## Chapter 9. General Programming（通用程序设计）
### Item 58: Prefer for-each loops to traditional for loops（for-each 循环优于传统的 for 循环）
As discussed in Item 45, some tasks are best accomplished with streams, others with iteration. Here is a traditional for loop to iterate over a collection:
正如在 [Item-45](/Chapter-7/Chapter-7-Item-45-Use-streams-judiciously.md) 中所讨论的，一些任务最好使用流来完成，其他任务最好使用 iteration。下面是使用一个传统的 for 循环来遍历一个集合：
```
// Not the best way to iterate over a collection!
for (Iterator<Element> i = c.iterator(); i.hasNext(); ) {
Element e = i.next();
... // Do something with e
}
```
and here is a traditional for loop to iterate over an array:
这是使用传统的 for 循环来遍历数组：
```
// Not the best way to iterate over an array!
for (int i = 0; i < a.length; i++) {
... // Do something with a[i]
}
```
These idioms are better than while loops (Item 57), but they aren’t perfect. The iterator and the index variables are both just clutter—all you need are the elements. Furthermore, they represent opportunities for error. The iterator occurs three times in each loop and the index variable four, which gives you many chances to use the wrong variable. If you do, there is no guarantee that the compiler will catch the problem. Finally, the two loops are quite different, drawing unnecessary attention to the type of the container and adding a (minor) hassle to changing that type.
这些习惯用法比 while 循环更好（[Item-57](/Chapter-9/Chapter-9-Item-57-Minimize-the-scope-of-local-variables.md)），但是它们并不完美。迭代器和索引变量都很混乱（你只需要元素）。此外，它们有出错的可能。迭代器在每个循环中出现三次，索引变量出现四次，这使得有很多机会使用到错误的变量。如果这样做，就不能保证编译器会捕捉到问题。最后，这两个循环区别很大，（第一个例子）还需要额外注意容器类型，并给类型转换增加小麻烦。
The for-each loop (officially known as the “enhanced for statement”) solves all of these problems. It gets rid of the clutter and the opportunity for error by hiding the iterator or index variable. The resulting idiom applies equally to collections and arrays, easing the process of switching the implementation type of a container from one to the other:
for-each 循环（官方称为「enhanced for 语句」）解决了所有这些问题。它通过隐藏迭代器或索引变量来消除混乱和出错的机会。由此产生的习惯用法同样适用于集合和数组，从而简化了将容器的实现类型从一种转换为另一种的过程：
```
// The preferred idiom for iterating over collections and arrays
for (Element e : elements) {
... // Do something with e
}
```
When you see the colon (:), read it as “in.” Thus, the loop above reads as “for each element e in elements.” There is no performance penalty for using for-each loops, even for arrays: the code they generate is essentially identical to the code you would write by hand.
当你看到冒号 `(:)` 时，请将其读作「in」。因此，上面的循环读作「对元素集的每个元素 e 进行操作」。使用 for-each 循环不会降低性能，对于数组也是如此：它们生成的代码本质上与你手工编写的 for 循环代码相同。
The advantages of the for-each loop over the traditional for loop are even greater when it comes to nested iteration. Here is a common mistake that people make when doing nested iteration:
当涉及到嵌套迭代时，for-each 循环相对于传统 for 循环的优势甚至更大。下面是人们在进行嵌套迭代时经常犯的一个错误：
```
// Can you spot the bug?
enum Suit { CLUB, DIAMOND, HEART, SPADE }
enum Rank { ACE, DEUCE, THREE, FOUR, FIVE, SIX, SEVEN, EIGHT,NINE, TEN, JACK, QUEEN, KING }
...
static Collection<Suit> suits = Arrays.asList(Suit.values());
static Collection<Rank> ranks = Arrays.asList(Rank.values());
List<Card> deck = new ArrayList<>();
for (Iterator<Suit> i = suits.iterator(); i.hasNext(); )
for (Iterator<Rank> j = ranks.iterator(); j.hasNext(); )
deck.add(new Card(i.next(), j.next()));
```
Don’t feel bad if you didn’t spot the bug. Many expert programmers have made this mistake at one time or another. The problem is that the next method is called too many times on the iterator for the outer collection (suits). It should be called from the outer loop so that it is called once per suit, but instead it is called from the inner loop, so it is called once per card. After you run out of suits, the loop throws a NoSuchElementException.
如果你没有发现这个bug，不要感到难过。许多专业程序员都曾犯过这样的错误。问题是，迭代器对外部的集合 suits 调用了太多次 next 方法。它应该从外部循环调用，因此每种花色调用一次，但它是从内部循环调用的，因此每一张牌调用一次。在用完所有花色之后，循环抛出 NoSuchElementException。
If you’re really unlucky and the size of the outer collection is a multiple of the size of the inner collection—perhaps because they’re the same collection—the loop will terminate normally, but it won’t do what you want. For example, consider this ill-conceived attempt to print all the possible rolls of a pair of dice:
如果真的很不幸，外部集合的大小是内部集合大小的几倍（可能因为它们是相同的集合），循环将正常终止，但是它不会执行你想要的操作。例如，考虑一个打印一对骰子所有可能的组合值的错误尝试：
```
// Same bug, different symptom!
enum Face { ONE, TWO, THREE, FOUR, FIVE, SIX }
...
Collection<Face> faces = EnumSet.allOf(Face.class);
for (Iterator<Face> i = faces.iterator(); i.hasNext(); )
for (Iterator<Face> j = faces.iterator(); j.hasNext(); )
System.out.println(i.next() + " " + j.next());
```
The program doesn’t throw an exception, but it prints only the six “doubles” (from “ONE ONE” to “SIX SIX”), instead of the expected thirty-six combinations.
程序不会抛出异常，但它只打印 6 个重复数值（从「ONE ONE」到「SIX SIX」），而不是预期的 36 个组合。
To fix the bugs in these examples, you must add a variable in the scope of the outer loop to hold the outer element:
要修复这些例子中的错误，必须在外部循环的作用域内添加一个变量来保存外部元素：
```
// Fixed, but ugly - you can do better!
for (Iterator<Suit> i = suits.iterator(); i.hasNext(); ) {
Suit suit = i.next();
for (Iterator<Rank> j = ranks.iterator(); j.hasNext(); )
deck.add(new Card(suit, j.next()));
}
```
If instead you use a nested for-each loop, the problem simply disappears. The resulting code is as succinct as you could wish for:
相反，如果使用嵌套 for-each 循环，问题就会消失。生成的代码更简洁：
```
// Preferred idiom for nested iteration on collections and arrays
for (Suit suit : suits)
for (Rank rank : ranks)
deck.add(new Card(suit, rank));
```
Unfortunately, there are three common situations where you can’t use foreach:
不幸的是，有三种常见的情况你不应使用 for-each：
- **Destructive filtering** —If you need to traverse a collection removing selected elements, then you need to use an explicit iterator so that you can call its remove method. You can often avoid explicit traversal by using Collection’s removeIf method, added in Java 8.
**破坏性过滤**，如果需要遍历一个集合并删除选定元素，则需要使用显式的迭代器，以便调用其 remove 方法。通过使用 Collection 在 Java 8 中添加的 removeIf 方法，通常可以避免显式遍历。
- **Transforming** —If you need to traverse a list or array and replace some or all of the values of its elements, then you need the list iterator or array index in order to replace the value of an element.
**转换**，如果需要遍历一个 List 或数组并替换其中部分或全部元素的值，那么需要 List 迭代器或数组索引来替换元素的值。
- **Parallel iteration** —If you need to traverse multiple collections in parallel, then you need explicit control over the iterator or index variable so that all iterators or index variables can be advanced in lockstep (as demonstrated unintentionally in the buggy card and dice examples above). If you find yourself in any of these situations, use an ordinary for loop and be wary of the traps mentioned in this item.
**并行迭代**，如果需要并行遍历多个集合，那么需要显式地控制迭代器或索引变量，以便所有迭代器或索引变量都可以同步执行（如上述牌和骰子示例中无意中演示的错误那样）。如果发现自己处于这些情况中的任何一种，请使用普通的 for 循环，并警惕本条目中提到的陷阱。
Not only does the for-each loop let you iterate over collections and arrays, it lets you iterate over any object that implements the Iterable interface, which consists of a single method. Here is how the interface looks:
for-each 循环不仅允许遍历集合和数组，还允许遍历实现 Iterable 接口的任何对象，该接口由一个方法组成。如下所示：
```
public interface Iterable<E> {
// Returns an iterator over the elements in this iterable
Iterator<E> iterator();
}
```
It is a bit tricky to implement Iterable if you have to write your own Iterator implementation from scratch, but if you are writing a type that represents a group of elements, you should strongly consider having it implement Iterable, even if you choose not to have it implement Collection. This will allow your users to iterate over your type using the foreach loop, and they will be forever grateful.
如果必须从头开始编写自己的 Iterator 实现，确实有点棘手，但是如果正在编写的类型表示一组元素，即使选择不让它实现 Collection，那么也应该强烈考虑让它实现 Iterable。这将允许用户使用 foreach 循环遍历类型，他们将永远感激不尽。
In summary, the for-each loop provides compelling advantages over the traditional for loop in clarity, flexibility, and bug prevention, with no performance penalty. Use for-each loops in preference to for loops wherever you can.
总之，for-each 循环在清晰度、灵活性和 bug 预防方面比传统的 for 循环更有优势，并且没有性能损失。尽可能使用 for-each 循环而不是 for 循环。
---
**[Back to contents of the chapter（返回章节目录）](/Chapter-9/Chapter-9-Introduction.md)**
- **Previous Item（上一条目）：[Item 57: Minimize the scope of local variables（将局部变量的作用域最小化）](/Chapter-9/Chapter-9-Item-57-Minimize-the-scope-of-local-variables.md)**
- **Next Item（下一条目）：[Item 59: Know and use the libraries（了解并使用库）](/Chapter-9/Chapter-9-Item-59-Know-and-use-the-libraries.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）