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                ## 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)**
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