## Chapter 5. Generics（泛型）
### Item 30: Favor generic methods（优先使用泛型方法）
Just as classes can be generic, so can methods. Static utility methods that operate on parameterized types are usually generic. All of the “algorithm” methods in Collections (such as binarySearch and sort) are generic.
类可以是泛型的，方法也可以是泛型的。操作参数化类型的静态实用程序方法通常是泛型的。Collections 类中的所有「算法」方法（如 binarySearch 和 sort）都是泛型的。
Writing generic methods is similar to writing generic types. Consider this deficient method, which returns the union of two sets:
编写泛型方法类似于编写泛型类型。考虑这个有缺陷的方法，它返回两个集合的并集：
```
// Uses raw types - unacceptable! (Item 26)
public static Set union(Set s1, Set s2) {
Set result = new HashSet(s1);
result.addAll(s2);
return result;
}
```
This method compiles but with two warnings:
该方法可进行编译，但有两个警告：
```
Union.java:5: warning: [unchecked] unchecked call to
HashSet(Collection<? extends E>) as a member of raw type HashSet
Set result = new HashSet(s1);
^
Union.java:6: warning: [
unchecked] unchecked call to
addAll(Collection<? extends E>) as a member of raw type Set
result.addAll(s2);
^
```
To fix these warnings and make the method typesafe, modify its declaration to declare a type parameter representing the element type for the three sets (the two arguments and the return value) and use this type parameter throughout the method. **The type parameter list, which declares the type parameters, goes between a method’s modifiers and its return type.** In this example, the type parameter list is `<E>`, and the return type is `Set<E>`. The naming conventions for type parameters are the same for generic methods and generic types (Items 29, 68):
要修复这些警告并使方法类型安全，请修改其声明，以声明表示三个集合（两个参数和返回值）的元素类型的类型参数，并在整个方法中使用该类型参数。类型参数列表声明类型参数，它位于方法的修饰符与其返回类型之间。在本例中，类型参数列表为 `<E>`，返回类型为 `Set<E>`。类型参数的命名约定与泛型方法和泛型类型的命名约定相同（[Item-29](/Chapter-5/Chapter-5-Item-29-Favor-generic-types.md)、[Item-68](/Chapter-9/Chapter-9-Item-68-Adhere-to-generally-accepted-naming-conventions.md)）:
```
// Generic method
public static <E> Set<E> union(Set<E> s1, Set<E> s2) {
Set<E> result = new HashSet<>(s1);
result.addAll(s2);
return result;
}
```
At least for simple generic methods, that’s all there is to it. This method compiles without generating any warnings and provides type safety as well as ease of use. Here’s a simple program to exercise the method. This program contains no casts and compiles without errors or warnings:
至少对于简单的泛型方法，这就是（要注意细节的）全部。该方法编译时不生成任何警告，并且提供了类型安全性和易用性。这里有一个简单的程序来演示。这个程序不包含转换，编译时没有错误或警告：
```
// Simple program to exercise generic method
public static void main(String[] args) {
Set<String> guys = Set.of("Tom", "Dick", "Harry");
Set<String> stooges = Set.of("Larry", "Moe", "Curly");
Set<String> aflCio = union(guys, stooges);
System.out.println(aflCio);
}
```
When you run the program, it prints [Moe, Tom, Harry, Larry, Curly, Dick]. (The order of the elements in the output is implementation-dependent.)
当你运行程序时，它会打印出 [Moe, Tom, Harry, Larry, Curly, Dick]。（输出元素的顺序可能不同）。
A limitation of the union method is that the types of all three sets (both input parameters and the return value) have to be exactly the same. You can make the method more flexible by using bounded wildcard types (Item 31).
union 方法的一个限制是，所有三个集合（输入参数和返回值）的类型必须完全相同。你可以通过使用有界通配符类型（[Item-31](/Chapter-5/Chapter-5-Item-31-Use-bounded-wildcards-to-increase-API-flexibility.md)）使方法更加灵活。
On occasion, you will need to create an object that is immutable but applicable to many different types. Because generics are implemented by erasure (Item 28), you can use a single object for all required type parameterizations, but you need to write a static factory method to repeatedly dole out the object for each requested type parameterization. This pattern, called the generic singleton factory, is used for function objects (Item 42) such as Collections.reverseOrder, and occasionally for collections such as Collections.emptySet.
有时，你需要创建一个对象，该对象是不可变的，但适用于许多不同类型。因为泛型是由擦除（[Item-28](/Chapter-5/Chapter-5-Item-28-Prefer-lists-to-arrays.md)）实现的，所以你可以为所有需要的类型参数化使用单个对象，但是你需要编写一个静态工厂方法，为每个请求的类型参数化重复分配对象。这种模式称为泛型单例工厂，可用于函数对象（[Item-42](/Chapter-7/Chapter-7-Item-42-Prefer-lambdas-to-anonymous-classes.md)），如 Collections.reverseOrder，偶尔也用于集合，如 Collections.emptySet。
Suppose that you want to write an identity function dispenser. The libraries provide Function.identity, so there’s no reason to write your own (Item 59), but it is instructive. It would be wasteful to create a new identity function object time one is requested, because it’s stateless. If Java’s generics were reified, you would need one identity function per type, but since they’re erased a generic singleton will suffice. Here’s how it looks:
假设你想要编写一个恒等函数分发器。这些库提供 Function.identity，所以没有理由编写自己的库（[Item-59](/Chapter-9/Chapter-9-Item-59-Know-and-use-the-libraries.md)），但是它很有指导意义。在请求标识函数对象时创建一个新的标识函数对象是浪费时间的，因为它是无状态的。如果 Java 的泛型被具体化了，那么每个类型都需要一个标识函数，但是由于它们已经被擦除，一个泛型单例就足够了。它是这样的：
```
// Generic singleton factory pattern
private static UnaryOperator<Object> IDENTITY_FN = (t) -> t;
@SuppressWarnings("unchecked")
public static <T> UnaryOperator<T> identityFunction() {
return (UnaryOperator<T>) IDENTITY_FN;
}
```
The cast of IDENTITY_FN to (`UnaryFunction<T>`) generates an unchecked cast warning, as `UnaryOperator<Object>` is not a `UnaryOperator<T>` for every T. But the identity function is special: it returns its argument unmodified, so we know that it is typesafe to use it as a `UnaryFunction<T>`, whatever the value of T. Therefore, we can confidently suppress the unchecked cast warning generated by this cast. Once we’ve done this, the code compiles without error or warning.
IDENTITY_FN 到（`UnaryFunction<T>`）的转换会生成一个 unchecked 转换警告，因为 `UnaryOperator<Object>` 并不是每个 T 都是 `UnaryOperator<T>`，但是恒等函数是特殊的：它会返回未修改的参数，所以我们知道，无论 T 的值是多少，都可以将其作为 `UnaryFunction<T>` 使用，这是类型安全的。一旦我们这样做了，代码编译就不会出现错误或警告。
Here is a sample program that uses our generic singleton as a `UnaryOperator<String>` and a `UnaryOperator<Number>`. As usual, it contains no casts and compiles without errors or warnings:
下面是一个示例程序，它使用我们的泛型单例作为 `UnaryOperator<String>` 和 `UnaryOperator<Number>`。像往常一样，它不包含类型转换和编译，没有错误或警告：
```
// Sample program to exercise generic singleton
public static void main(String[] args) {
String[] strings = { "jute", "hemp", "nylon" };
UnaryOperator<String> sameString = identityFunction();
for (String s : strings)
System.out.println(sameString.apply(s));
Number[] numbers = { 1, 2.0, 3L };
UnaryOperator<Number> sameNumber = identityFunction();
for (Number n : numbers)
System.out.println(sameNumber.apply(n));
}
```
It is permissible, though relatively rare, for a type parameter to be bounded by some expression involving that type parameter itself. This is what’s known as a recursive type bound. A common use of recursive type bounds is in connection with the Comparable interface, which defines a type’s natural ordering (Item 14). This interface is shown here:
允许类型参数被包含该类型参数本身的表达式限制，尽管这种情况比较少见。这就是所谓的递归类型限定。递归类型边界的一个常见用法是与 Comparable 接口相关联，后者定义了类型的自然顺序（[Item-14](/Chapter-3/Chapter-3-Item-14-Consider-implementing-Comparable.md)）。该界面如下图所示：
```
public interface Comparable<T> {
int compareTo(T o);
}
```
The type parameter T defines the type to which elements of the type implementing `Comparable<T>` can be compared. In practice, nearly all types can be compared only to elements of their own type. So, for example, String implements `Comparable<String>`, Integer implements `Comparable<Integer>`, and so on.
类型参数 T 定义了实现 `Comparable<T>` 的类型的元素可以与之进行比较的类型。在实践中，几乎所有类型都只能与它们自己类型的元素进行比较。例如，String 实现 `Comparable<String>`， Integer 实现 `Comparable<Integer>`，等等。
Many methods take a collection of elements implementing Comparable to sort it, search within it, calculate its minimum or maximum, and the like. To do these things, it is required that every element in the collection be comparable to every other element in it, in other words, that the elements of the list be mutually comparable. Here is how to express that constraint:
许多方法采用实现 Comparable 的元素集合，在其中进行搜索，计算其最小值或最大值，等等。要做到这些，需要集合中的每个元素与集合中的每个其他元素相比较，换句话说，就是列表中的元素相互比较。下面是如何表达这种约束（的示例）：
```
// Using a recursive type bound to express mutual comparability
public static <E extends Comparable<E>> E max(Collection<E> c);
```
The type bound `<E extends Comparable<E>>` may be read as “any type E that can be compared to itself,” which corresponds more or less precisely to the notion of mutual comparability.
类型限定 `<E extends Comparable<E>>` 可以被理解为「可以与自身进行比较的任何类型 E」，这或多或少与相互可比性的概念相对应。
Here is a method to go with the previous declaration. It calculates the maximum value in a collection according to its elements’ natural order, and it compiles without errors or warnings:
下面是一个与前面声明相同的方法。它根据元素的自然顺序计算集合中的最大值，编译时没有错误或警告：
```
// Returns max value in a collection - uses recursive type bound
public static <E extends Comparable<E>> E max(Collection<E> c) {
if (c.isEmpty())
throw new IllegalArgumentException("Empty collection");
E result = null;
for (E e : c)
if (result == null || e.compareTo(result) > 0)
result = Objects.requireNonNull(e);
return result;
}
```
Note that this method throws IllegalArgumentException if the list is empty. A better alternative would be to return an `Optional<E>` (Item 55).
注意，如果列表为空，该方法将抛出 IllegalArgumentException。更好的选择是返回一个 `Optional<E>`（[Item-55](/Chapter-8/Chapter-8-Item-55-Return-optionals-judiciously.md)）。
Recursive type bounds can get much more complex, but luckily they rarely do. If you understand this idiom, its wildcard variant (Item 31), and the simulated self-type idiom (Item 2), you’ll be able to deal with most of the recursive type bounds you encounter in practice.
递归类型限定可能会变得复杂得多，但幸运的是，这种情况很少。如果你理解这个习惯用法、它的通配符变量（[Item-31](/Chapter-5/Chapter-5-Item-31-Use-bounded-wildcards-to-increase-API-flexibility.md)）和模拟的自类型习惯用法（[Item-2](/Chapter-2/Chapter-2-Item-2-Consider-a-builder-when-faced-with-many-constructor-parameters.md)），你就能够处理在实践中遇到的大多数递归类型限定。
In summary, generic methods, like generic types, are safer and easier to use than methods requiring their clients to put explicit casts on input parameters and return values. Like types, you should make sure that your methods can be used without casts, which often means making them generic. And like types, you should generify existing methods whose use requires casts. This makes life easier for new users without breaking existing clients (Item 26).
总之，与要求客户端对输入参数和返回值进行显式转换的方法相比，泛型方法与泛型一样，更安全、更容易使用。与类型一样，你应该确保你的方法可以在不使用类型转换的情况下使用，这通常意味着要使它们具有通用性。与类型类似，你应该将需要强制类型转换的现有方法泛型化。这使得新用户在不破坏现有客户端的情况下更容易使用（[Item-26](/Chapter-5/Chapter-5-Item-26-Do-not-use-raw-types.md)）。
---
**[Back to contents of the chapter（返回章节目录）](/Chapter-5/Chapter-5-Introduction.md)**
- **Previous Item（上一条目）：[Item 29: Favor generic types（优先使用泛型）](/Chapter-5/Chapter-5-Item-29-Favor-generic-types.md)**
- **Next Item（下一条目）：[Item 31: Use bounded wildcards to increase API flexibility（使用有界通配符增加 API 的灵活性）](/Chapter-5/Chapter-5-Item-31-Use-bounded-wildcards-to-increase-API-flexibility.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）