：
java.lang.IllegalArgumentException: No enum constant com.nio.uds.user.service.Color.red
myColor: RED
myColor2: BLUE
深入理解Java枚举类型(enum)
https://blog.csdn.net/javazejian/article/details/71333103
Java 枚举(enum) 详解7种常见的用法
https://blog.csdn.net/qq_27093465/article/details/52180865
Java 中的枚举 (enum)
https://www.jianshu.com/p/46dbd930f6a2
深度分析Java的枚举类型—-枚举的线程安全性及序列化问题
https://www.hollischuang.com/archives/197
name() 和 toString()
name() 和 toString() 之间的主要区别是name() 是一个 final 方法，因此它不能被覆盖。toString() 方法返回的值与 name() 默认值相同，但 toString() 可以被 Enum 的子类覆盖。
因此，如果需要字段本身的名称，请使用 name()。如果需要字段值的字符串表示，请使用 toString()。
例如：
public enum WeekDay {
MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY;
public String toString() {
return name().charAt(0) + name().substring(1).toLowerCase();
}
}
在这个例子中，WeekDay.MONDAY.name() 返回 MONDAYWeekDay.MONDAY.toString() 返回 Monday
WeekDay.valueOf(WeekDay.MONDAY.name()) 返回WeekDay.MONDAY
但WeekDay.valueOf(WeekDay.MONDAY.toString())会抛出IllegalArgumentException。
What is the difference between Enum.name() and Enum.toString()? [duplicate]
https://stackoverflow.com/questions/18031125/what-is-the-difference-between-enum-name-and-enum-tostring/18031576
枚举比较
使用 == 和使用equals方法的执行结果是一样的。
因为在Enum类里面，已经重写了equals方法，而方法里面比较就是直接使用==，来比较2个对象的。所以，你在外边直接使用==也是可以的。
/**
* 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;
}
为什么枚举可以用==比较？
JLS 8.9 Enums 一个枚举类型除了定义的那些枚举常量外没有其他实例了。 试图明确地说明一种枚举类型是会导致编译期异常。在枚举中final clone方法确保枚举常量从不会被克隆，而且序列化机制会确保从不会因为反序列化而创造复制的实例。枚举类型的反射实例化也是被禁止的。总之，以上内容确保了除了定义的枚举常量之外，没有枚举类型实例。
因为每个枚举常量只有一个实例，所以如果在比较两个参考值，至少有一个涉及到枚举常量时，允许使用“==”代替equals()。
什么时候 == 和 equals 不一样？
== 不会抛出 NullPointerException
enum Color { BLACK, WHITE };
Color nothing = null;
if (nothing == Color.BLACK); // runs fine
if (nothing.equals(Color.BLACK)); // throws NullPointerException
== 在编译期检测类型兼容性
enum Color { BLACK, WHITE };
enum Chiral { LEFT, RIGHT };
if (Color.BLACK.equals(Chiral.LEFT)); // compiles fine
if (Color.BLACK == Chiral.LEFT); // DOESN'T COMPILE!!! Incompatible types!
比较java枚举成员使用equal还是==
https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/comparing-java-enum-members-or-equals.md
枚举使用举例
兼容Jackson序列化的枚举类
package com.madaimeng;
import com.fasterxml.jackson.annotation.JsonCreator;
import com.google.common.collect.Maps;
import java.util.Collections;
import java.util.Map;
import org.apache.commons.lang3.StringUtils;
public enum AuthorizationType {
UNKNOWN((byte) 0, "", "未知"),
AUTHORIZED((byte) 1, "granted", "授权"),
REVOKE((byte) 2, "revoked", "取消授权");
private byte code; // 枚举的code，可用于和数据库字段映射
private String mappingCode; // 对应的外系统code，用于接收外系统json数据时映射
private String description; // 描述
// 枚举的构造方法
VehicleAuthorizationType(byte code, String mappingCode, String description) {
this.code = code;
this.mappingCode = mappingCode;
this.description = description;
}
public byte getCode() {
return code;
}
public String getMappingCode() {
return mappingCode;
}
public String getDescription() {
return description;
}
// 枚举内定义了两个map，分别用于根据外系统code查找对应的枚举值，和根据code查找对应的枚举值
private static Map<Byte, VehicleAuthorizationType> codeMap = Maps.newHashMap();
private static Map<String, VehicleAuthorizationType> mappingCodeMap = Maps.newHashMap();
// 初始化两个map
static {
for (VehicleAuthorizationType value : VehicleAuthorizationType.values()) {
codeMap.put(value.getCode(), value);
mappingCodeMap.put(value.mappingCode, value);
}
codeMap = Collections.unmodifiableMap(codeMap);
mappingCodeMap = Collections.unmodifiableMap(mappingCodeMap);
}
// 通过 @JsonCreator 标记为 jackson 反序列化构造方法，入参str是外系统code，即mappingCode
@JsonCreator
public static VehicleAuthorizationType getVehicleAuthorizationType(String str) {
if (StringUtils.isBlank(str)) {
return UNKNOWN;
}
return mappingCodeMap.get(str);
}
// 根据code查找对应的枚举类型
public static VehicleAuthorizationType getByCode(byte code) {
return codeMap.get(code);
}
// 根据外系统code查找对应的枚举类型
public static VehicleAuthorizationType getByMappingCode(String mappingCode) {
return mappingCodeMap.get(mappingCode);
}
}
通过一个接口IEnum规范枚举类行为
通过一个接口 IEnum 规范枚举类行为
package com.masikkk.enums;
public interface IEnum<T> {
public T getValue();
public String getDesc();
}
所以定义的枚举类都实现这个接口
package com.masikkk.enums;
import com.fasterxml.jackson.annotation.JsonCreator;
import com.masikkk.enums.IEnum;
import org.apache.commons.lang3.StringUtils;
public enum MobileType implements IEnum<String> {
PERSONAL("私人手机"),
WORK("工作手机");
private String description;
MobileType(String description) {
this.description = description;
}
@Override
public String getValue() {
return name();
}
@Override
public String getDesc() {
return description;
}
@JsonCreator
public static MobileType getByName(String str) {
return StringUtils.isBlank(str) ? null : MobileType.valueOf(str.trim().toUpperCase());
}
}
java.lang.Enum 抽象类源码
package java.lang;
import java.io.Serializable;
import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectStreamException;
/**
* 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
* @author Josh Bloch
* @author Neal Gafter
* @see Class#getEnumConstants()
* @see java.util.EnumSet
* @see java.util.EnumMap
* @since 1.5
*/
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; //枚举字符串名称
/**
* 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; //枚举顺序值，从0开始
/**
* 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 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");
}
}
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