## Chapter 6. Enums and Annotations（枚举和注解）
### Item 34: Use enums instead of int constants（用枚举类型代替 int 常量）
An enumerated type is a type whose legal values consist of a fixed set of constants, such as the seasons of the year, the planets in the solar system, or the suits in a deck of playing cards. Before enum types were added to the language, a common pattern for representing enumerated types was to declare a group of named int constants, one for each member of the type:
枚举类型是这样一种类型：它合法的值由一组固定的常量组成，如：一年中的季节、太阳系中的行星或扑克牌中的花色。在枚举类型被添加到 JAVA 之前，表示枚举类型的一种常见模式是声明一组 int 的常量，每个类型的成员都有一个：
```
// The int enum pattern - severely deficient!
public static final int APPLE_FUJI = 0;
public static final int APPLE_PIPPIN = 1;
public static final int APPLE_GRANNY_SMITH = 2;
public static final int ORANGE_NAVEL = 0;
public static final int ORANGE_TEMPLE = 1;
public static final int ORANGE_BLOOD = 2;
```
This technique, known as the int enum pattern, has many shortcomings. It provides nothing in the way of type safety and little in the way of expressive power. The compiler won’t complain if you pass an apple to a method that expects an orange, compare apples to oranges with the == operator, or worse:
这种技术称为 int 枚举模式，它有许多缺点。它没有提供任何类型安全性，并且几乎不具备表现力。如果你传递一个苹果给方法，希望得到一个橘子，使用 == 操作符比较苹果和橘子时编译器并不会提示错误，或更糟的情况：
```
// Tasty citrus flavored applesauce!
int i = (APPLE_FUJI - ORANGE_TEMPLE) / APPLE_PIPPIN;
```
Note that the name of each apple constant is prefixed with APPLE_ and the name of each orange constant is prefixed with ORANGE_. This is because Java doesn’t provide namespaces for int enum groups. Prefixes prevent name clashes when two int enum groups have identically named constants, for example between ELEMENT_MERCURY and PLANET_MERCURY.
注意，每个 apple 常量的名称都以 APPLE_ 为前缀，每个 orange 常量的名称都以 ORANGE_ 为前缀。这是因为 Java 不为这些 int 枚举提供名称空间。当两组 int 枚举具有相同的命名常量时，前缀可以防止名称冲突，例如 ELEMENT_MERCURY 和 PLANET_MERCURY 之间的冲突。
Programs that use int enums are brittle. Because int enums are constant variables [JLS, 4.12.4], their int values are compiled into the clients that use them [JLS, 13.1]. If the value associated with an int enum is changed, its clients must be recompiled. If not, the clients will still run, but their behavior will be incorrect.
使用 int 枚举的程序很脆弱。因为 int 枚举是常量变量 [JLS, 4.12.4]，所以它们的值被编译到使用它们的客户端中 [JLS, 13.1]。如果与 int 枚举关联的值发生了更改，则必须重新编译客户端。如果不重新编译，客户端仍然可以运行，但是他们的行为将是错误的。
There is no easy way to translate int enum constants into printable strings. If you print such a constant or display it from a debugger, all you see is a number, which isn’t very helpful. There is no reliable way to iterate over all the int enum constants in a group, or even to obtain the size of an int enum group.
没有一种简单的方法可以将 int 枚举常量转换为可打印的字符串。如果你打印这样的常量或从调试器中显示它，你所看到的只是一个数字，这不是很有帮助。没有可靠的方法可以遍历组中的所有 int 枚举常量，甚至无法获得组的大小。
You may encounter a variant of this pattern in which String constants are used in place of int constants. This variant, known as the String enum pattern, is even less desirable. While it does provide printable strings for its constants, it can lead naive users to hard-code string constants into client code instead of using field names. If such a hard-coded string constant contains a typographical error, it will escape detection at compile time and result in bugs at runtime. Also, it might lead to performance problems, because it relies on string comparisons.
可能会遇到这种模式的另一种形式：使用 String 常量代替 int 常量。这种称为 String 枚举模式的变体甚至更不可取。虽然它确实为常量提供了可打印的字符串，但是它可能会导致不知情的用户将字符串常量硬编码到客户端代码中，而不是使用字段名。如果这样一个硬编码的 String 常量包含一个排版错误，它将在编译时躲过检测，并在运行时导致错误。此外，它可能会导致性能问题，因为它依赖于字符串比较。
Luckily, Java provides an alternative that avoids all the shortcomings of the int and string enum patterns and provides many added benefits. It is the enum type [JLS, 8.9]. Here’s how it looks in its simplest form:
幸运的是，Java 提供了一种替代方案，它避免了 int 和 String 枚举模式的所有缺点，并提供了许多额外的好处。它就是枚举类型 [JLS, 8.9]。下面是它最简单的形式：
```
public enum Apple { FUJI, PIPPIN, GRANNY_SMITH }
public enum Orange { NAVEL, TEMPLE, BLOOD }
```
On the surface, these enum types may appear similar to those of other languages, such as C, C++, and C#, but appearances are deceiving. Java’s enum types are full-fledged classes, far more powerful than their counterparts in these other languages, where enums are essentially int values.
从表面上看，这些枚举类型可能与其他语言（如 C、c++ 和 c#）的枚举类型类似，但不能只看表象。Java 的枚举类型是成熟的类，比其他语言中的枚举类型功能强大得多，在其他语言中的枚举本质上是 int 值。
The basic idea behind Java’s enum types is simple: they are classes that export one instance for each enumeration constant via a public static final field. Enum types are effectively final, by virtue of having no accessible constructors. Because clients can neither create instances of an enum type nor extend it, there can be no instances but the declared enum constants. In other words, enum types are instance-controlled (page 6). They are a generalization of singletons (Item 3), which are essentially single-element enums.
Java 枚举类型背后的基本思想很简单：它们是通过 public static final 修饰的字段为每个枚举常量导出一个实例的类。枚举类型实际上是 final 类型，因为没有可访问的构造函数。客户端既不能创建枚举类型的实例，也不能继承它，所以除了声明的枚举常量之外，不能有任何实例。换句话说，枚举类型是实例受控的类（参阅第 6 页，[Item-1](/Chapter-2/Chapter-2-Item-1-Consider-static-factory-methods-instead-of-constructors.md)）。它们是单例（[Item-3](/Chapter-2/Chapter-2-Item-3-Enforce-the-singleton-property-with-a-private-constructor-or-an-enum-type.md)）的推广应用，单例本质上是单元素的枚举。
Enums provide compile-time type safety. If you declare a parameter to be of type Apple, you are guaranteed that any non-null object reference passed to the parameter is one of the three valid Apple values. Attempts to pass values of the wrong type will result in compile-time errors, as will attempts to assign an expression of one enum type to a variable of another, or to use the == operator to compare values of different enum types.
枚举提供编译时类型的安全性。如果将参数声明为 Apple 枚举类型，则可以保证传递给该参数的任何非空对象引用都是三个有效 Apple 枚举值之一。尝试传递错误类型的值将导致编译时错误，将一个枚举类型的表达式赋值给另一个枚举类型的变量，或者使用 == 运算符比较不同枚举类型的值同样会导致错误。
Enum types with identically named constants coexist peacefully because each type has its own namespace. You can add or reorder constants in an enum type without recompiling its clients because the fields that export the constants provide a layer of insulation between an enum type and its clients: constant values are not compiled into the clients as they are in the int enum patterns. Finally, you can translate enums into printable strings by calling their toString method.
名称相同的枚举类型常量能和平共存，因为每种类型都有自己的名称空间。你可以在枚举类型中添加或重新排序常量，而无需重新编译其客户端，因为导出常量的字段在枚举类型及其客户端之间提供了一层隔离：常量值不会像在 int 枚举模式中那样编译到客户端中。最后，你可以通过调用枚举的 toString 方法将其转换为可打印的字符串。
In addition to rectifying the deficiencies of int enums, enum types let you add arbitrary methods and fields and implement arbitrary interfaces. They provide high-quality implementations of all the Object methods (Chapter 3), they implement Comparable (Item 14) and Serializable (Chapter 12), and their serialized form is designed to withstand most changes to the enum type.
除了纠正 int 枚举的不足之外，枚举类型还允许添加任意方法和字段并实现任意接口。它们提供了所有 Object 方法的高质量实现（参阅 Chapter 3），还实现了 Comparable（[Item-14](/Chapter-3/Chapter-3-Item-14-Consider-implementing-Comparable.md)）和 Serializable（参阅 Chapter 12），并且它们的序列化形式被设计成能够适应枚举类型的可变性。
So why would you want to add methods or fields to an enum type? For starters, you might want to associate data with its constants. Our Apple and Orange types, for example, might benefit from a method that returns the color of the fruit, or one that returns an image of it. You can augment an enum type with any method that seems appropriate. An enum type can start life as a simple collection of enum constants and evolve over time into a full-featured abstraction.
那么，为什么要向枚举类型添加方法或字段呢？首先，你可能希望将数据与其常量关联起来。例如，我们的 Apple 和 Orange 类型可能受益于返回水果颜色的方法，或者返回水果图像的方法。你可以使用任何适当的方法来扩充枚举类型。枚举类型可以从枚举常量的简单集合开始，并随着时间的推移演变为功能齐全的抽象。
For a nice example of a rich enum type, consider the eight planets of our solar system. Each planet has a mass and a radius, and from these two attributes you can compute its surface gravity. This in turn lets you compute the weight of an object on the planet’s surface, given the mass of the object. Here’s how this enum looks. The numbers in parentheses after each enum constant are parameters that are passed to its constructor. In this case, they are the planet’s mass and radius:
对于富枚举类型来说，有个很好的例子，考虑我们太阳系的八颗行星。每颗行星都有质量和半径，通过这两个属性你可以计算出它的表面引力。反过来，可以给定物体的质量，让你计算出一个物体在行星表面的重量。这个枚举是这样的。每个枚举常量后括号中的数字是传递给其构造函数的参数。在本例中，它们是行星的质量和半径：
```
// Enum type with data and behavior
public enum Planet {
MERCURY(3.302e+23, 2.439e6),
VENUS (4.869e+24, 6.052e6),
EARTH (5.975e+24, 6.378e6),
MARS (6.419e+23, 3.393e6),
JUPITER(1.899e+27, 7.149e7),
SATURN (5.685e+26, 6.027e7),
URANUS (8.683e+25, 2.556e7),
NEPTUNE(1.024e+26, 2.477e7);
private final double mass; // In kilograms
private final double radius; // In meters
private final double surfaceGravity; // In m / s^2
// Universal gravitational constant in m^3 / kg s^2
private static final double G = 6.67300E-11;
// Constructor
Planet(double mass, double radius) {
this.mass = mass;
this.radius = radius;
surfaceGravity = G * mass / (radius * radius);
}
public double mass() { return mass; }
public double radius() { return radius; }
public double surfaceGravity() { return surfaceGravity; }
public double surfaceWeight(double mass) {
return mass * surfaceGravity; // F = ma
}
}
```
It is easy to write a rich enum type such as Planet. **To associate data with enum constants, declare instance fields and write a constructor that takes the data and stores it in the fields.** Enums are by their nature immutable, so all fields should be final (Item 17). Fields can be public, but it is better to make them private and provide public accessors (Item 16). In the case of Planet, the constructor also computes and stores the surface gravity, but this is just an optimization. The gravity could be recomputed from the mass and radius each time it was used by the surfaceWeight method, which takes an object’s mass and returns its weight on the planet represented by the constant. While the Planet enum is simple, it is surprisingly powerful. Here is a short program that takes the earth weight of an object (in any unit) and prints a nice table of the object’s weight on all eight planets (in the same unit):
编写一个富枚举类型很容易，如上述的 Planet。**要将数据与枚举常量关联，可声明实例字段并编写一个构造函数，该构造函数接受数据并将其存储在字段中。** 枚举本质上是不可变的，因此所有字段都应该是 final（[Item-17](/Chapter-4/Chapter-4-Item-17-Minimize-mutability.md)）。字段可以是公共的，但是最好将它们设置为私有并提供公共访问器（[Item-16](/Chapter-4/Chapter-4-Item-16-In-public-classes-use-accessor-methods-not-public-fields.md)）。在 Planet 的例子中，构造函数还计算和存储表面重力，但这只是一个优化。每一次使用 surfaceWeight 方法时，都可以通过质量和半径重新计算重力。surfaceWeight 方法获取一个物体的质量，并返回其在该常数所表示的行星上的重量。虽然 Planet 枚举很简单，但它的力量惊人。下面是一个简短的程序，它获取一个物体的地球重量（以任何单位表示），并打印一个漂亮的表格，显示该物体在所有 8 个行星上的重量（以相同的单位表示）：
```
public class WeightTable {
public static void main(String[] args) {
double earthWeight = Double.parseDouble(args[0]);
double mass = earthWeight / Planet.EARTH.surfaceGravity();
for (Planet p : Planet.values())
System.out.printf("Weight on %s is %f%n",p, p.surfaceWeight(mass));
}
}
```
Note that Planet, like all enums, has a static values method that returns an array of its values in the order they were declared. Note also that the toString method returns the declared name of each enum value, enabling easy printing by println and printf. If you’re dissatisfied with this string representation, you can change it by overriding the toString method. Here is the result of running our WeightTable program (which doesn’t override toString) with the command line argument 185:
请注意，Planet 和所有枚举一样，有一个静态 values() 方法，该方法按照声明值的顺序返回其值的数组。还要注意的是，toString 方法返回每个枚举值的声明名称，这样就可以通过 println 和 printf 轻松打印。如果你对这个字符串表示不满意，可以通过重写 toString 方法来更改它。下面是用命令行运行我们的 WeightTable 程序（未覆盖 toString）的结果：
```
Weight on MERCURY is 69.912739
Weight on VENUS is 167.434436
Weight on EARTH is 185.000000
Weight on MARS is 70.226739
Weight on JUPITER is 467.990696
Weight on SATURN is 197.120111
Weight on URANUS is 167.398264
Weight on NEPTUNE is 210.208751
```
Until 2006, two years after enums were added to Java, Pluto was a planet. This raises the question “what happens when you remove an element from an enum type?” The answer is that any client program that doesn’t refer to the removed element will continue to work fine. So, for example, our WeightTable program would simply print a table with one fewer row. And what of a client program that refers to the removed element (in this case, Planet.Pluto)? If you recompile the client program, the compilation will fail with a helpful error message at the line that refers to the erstwhile planet; if you fail to recompile the client, it will throw a helpful exception from this line at runtime. This is the best behavior you could hope for, far better than what you’d get with the int enum pattern.
直到 2006 年，也就是枚举被添加到 Java 的两年后，冥王星还是一颗行星。这就提出了一个问题：「从枚举类型中删除元素时会发生什么?」答案是，任何不引用被删除元素的客户端程序将继续正常工作。例如，我们的 WeightTable 程序只需打印一个少一行的表。那么引用被删除元素（在本例中是 Planet.Pluto）的客户端程序又如何呢？如果重新编译客户端程序，编译将失败，并在引用该「过时」行星的行中显示一条有用的错误消息；如果你未能重新编译客户端，它将在运行时从这行抛出一个有用的异常。这是你所希望的最佳行为，比 int 枚举模式要好得多。
Some behaviors associated with enum constants may need to be used only from within the class or package in which the enum is defined. Such behaviors are best implemented as private or package-private methods. Each constant then carries with it a hidden collection of behaviors that allows the class or package containing the enum to react appropriately when presented with the constant. Just as with other classes, unless you have a compelling reason to expose an enum method to its clients, declare it private or, if need be, package-private (Item 15).
与枚举常量相关的一些行为可能只需要在定义枚举的类或包中使用。此类行为最好以私有或包私有方法来实现。然后，每个常量都带有一个隐藏的行为集合，允许包含枚举的类或包在使用该常量时做出适当的反应。与其他类一样，除非你有充分的理由向其客户端公开枚举方法，否则将其声明为私有的，或者在必要时声明为包私有（[Item-15](/Chapter-4/Chapter-4-Item-15-Minimize-the-accessibility-of-classes-and-members.md)）。
If an enum is generally useful, it should be a top-level class; if its use is tied to a specific top-level class, it should be a member class of that top-level class (Item 24). For example, the java.math.RoundingMode enum represents a rounding mode for decimal fractions. These rounding modes are used by the BigDecimal class, but they provide a useful abstraction that is not fundamentally tied to BigDecimal. By making RoundingMode a top-level enum, the library designers encourage any programmer who needs rounding modes to reuse this enum, leading to increased consistency across APIs.
通常，如果一个枚举用途广泛，那么它应该是顶级类；如果它被绑定到一个特定的顶级类使用，那么它应该是这个顶级类（[Item-24](/Chapter-4/Chapter-4-Item-24-Favor-static-member-classes-over-nonstatic.md)）的成员类。例如，java.math.RoundingMode 枚举表示小数部分的舍入模式。BigDecimal 类使用这些四舍五入模式，但是它们提供了一个有用的抽象，这个抽象与 BigDecimal 没有本质上的联系。通过使 RoundingMode 成为顶级枚举，库设计人员支持任何需要舍入模式的程序员复用该枚举，从而提高 API 之间的一致性。
The techniques demonstrated in the Planet example are sufficient for most enum types, but sometimes you need more. There is different data associated with each Planet constant, but sometimes you need to associate fundamentally different behavior with each constant. For example, suppose you are writing an enum type to represent the operations on a basic four-function calculator and you want to provide a method to perform the arithmetic operation represented by each constant. One way to achieve this is to switch on the value of the enum:
Planet 示例中演示的技术对于大多数枚举类型来说已经足够了，但有时还需要更多。每个行星常数都有不同的数据，但有时你需要将基本不同的行为与每个常数联系起来。例如，假设你正在编写一个枚举类型来表示一个基本的四则运算计算器上的操作，并且你希望提供一个方法来执行由每个常量表示的算术操作。实现这一点的一种方式是用 switch 接收枚举值：
```
// Enum type that switches on its own value - questionable
public enum Operation {
PLUS, MINUS, TIMES, DIVIDE;
// Do the arithmetic operation represented by this constant
public double apply(double x, double y) {
switch(this) {
case PLUS: return x + y;
case MINUS: return x - y;
case TIMES: return x * y;
case DIVIDE: return x / y;
}
throw new AssertionError("Unknown op: "+ this);
}
}
```
This code works, but it isn’t very pretty. It won’t compile without the throw statement because the end of the method is technically reachable, even though it will never be reached [JLS, 14.21]. Worse, the code is fragile. If you add a new enum constant but forget to add a corresponding case to the switch, the enum will still compile, but it will fail at runtime when you try to apply the new operation.
这段代码可以工作，但不是很漂亮。如果没有 throw 语句，它将无法编译，因为从理论上讲，方法的结尾是可到达的，尽管它确实永远不会到达 [JLS, 14.21]。更糟糕的是，代码很脆弱。如果你添加了一个新的枚举常量，但忘记向 switch 添加相应的 case，则枚举仍将编译，但在运行时尝试应用新操作时将失败。
Luckily, there is a better way to associate a different behavior with each enum constant: declare an abstract apply method in the enum type, and override it with a concrete method for each constant in a constant-specific class body. Such methods are known as constant-specific method implementations:
幸运的是，有一种更好的方法可以将不同的行为与每个枚举常量关联起来：在枚举类型中声明一个抽象的 apply 方法，并用一个特定于常量的类体中的每个常量的具体方法覆盖它。这些方法称为特定常量方法实现：
```
// Enum type with constant-specific method implementations
public enum Operation {
PLUS {public double apply(double x, double y){return x + y;}},
MINUS {public double apply(double x, double y){return x - y;}},
TIMES {public double apply(double x, double y){return x * y;}},
DIVIDE{public double apply(double x, double y){return x / y;}};
public abstract double apply(double x, double y);
}
```
If you add a new constant to the second version of Operation, it is unlikely that you’ll forget to provide an apply method, because the method immediately follows each constant declaration. In the unlikely event that you do forget, the compiler will remind you because abstract methods in an enum type must be overridden with concrete methods in all of its constants.
如果你在 Operation 枚举的第二个版本中添加一个新常量，那么你不太可能忘记提供一个 apply 方法，因为该方法紧跟每个常量声明。在不太可能忘记的情况下，编译器会提醒你，因为枚举类型中的抽象方法必须用其所有常量中的具体方法覆盖。
Constant-specific method implementations can be combined with constantspecific data. For example, here is a version of Operation that overrides the toString method to return the symbol commonly associated with the operation:
特定常量方法实现可以与特定于常量的数据相结合。例如，下面是 Operation 枚举的一个版本，它重写 toString 方法来返回与操作相关的符号：
**译注：原文 constantspecific data 应修改为 constant-specific data ，译为「特定常量数据」**
```
// Enum type with constant-specific class bodies and data
public enum Operation {
PLUS("+") {
public double apply(double x, double y) { return x + y; }
},
MINUS("-") {
public double apply(double x, double y) { return x - y; }
},
TIMES("*") {
public double apply(double x, double y) { return x * y; }
},
DIVIDE("/") {
public double apply(double x, double y) { return x / y; }
};
private final String symbol;
Operation(String symbol) { this.symbol = symbol; }
@Override
public String toString() { return symbol; }
public abstract double apply(double x, double y);
}
```
The toString implementation shown makes it easy to print arithmetic expressions, as demonstrated by this little program:
重写的 toString 实现使得打印算术表达式变得很容易，如下面的小程序所示：
```
public static void main(String[] args) {
double x = Double.parseDouble(args[0]);
double y = Double.parseDouble(args[1]);
for (Operation op : Operation.values())
System.out.printf("%f %s %f = %f%n",x, op, y, op.apply(x, y));
}
```
Running this program with 2 and 4 as command line arguments produces the following output:
以 2 和 4 作为命令行参数运行这个程序将产生以下输出：
```
2.000000 + 4.000000 = 6.000000
2.000000 - 4.000000 = -2.000000
2.000000 * 4.000000 = 8.000000
2.000000 / 4.000000 = 0.500000
```
Enum types have an automatically generated valueOf(String) method that translates a constant’s name into the constant itself. If you override the toString method in an enum type, consider writing a fromString method to translate the custom string representation back to the corresponding enum. The following code (with the type name changed appropriately) will do the trick for any enum, so long as each constant has a unique string representation:
枚举类型有一个自动生成的 valueOf(String) 方法，该方法将常量的名称转换为常量本身。如果在枚举类型中重写 toString 方法，可以考虑编写 fromString 方法将自定义字符串表示形式转换回相应的枚举。只要每个常量都有唯一的字符串表示形式，下面的代码（类型名称适当更改）就可以用于任何枚举：
```
// Implementing a fromString method on an enum type
private static final Map<String, Operation> stringToEnum =Stream.of(values()).collect(toMap(Object::toString, e -> e));
// Returns Operation for string, if any
public static Optional<Operation> fromString(String symbol) {
return Optional.ofNullable(stringToEnum.get(symbol));
}
```
Note that the Operation constants are put into the stringToEnum map from a static field initialization that runs after the enum constants have been created. The previous code uses a stream (Chapter 7) over the array returned by the values() method; prior to Java 8, we would have created an empty hash map and iterated over the values array inserting the string-to-enum mappings into the map, and you can still do it that way if you prefer. But note that attempting to have each constant put itself into a map from its own constructor does not work. It would cause a compilation error, which is good thing because if it were legal, it would cause a NullPointerException at runtime. Enum constructors aren’t permitted to access the enum’s static fields, with the exception of constant variables (Item 34). This restriction is necessary because static fields have not yet been initialized when enum constructors run. A special case of this restriction is that enum constants cannot access one another from their constructors.
注意，Operation 枚举的常量是从创建枚举常量之后运行的静态字段初始化中放入 stringToEnum 的。上述代码在 values() 方法返回的数组上使用流（参阅第 7 章）；在 Java 8 之前，我们将创建一个空 HashMap，并遍历值数组，将自定义字符串与枚举的映射插入到 HashMap 中，如果你愿意，你仍然可以这样做。但是请注意，试图让每个常量通过构造函数将自身放入 HashMap 中是行不通的。它会导致编译错误，这是好事，因为如果合法，它会在运行时导致 NullPointerException。枚举构造函数不允许访问枚举的静态字段，常量变量除外（[Item-34](/Chapter-6/Chapter-6-Item-34-Use-enums-instead-of-int-constants.md)）。这个限制是必要的，因为在枚举构造函数运行时静态字段还没有初始化。这种限制的一个特殊情况是枚举常量不能从它们的构造函数中相互访问。
Also note that the fromString method returns an `Optional<String>`. This allows the method to indicate that the string that was passed in does not represent a valid operation, and it forces the client to confront this possibility (Item 55).
还要注意 fromString 方法返回一个 `Optional<String>`。这允许该方法提示传入的字符串并非有效操作，并强制客户端处理这种可能（[Item-55](/Chapter-8/Chapter-8-Item-55-Return-optionals-judiciously.md)）。
A disadvantage of constant-specific method implementations is that they make it harder to share code among enum constants. For example, consider an enum representing the days of the week in a payroll package. This enum has a method that calculates a worker’s pay for that day given the worker’s base salary (per hour) and the number of minutes worked on that day. On the five weekdays, any time worked in excess of a normal shift generates overtime pay; on the two weekend days, all work generates overtime pay. With a switch statement, it’s easy to do this calculation by applying multiple case labels to each of two code fragments:
特定常量方法实现的一个缺点是，它们使得在枚举常量之间共享代码变得更加困难。例如，考虑一个表示一周当中计算工资发放的枚举。枚举有一个方法，该方法根据工人的基本工资（每小时）和当天的工作分钟数计算工人当天的工资。在五个工作日内，任何超过正常轮班时间的工作都会产生加班费；在两个周末，所有的工作都会产生加班费。使用 switch 语句，通过多个 case 标签应用于每一类情况，可以很容易地进行计算：
```
// Enum that switches on its value to share code - questionable
enum PayrollDay {
MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY,SATURDAY, SUNDAY;
private static final int MINS_PER_SHIFT = 8 * 60;
int pay(int minutesWorked, int payRate) {
int basePay = minutesWorked * payRate;
int overtimePay;
switch(this) {
case SATURDAY:
case SUNDAY: // Weekend
overtimePay = basePay / 2;
break;
default: // Weekday
overtimePay = minutesWorked <= MINS_PER_SHIFT ?0 : (minutesWorked - MINS_PER_SHIFT) * payRate / 2;
}
return basePay + overtimePay;
}
}
```
**译注 1：该例子中，加班的每分钟工资为工作日每分钟工资（payRate）的一半**
**译注 2：原文中 pay 方法存在问题，说明如下：**
```
// 基本工资 basePay 不应该直接将工作时间参与计算，如果工作日存在加班的情况，会将加班时间也计入基本工资计算。假设在周一工作 10 小时，假设每分钟 1 元：
/*
修改前：
基本工资 basePay = minutesWorked * payRate=10*60*1=600（不应该将 2 小时加班也计入正常工作时间）
加班工资 overtimePay = (minutesWorked - MINS_PER_SHIFT) * payRate / 2=2*60*1/2=60
合计= basePay + overtimePay=660
修改后：
基本工资 basePay = MINS_PER_SHIFT * payRate=8*60*1=480（基本工资最高只能按照 8 小时计算）
加班工资 overtimePay = (minutesWorked - MINS_PER_SHIFT) * payRate / 2=2*60*1/2=60
合计= basePay + overtimePay=540
*/
// 修改后代码：
int pay(int minutesWorked, int payRate) {
int basePay = 0;
int overtimePay;
switch (this) {
case SATURDAY:
case SUNDAY: // Weekend
overtimePay = minutesWorked * payRate / 2;
break;
default: // Weekday
basePay = minutesWorked <= MINS_PER_SHIFT ? minutesWorked * payRate : MINS_PER_SHIFT * payRate;
overtimePay = minutesWorked <= MINS_PER_SHIFT ? 0 : (minutesWorked - MINS_PER_SHIFT) * payRate / 2;
}
return basePay + overtimePay;
}
```
This code is undeniably concise, but it is dangerous from a maintenance perspective. Suppose you add an element to the enum, perhaps a special value to represent a vacation day, but forget to add a corresponding case to the switch statement. The program will still compile, but the pay method will silently pay the worker the same amount for a vacation day as for an ordinary weekday.
不可否认，这段代码非常简洁，但是从维护的角度来看，它是危险的。假设你向枚举中添加了一个元素，可能是一个表示假期的特殊值，但是忘记向 switch 语句添加相应的 case。这个程序仍然会被编译，但是 pay 方法会把假期默认当做普通工作日并支付工资。
To perform the pay calculation safely with constant-specific method implementations, you would have to duplicate the overtime pay computation for each constant, or move the computation into two helper methods, one for weekdays and one for weekend days, and invoke the appropriate helper method from each constant. Either approach would result in a fair amount of boilerplate code, substantially reducing readability and increasing the opportunity for error.
为了使用特定常量方法实现安全地执行工资计算，你必须为每个常量复制加班费计算，或者将计算移动到两个辅助方法中，一个用于工作日，一个用于周末，再从每个常量调用适当的辅助方法。任何一种方法都会导致相当数量的样板代码，极大地降低可读性并增加出错的机会。
The boilerplate could be reduced by replacing the abstract overtimePay method on PayrollDay with a concrete method that performs the overtime calculation for weekdays. Then only the weekend days would have to override the method. But this would have the same disadvantage as the switch statement: if you added another day without overriding the overtimePay method, you would silently inherit the weekday calculation.
用工作日加班计算的具体方法代替发薪日的抽象加班法，可以减少样板。那么只有周末才需要重写该方法。但是这与 switch 语句具有相同的缺点：如果你在不覆盖 overtimePay 方法的情况下添加了另一天，那么你将默默地继承工作日的计算。
What you really want is to be forced to choose an overtime pay strategy each time you add an enum constant. Luckily, there is a nice way to achieve this. The idea is to move the overtime pay computation into a private nested enum, and to pass an instance of this strategy enum to the constructor for the PayrollDay enum. The PayrollDay enum then delegates the overtime pay calculation to the strategy enum, eliminating the need for a switch statement or constantspecific method implementation in PayrollDay. While this pattern is less concise than the switch statement, it is safer and more flexible:
你真正想要的是在每次添加枚举常量时被迫选择加班费策略。幸运的是，有一个很好的方法可以实现这一点。其思想是将加班费计算移到私有嵌套枚举中，并将此策略枚举的实例传递给 PayrollDay 枚举的构造函数。然后 PayrollDay 枚举将加班费计算委托给策略枚举，从而消除了在 PayrollDay 中使用 switch 语句或特定于常量的方法实现的需要。虽然这种模式不如 switch 语句简洁，但它更安全，也更灵活：
```
// The strategy enum pattern
enum PayrollDay {
MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY,SATURDAY(PayType.WEEKEND), SUNDAY(PayType.WEEKEND);
private final PayType payType;
PayrollDay(PayType payType) { this.payType = payType; }
PayrollDay() { this(PayType.WEEKDAY); } // Default
int pay(int minutesWorked, int payRate) {
return payType.pay(minutesWorked, payRate);
}
// The strategy enum type
private enum PayType {
WEEKDAY {
int overtimePay(int minsWorked, int payRate) {
return minsWorked <= MINS_PER_SHIFT ? 0 :(minsWorked - MINS_PER_SHIFT) * payRate / 2;
}
},
WEEKEND {
int overtimePay(int minsWorked, int payRate) {
return minsWorked * payRate / 2;
}
};
abstract int overtimePay(int mins, int payRate);
private static final int MINS_PER_SHIFT = 8 * 60;
int pay(int minsWorked, int payRate) {
int basePay = minsWorked * payRate;
return basePay + overtimePay(minsWorked, payRate);
}
}
}
```
**译注：上述代码 pay 方法也存将加班时间计入基本工资计算的问题，修改如下：**
```
int pay(int minsWorked, int payRate) {
int basePay = minsWorked <= MINS_PER_SHIFT ? minsWorked * payRate : MINS_PER_SHIFT * payRate;
return basePay + overtimePay(minsWorked, payRate);
}
```
If switch statements on enums are not a good choice for implementing constant-specific behavior on enums, what are they good for? **Switches on enums are good for augmenting enum types with constant-specific behavior.** For example, suppose the Operation enum is not under your control and you wish it had an instance method to return the inverse of each operation. You could simulate the effect with the following static method:
如果在枚举上实现特定常量的行为时 switch 语句不是一个好的选择，那么它们有什么用呢？**枚举中的 switch 有利于扩展具有特定常量行为的枚举类型。** 例如，假设 Operation 枚举不在你的控制之下，你希望它有一个实例方法来返回每个操作的逆操作。你可以用以下静态方法模拟效果：
```
// Switch on an enum to simulate a missing method
public static Operation inverse(Operation op) {
switch(op) {
case PLUS: return Operation.MINUS;
case MINUS: return Operation.PLUS;
case TIMES: return Operation.DIVIDE;
case DIVIDE: return Operation.TIMES;
default: throw new AssertionError("Unknown op: " + op);
}
}
```
You should also use this technique on enum types that are under your control if a method simply doesn’t belong in the enum type. The method may be required for some use but is not generally useful enough to merit inclusion in the enum type.
如果一个方法不属于枚举类型，那么还应该在你控制的枚举类型上使用这种技术。该方法可能适用于某些特殊用途，但通常如果没有足够的好处，就不值得包含在枚举类型中。
Enums are, generally speaking, comparable in performance to int constants. A minor performance disadvantage of enums is that there is a space and time cost to load and initialize enum types, but it is unlikely to be noticeable in practice.
一般来说，枚举在性能上可与 int 常量相比。枚举在性能上有一个小缺点，加载和初始化枚举类型需要花费空间和时间，但是在实际应用中这一点可能不太明显。
So when should you use enums? **Use enums any time you need a set of constants whose members are known at compile time.** Of course, this includes “natural enumerated types,” such as the planets, the days of the week, and the chess pieces. But it also includes other sets for which you know all the possible values at compile time, such as choices on a menu, operation codes, and command line flags. **It is not necessary that the set of constants in an enum type stay fixed for all time.** The enum feature was specifically designed to allow for binary compatible evolution of enum types.
那么什么时候应该使用枚举呢？**在需要一组常量时使用枚举，这些常量的成员在编译时是已知的。** 当然，这包括「自然枚举类型」，如行星、星期几和棋子。但是它还包括其他在编译时已知所有可能值的集合，例如菜单上的选项、操作代码和命令行标志。**枚举类型中的常量集没有必要一直保持固定。** 枚举的特性是专门为枚举类型的二进制兼容进化而设计的。
In summary, the advantages of enum types over int constants are compelling. Enums are more readable, safer, and more powerful. Many enums require no explicit constructors or members, but others benefit from associating data with each constant and providing methods whose behavior is affected by this data. Fewer enums benefit from associating multiple behaviors with a single method. In this relatively rare case, prefer constant-specific methods to enums that switch on their own values. Consider the strategy enum pattern if some, but not all, enum constants share common behaviors.
总之，枚举类型相对于 int 常量的优势是毋庸置疑的。枚举更易于阅读、更安全、更强大。许多枚举不需要显式构造函数或成员，但有些枚举则受益于将数据与每个常量关联，并提供行为受数据影响的方法。将多个行为与一个方法关联起来，这样的枚举更少。在这种相对少见的情况下，相对于使用 switch 的枚举，特定常量方法更好。如果枚举常量有一些（但不是全部）共享公共行为，请考虑策略枚举模式。
---
**[Back to contents of the chapter（返回章节目录）](/Chapter-6/Chapter-6-Introduction.md)**
- **Next Item（下一条目）：[Item 35: Use instance fields instead of ordinals（使用实例字段替代序数）](/Chapter-6/Chapter-6-Item-35-Use-instance-fields-instead-of-ordinals.md)**
- Chapter 2. Creating and Destroying Objects（创建和销毁对象）
- Item 1: Consider static factory methods instead of constructors（考虑以静态工厂方法代替构造函数）
- Item 2: Consider a builder when faced with many constructor parameters（在面对多个构造函数参数时，请考虑构建器）
- Item 3: Enforce the singleton property with a private constructor or an enum type（使用私有构造函数或枚举类型实施单例属性）
- Item 4: Enforce noninstantiability with a private constructor（用私有构造函数实施不可实例化）
- Item 5: Prefer dependency injection to hardwiring resources（依赖注入优于硬连接资源）
- Item 6: Avoid creating unnecessary objects（避免创建不必要的对象）
- Item 7: Eliminate obsolete object references（排除过时的对象引用）
- Item 8: Avoid finalizers and cleaners（避免使用终结器和清除器）
- Item 9: Prefer try with resources to try finally（使用 try-with-resources 优于 try-finally）
- Chapter 3. Methods Common to All Objects（对象的通用方法）
- Item 10: Obey the general contract when overriding equals（覆盖 equals 方法时应遵守的约定）
- Item 11: Always override hashCode when you override equals（当覆盖 equals 方法时，总要覆盖 hashCode 方法）
- Item 12: Always override toString（始终覆盖 toString 方法）
- Item 13: Override clone judiciously（明智地覆盖 clone 方法）
- Item 14: Consider implementing Comparable（考虑实现 Comparable 接口）
- Chapter 4. Classes and Interfaces（类和接口）
- Item 15: Minimize the accessibility of classes and members（尽量减少类和成员的可访问性）
- Item 16: In public classes use accessor methods not public fields（在公共类中，使用访问器方法，而不是公共字段）
- Item 17: Minimize mutability（减少可变性）
- Item 18: Favor composition over inheritance（优先选择复合而不是继承）
- Item 19: Design and document for inheritance or else prohibit it（继承要设计良好并且具有文档，否则禁止使用）
- Item 20: Prefer interfaces to abstract classes（接口优于抽象类）
- Item 21: Design interfaces for posterity（为后代设计接口）
- Item 22: Use interfaces only to define types（接口只用于定义类型）
- Item 23: Prefer class hierarchies to tagged classes（类层次结构优于带标签的类）
- Item 24: Favor static member classes over nonstatic（静态成员类优于非静态成员类）
- Item 25: Limit source files to a single top level class（源文件仅限有单个顶层类）
- Chapter 5. Generics（泛型）
- Item 26: Do not use raw types（不要使用原始类型）
- Item 27: Eliminate unchecked warnings（消除 unchecked 警告）
- Item 28: Prefer lists to arrays（list 优于数组）
- Item 29: Favor generic types（优先使用泛型）
- Item 30: Favor generic methods（优先使用泛型方法）
- Item 31: Use bounded wildcards to increase API flexibility（使用有界通配符增加 API 的灵活性）
- Item 32: Combine generics and varargs judiciously（明智地合用泛型和可变参数）
- Item 33: Consider typesafe heterogeneous containers（考虑类型安全的异构容器）
- Chapter 6. Enums and Annotations（枚举和注解）
- Item 34: Use enums instead of int constants（用枚举类型代替 int 常量）
- Item 35: Use instance fields instead of ordinals（使用实例字段替代序数）
- Item 36: Use EnumSet instead of bit fields（用 EnumSet 替代位字段）
- Item 37: Use EnumMap instead of ordinal indexing（使用 EnumMap 替换序数索引）
- Item 38: Emulate extensible enums with interfaces（使用接口模拟可扩展枚举）
- Item 39: Prefer annotations to naming patterns（注解优于命名模式）
- Item 40: Consistently use the Override annotation（坚持使用 @Override 注解）
- Item 41: Use marker interfaces to define types（使用标记接口定义类型）
- Chapter 7. Lambdas and Streams（λ 表达式和流）
- Item 42: Prefer lambdas to anonymous classes（λ 表达式优于匿名类）
- Item 43: Prefer method references to lambdas（方法引用优于 λ 表达式）
- Item 44: Favor the use of standard functional interfaces（优先使用标准函数式接口）
- Item 45: Use streams judiciously（明智地使用流）
- Item 46: Prefer side effect free functions in streams（在流中使用无副作用的函数）
- Item 47: Prefer Collection to Stream as a return type（优先选择 Collection 而不是流作为返回类型）
- Item 48: Use caution when making streams parallel（谨慎使用并行流）
- Chapter 8. Methods（方法）
- Item 49: Check parameters for validity（检查参数的有效性）
- Item 50: Make defensive copies when needed（在需要时制作防御性副本）
- Item 51: Design method signatures carefully（仔细设计方法签名）
- Item 52: Use overloading judiciously（明智地使用重载）
- Item 53: Use varargs judiciously（明智地使用可变参数）
- Item 54: Return empty collections or arrays, not nulls（返回空集合或数组，而不是 null）
- Item 55: Return optionals judiciously（明智地的返回 Optional）
- Item 56: Write doc comments for all exposed API elements（为所有公开的 API 元素编写文档注释）
- Chapter 9. General Programming（通用程序设计）
- Item 57: Minimize the scope of local variables（将局部变量的作用域最小化）
- Item 58: Prefer for-each loops to traditional for loops（for-each 循环优于传统的 for 循环）
- Item 59: Know and use the libraries（了解并使用库）
- Item 60: Avoid float and double if exact answers are required（若需要精确答案就应避免使用 float 和 double 类型）
- Item 61: Prefer primitive types to boxed primitives（基本数据类型优于包装类）
- Item 62: Avoid strings where other types are more appropriate（其他类型更合适时应避免使用字符串）
- Item 63: Beware the performance of string concatenation（当心字符串连接引起的性能问题）
- Item 64: Refer to objects by their interfaces（通过接口引用对象）
- Item 65: Prefer interfaces to reflection（接口优于反射）
- Item 66: Use native methods judiciously（明智地使用本地方法）
- Item 67: Optimize judiciously（明智地进行优化）
- Item 68: Adhere to generally accepted naming conventions（遵守被广泛认可的命名约定）
- Chapter 10. Exceptions（异常）
- Item 69: Use exceptions only for exceptional conditions（仅在确有异常条件下使用异常）
- Item 70: Use checked exceptions for recoverable conditions and runtime exceptions for programming errors（对可恢复情况使用 checked 异常，对编程错误使用运行时异常）
- Item 71: Avoid unnecessary use of checked exceptions（避免不必要地使用 checked 异常）
- Item 72: Favor the use of standard exceptions（鼓励复用标准异常）
- Item 73: Throw exceptions appropriate to the abstraction（抛出能用抽象解释的异常）
- Item 74: Document all exceptions thrown by each method（为每个方法记录会抛出的所有异常）
- Item 75: Include failure capture information in detail messages（异常详细消息中应包含捕获失败的信息）
- Item 76: Strive for failure atomicity（尽力保证故障原子性）
- Item 77: Don’t ignore exceptions（不要忽略异常）
- Chapter 11. Concurrency（并发）
- Item 78: Synchronize access to shared mutable data（对共享可变数据的同步访问）
- Item 79: Avoid excessive synchronization（避免过度同步）
- Item 80: Prefer executors, tasks, and streams to threads（Executor、task、流优于直接使用线程）
- Item 81: Prefer concurrency utilities to wait and notify（并发实用工具优于 wait 和 notify）
- Item 82: Document thread safety（文档应包含线程安全属性）
- Item 83: Use lazy initialization judiciously（明智地使用延迟初始化）
- Item 84: Don’t depend on the thread scheduler（不要依赖线程调度器）
- Chapter 12. Serialization（序列化）
- Item 85: Prefer alternatives to Java serialization（优先选择 Java 序列化的替代方案）
- Item 86: Implement Serializable with great caution（非常谨慎地实现 Serializable）
- Item 87: Consider using a custom serialized form（考虑使用自定义序列化形式）
- Item 88: Write readObject methods defensively（防御性地编写 readObject 方法）
- Item 89: For instance control, prefer enum types to readResolve（对于实例控制，枚举类型优于 readResolve）
- Item 90: Consider serialization proxies instead of serialized instances（考虑以序列化代理代替序列化实例）