## Chapter 8. Methods(方法)
### Item 54: Return empty collections or arrays, not nulls(返回空集合或數組,而不是 null)
It is not uncommon to see methods that look something like this:
如下的方法很常見:
```
// Returns null to indicate an empty collection. Don't do this!
private final List<Cheese> cheesesInStock = ...;
/**
* @return a list containing all of the cheeses in the shop,
* or null if no cheeses are available for purchase.
*/
public List<Cheese> getCheeses() {
return cheesesInStock.isEmpty() ? null: new ArrayList<>(cheesesInStock);
}
```
There is no reason to special-case the situation where no cheeses are available for purchase. Doing so requires extra code in the client to handle the possibly null return value, for example:
沒有理由對沒有奶酪可供購買的情況進行特殊處理。如果這樣做,在客戶端需要額外的代碼處理可能為空的返回值,例如:
```
List<Cheese> cheeses = shop.getCheeses();
if (cheeses != null && cheeses.contains(Cheese.STILTON))
System.out.println("Jolly good, just the thing.");
```
This sort of circumlocution is required in nearly every use of a method that returns null in place of an empty collection or array. It is error-prone, because the programmer writing the client might forget to write the special-case code to handle a null return. Such an error may go unnoticed for years because such methods usually return one or more objects. Also, returning null in place of an empty container complicates the implementation of the method returning the container.
在幾乎每次使用返回 null 來代替空集合或數組的方法時,都需要使用這種權宜之計。它很容易出錯,因為編寫客戶端的程序員可能忘記編寫特殊情況的代碼來處理 null 返回。這樣的錯誤可能會被忽略多年,因為這樣的方法通常返回一個或多個對象。此外,在空容器中返回 null 會使返回容器的方法的實現復雜化。
It is sometimes argued that a null return value is preferable to an empty collection or array because it avoids the expense of allocating the empty container. This argument fails on two counts. First, it is inadvisable to worry about performance at this level unless measurements have shown that the allocation in question is a real contributor to performance problems (Item 67). Second, it is possible to return empty collections and arrays without allocating them. Here is the typical code to return a possibly empty collection. Usually, this is all you need:
有時有人認為,空返回值比空集合或數組更可取,因為它避免了分配空容器的開銷。這個論點有兩點是不成立的。首先,在這個級別上擔心性能是不明智的,除非分析表明這個方法正是造成性能問題的真正源頭([Item-67](/Chapter-9/Chapter-9-Item-67-Optimize-judiciously.md))。第二,返回空集合和數組而不分配它們是可能的。下面是返回可能為空的集合的典型代碼。通常,這就是你所需要的:
```
//The right way to return a possibly empty collection
public List<Cheese> getCheeses() {
return new ArrayList<>(cheesesInStock);
}
```
In the unlikely event that you have evidence suggesting that allocating empty collections is harming performance, you can avoid the allocations by returning the same immutable empty collection repeatedly, as immutable objects may be shared freely (Item 17). Here is the code to do it, using the Collections.emptyList method. If you were returning a set, you’d use Collections.emptySet; if you were returning a map, you’d use Collections.emptyMap. But remember, this is an optimization, and it’s seldom called for. If you think you need it, measure performance before and after, to ensure that it’s actually helping:
在不太可能的情況下,你有證據表明分配空集合會損害性能,你可以通過重復返回相同的不可變空集合來避免分配,因為不可變對象可以自由共享([Item-17](/Chapter-4/Chapter-4-Item-17-Minimize-mutability.md))。下面是使用 `Collections.emptyList` 完成此任務的代碼。如果你要返回一個 Set,你會使用 `Collections.emptySet`;如果要返回 Map,則使用 `Collections.emptyMap`。但是請記住,這是一個優化,很少真正需要它。如果你認為你需要它,測試一下前后的表現,確保它確實有幫助:
```
// Optimization - avoids allocating empty collections
public List<Cheese> getCheeses() {
return cheesesInStock.isEmpty() ? Collections.emptyList(): new ArrayList<>(cheesesInStock);
}
```
The situation for arrays is identical to that for collections. Never return null instead of a zero-length array. Normally, you should simply return an array of the correct length, which may be zero. Note that we’re passing a zero-length array into the toArray method to indicate the desired return type, which is Cheese[]:
數組的情況與集合的情況相同。永遠不要返回 null,而應該返回零長度的數組。通常,你應該簡單地返回一個正確長度的數組,它可能是零長度。注意,我們將一個零長度的數組傳遞到 toArray 方法中,以指示所需的返回類型,即 Cheese[0]:
```
//The right way to return a possibly empty array
public Cheese[] getCheeses() {
return cheesesInStock.toArray(new Cheese[0]);
}
```
If you believe that allocating zero-length arrays is harming performance, you can return the same zero-length array repeatedly because all zero-length arrays are immutable:
如果你認為分配零長度數組會損害性能,你可以重復返回相同的零長度數組,因為所有的零長度數組都是不可變的:
```
// Optimization - avoids allocating empty arrays
private static final Cheese[] EMPTY_CHEESE_ARRAY = new Cheese[0];
public Cheese[] getCheeses() {
return cheesesInStock.toArray(EMPTY_CHEESE_ARRAY);
}
```
In the optimized version, we pass the same empty array into every toArray call, and this array will be returned from getCheeses whenever cheesesInStock is empty. Do not preallocate the array passed to toArray in hopes of improving performance. Studies have shown that it is counterproductive [Shipil?v16]:
在優化版本中,我們將相同的空數組傳遞到每個 toArray 調用中,當 cheesesInStock 為空時,這個數組將從 getCheeses 返回。不要為了提高性能而預先分配傳遞給 toArray 的數組。研究表明,這樣做會適得其反 [Shipilev16]:
```
// Don’t do this - preallocating the array harms performance!
return cheesesInStock.toArray(new Cheese[cheesesInStock.size()]);
```
In summary, never return null in place of an empty array or collection. It makes your API more difficult to use and more prone to error, and it has no performance advantages.
總之,永遠不要用 null 來代替空數組或集合。它使你的 API 更難以使用,更容易出錯,并且沒有性能優勢。
---
**[Back to contents of the chapter(返回章節目錄)](/Chapter-8/Chapter-8-Introduction.md)**
- **Previous Item(上一條目):[Item 53: Use varargs judiciously(明智地使用可變參數)](/Chapter-8/Chapter-8-Item-53-Use-varargs-judiciously.md)**
- **Next Item(下一條目):[Item 55: Return optionals judiciously(明智地的返回 Optional)](/Chapter-8/Chapter-8-Item-55-Return-optionals-judiciously.md)**
- Chapter 2. Creating and Destroying Objects(創建和銷毀對象)
- Item 1: Consider static factory methods instead of constructors(考慮以靜態工廠方法代替構造函數)
- Item 2: Consider a builder when faced with many constructor parameters(在面對多個構造函數參數時,請考慮構建器)
- Item 3: Enforce the singleton property with a private constructor or an enum type(使用私有構造函數或枚舉類型實施單例屬性)
- Item 4: Enforce noninstantiability with a private constructor(用私有構造函數實施不可實例化)
- Item 5: Prefer dependency injection to hardwiring resources(依賴注入優于硬連接資源)
- Item 6: Avoid creating unnecessary objects(避免創建不必要的對象)
- Item 7: Eliminate obsolete object references(排除過時的對象引用)
- Item 8: Avoid finalizers and cleaners(避免使用終結器和清除器)
- Item 9: Prefer try with resources to try finally(使用 try-with-resources 優于 try-finally)
- Chapter 3. Methods Common to All Objects(對象的通用方法)
- Item 10: Obey the general contract when overriding equals(覆蓋 equals 方法時應遵守的約定)
- Item 11: Always override hashCode when you override equals(當覆蓋 equals 方法時,總要覆蓋 hashCode 方法)
- Item 12: Always override toString(始終覆蓋 toString 方法)
- Item 13: Override clone judiciously(明智地覆蓋 clone 方法)
- Item 14: Consider implementing Comparable(考慮實現 Comparable 接口)
- Chapter 4. Classes and Interfaces(類和接口)
- Item 15: Minimize the accessibility of classes and members(盡量減少類和成員的可訪問性)
- Item 16: In public classes use accessor methods not public fields(在公共類中,使用訪問器方法,而不是公共字段)
- Item 17: Minimize mutability(減少可變性)
- Item 18: Favor composition over inheritance(優先選擇復合而不是繼承)
- Item 19: Design and document for inheritance or else prohibit it(繼承要設計良好并且具有文檔,否則禁止使用)
- Item 20: Prefer interfaces to abstract classes(接口優于抽象類)
- Item 21: Design interfaces for posterity(為后代設計接口)
- Item 22: Use interfaces only to define types(接口只用于定義類型)
- Item 23: Prefer class hierarchies to tagged classes(類層次結構優于帶標簽的類)
- Item 24: Favor static member classes over nonstatic(靜態成員類優于非靜態成員類)
- Item 25: Limit source files to a single top level class(源文件僅限有單個頂層類)
- Chapter 5. Generics(泛型)
- Item 26: Do not use raw types(不要使用原始類型)
- Item 27: Eliminate unchecked warnings(消除 unchecked 警告)
- Item 28: Prefer lists to arrays(list 優于數組)
- Item 29: Favor generic types(優先使用泛型)
- Item 30: Favor generic methods(優先使用泛型方法)
- Item 31: Use bounded wildcards to increase API flexibility(使用有界通配符增加 API 的靈活性)
- Item 32: Combine generics and varargs judiciously(明智地合用泛型和可變參數)
- Item 33: Consider typesafe heterogeneous containers(考慮類型安全的異構容器)
- Chapter 6. Enums and Annotations(枚舉和注解)
- Item 34: Use enums instead of int constants(用枚舉類型代替 int 常量)
- Item 35: Use instance fields instead of ordinals(使用實例字段替代序數)
- 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 替換序數索引)
- Item 38: Emulate extensible enums with interfaces(使用接口模擬可擴展枚舉)
- Item 39: Prefer annotations to naming patterns(注解優于命名模式)
- Item 40: Consistently use the Override annotation(堅持使用 @Override 注解)
- Item 41: Use marker interfaces to define types(使用標記接口定義類型)
- Chapter 7. Lambdas and Streams(λ 表達式和流)
- Item 42: Prefer lambdas to anonymous classes(λ 表達式優于匿名類)
- Item 43: Prefer method references to lambdas(方法引用優于 λ 表達式)
- Item 44: Favor the use of standard functional interfaces(優先使用標準函數式接口)
- Item 45: Use streams judiciously(明智地使用流)
- Item 46: Prefer side effect free functions in streams(在流中使用無副作用的函數)
- Item 47: Prefer Collection to Stream as a return type(優先選擇 Collection 而不是流作為返回類型)
- Item 48: Use caution when making streams parallel(謹慎使用并行流)
- Chapter 8. Methods(方法)
- Item 49: Check parameters for validity(檢查參數的有效性)
- Item 50: Make defensive copies when needed(在需要時制作防御性副本)
- Item 51: Design method signatures carefully(仔細設計方法簽名)
- Item 52: Use overloading judiciously(明智地使用重載)
- Item 53: Use varargs judiciously(明智地使用可變參數)
- Item 54: Return empty collections or arrays, not nulls(返回空集合或數組,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(為所有公開的 API 元素編寫文檔注釋)
- Chapter 9. General Programming(通用程序設計)
- Item 57: Minimize the scope of local variables(將局部變量的作用域最小化)
- Item 58: Prefer for-each loops to traditional for loops(for-each 循環優于傳統的 for 循環)
- Item 59: Know and use the libraries(了解并使用庫)
- Item 60: Avoid float and double if exact answers are required(若需要精確答案就應避免使用 float 和 double 類型)
- Item 61: Prefer primitive types to boxed primitives(基本數據類型優于包裝類)
- Item 62: Avoid strings where other types are more appropriate(其他類型更合適時應避免使用字符串)
- Item 63: Beware the performance of string concatenation(當心字符串連接引起的性能問題)
- Item 64: Refer to objects by their interfaces(通過接口引用對象)
- Item 65: Prefer interfaces to reflection(接口優于反射)
- Item 66: Use native methods judiciously(明智地使用本地方法)
- Item 67: Optimize judiciously(明智地進行優化)
- Item 68: Adhere to generally accepted naming conventions(遵守被廣泛認可的命名約定)
- Chapter 10. Exceptions(異常)
- Item 69: Use exceptions only for exceptional conditions(僅在確有異常條件下使用異常)
- Item 70: Use checked exceptions for recoverable conditions and runtime exceptions for programming errors(對可恢復情況使用 checked 異常,對編程錯誤使用運行時異常)
- Item 71: Avoid unnecessary use of checked exceptions(避免不必要地使用 checked 異常)
- Item 72: Favor the use of standard exceptions(鼓勵復用標準異常)
- Item 73: Throw exceptions appropriate to the abstraction(拋出能用抽象解釋的異常)
- Item 74: Document all exceptions thrown by each method(為每個方法記錄會拋出的所有異常)
- Item 75: Include failure capture information in detail messages(異常詳細消息中應包含捕獲失敗的信息)
- Item 76: Strive for failure atomicity(盡力保證故障原子性)
- Item 77: Don’t ignore exceptions(不要忽略異常)
- Chapter 11. Concurrency(并發)
- Item 78: Synchronize access to shared mutable data(對共享可變數據的同步訪問)
- Item 79: Avoid excessive synchronization(避免過度同步)
- Item 80: Prefer executors, tasks, and streams to threads(Executor、task、流優于直接使用線程)
- Item 81: Prefer concurrency utilities to wait and notify(并發實用工具優于 wait 和 notify)
- Item 82: Document thread safety(文檔應包含線程安全屬性)
- Item 83: Use lazy initialization judiciously(明智地使用延遲初始化)
- Item 84: Don’t depend on the thread scheduler(不要依賴線程調度器)
- Chapter 12. Serialization(序列化)
- Item 85: Prefer alternatives to Java serialization(優先選擇 Java 序列化的替代方案)
- Item 86: Implement Serializable with great caution(非常謹慎地實現 Serializable)
- Item 87: Consider using a custom serialized form(考慮使用自定義序列化形式)
- Item 88: Write readObject methods defensively(防御性地編寫 readObject 方法)
- Item 89: For instance control, prefer enum types to readResolve(對于實例控制,枚舉類型優于 readResolve)
- Item 90: Consider serialization proxies instead of serialized instances(考慮以序列化代理代替序列化實例)