ardorleo的技術博客

4.1. Generic Types

Generic types and methods are the defining new feature of Java 5.0. A generic typeis defined using one or more type variablesand has one or more methods that use a type variable as a placeholder for an argument or return type. For example, the type java.util.List<E> is a generic type: a list that holds elements of some type represented by the placeholder E. This type has a method named add(), declared to take an argument of type E, and a method named get(), declared to return a value of type E.

4.1. 泛型

泛類型和泛型方法是 Java5.0 中的新特性。一種泛類型用一個或多個泛型變量定義，可以有一個或多個，泛型變量做參數占位符或做返回值的方法。例如，類型 java.util.List<E> 是一種泛類型：一個 list ，它的元素類型是 E 這個占位符表示的類型。這個類型有一個叫 add() 的方法，有一個參數類型為 E ，有一個名叫 get() 的方法，返回一個類型為 E 的值。

In order to use a generic type like this, you specify actual types for the type variable (or variables), producing a parameterized type such as List<String>.^[1] The reason to specify this extra type information is that the compiler can provide much stronger compile-time type checking for you, increasing the type safety of your programs. This type checking prevents you from adding a String[], for example, to a List that is intended to hold only String objects. Also, the additional type information enables the compiler to do some casting for you. The compiler knows that the get( ) method of a List<String> (for example) returns a String object: you are no longer required to cast a return value of type Object to a String.

為了可以象往常一樣使用泛類型，你需要為泛型變量（或是變量）指定一個實際的類型，產生一個參數化類型，就象 List<String> 。這樣做的原因是，為編譯器提供一個特定的類型信息，讓它可以在編譯期為你做類型檢查，這樣可以大大增加你程序的類型安全。例如，有一個 List 打算容納 String 類型的對象，這種類型安全檢查阻止你增加一個 String[] 的元素。而且，這附加的類型信息使編譯器幫你做些造型的活兒。例如，編譯器知道 List<String> 的 get() 方法返回一個 String 類型對象，你不再需要把 Object 類型的返回值造型成一個 String 類型的對象。

^[1] Throughout this chapter, I've tried to consistently use the term "generic type" to mean a type that declares one or more type variables and the term "parameterized type" to mean a generic type that has had actual type arguments substituted for its type varaiables. In common usage, however, the distinction is not a sharp one and the terms are sometimes used interchangeably.

^[1] 整個這章，我們將統一使用這些術語。 " 泛類型 " ：意味著一個類型，可以聲明一個或多個泛型變量。 " 參數化類型 " ，意味著一個（運行期的）泛類型，表示它的泛型變量被實際類型做為參數值替換了。但在通常的應用中，兩個術語的差別不是太明顯，有時還可以替換。

The collections classes of the java.util package have been made generic in Java 5.0, and you will probably use them frequently in your programs. Typesafe collections are the canonical use case for generic types. Even if you never define generic types of your own and never use generic types other than the collections classes in java.util, the benefits of typesafe collections are so significant that they justify the complexity of this major new language feature.

在 Java 5.0 中，包 java.util 中的 Collection 類都已經被泛化了，你可能會在程序中頻繁的用到它們。類型安全的 Collection 是應用泛類型的典范。可能你還從沒有定義過自己的泛類型，也從沒用過 Collection （在包 java.uitl 中）之外的泛類型，但類型安全的 Collection 的好處是顯而易見的，它將證實這個新的，復雜的，主要且重要的語言特性。

We begin by exploring the basic use of generics in typesafe collections, then delve into more complex details about the use of generic types. Next we cover type parameter wildcards and bounded wildcards. After describing how to use generic types, we explain how to write your own generic types and generic methods. Our coverage of generics concludes with a tour of important generic types in the core Java API. It explores these types and their use in depth in order to provide a deeper understanding of how generics work.

我們將從探索類型安全的 Collection 的基本泛化用法開始，然后深入泛類型用法更為復雜的細節。接下來，我們將覆蓋泛型參數通配符和邊界通配符。再說明怎么用泛類型，我們將闡述怎樣寫自己的泛類型和泛型方法。我們涉及的泛型知識包括了 Java API 中關于泛類型的主要部分。深入的探討了這些類型和他們的用法，為的是更深入的理解泛型是如何工作的。

4.1.1 . Typesafe Collections

4.1.1 . 類型安全的 Collection

The java.utilpackage includes the Java Collections Framework for working with sets and lists of objects and mappings from key objects to value objects. Collections are covered in Chapter 5. Here, we discuss the fact that in Java 5.0 the collections classes use type parameters to identify the type of the objects in the collection. This is not the case in Java 1.4 and earlier. Without generics, the use of collections requires the programmer to remember the proper element type for each collection. When you create a collection in Java 1.4, you know what type of objects you intend to store in that collection, but the compiler cannot know this. You must be careful to add elements of the appropriate type. And when querying elements from a collection, you must write explicit casts to convert them from Object to their actual type. Consider the following Java 1.4 code:

在包 java.util 中包含了 Java 的 Collection 框架 (set 、 list-- 關于對象、 map-- 關于關鍵字對象和值對象的對 ) 。 Collection 將在第五章討論。這里我們只討論有關在 Java 5.0 的 Collection 類中，用泛型參數標識 Collection 對象類型的內容。這在 Java 1.4 或是更早的 Java 版本中，沒有這個用法。在沒有泛型應用 Collection 的時候，需要程序員正確記得每個 Collection 的每個元素的類型。當我們建立一個 Java 1.4 的 Collection 時，你知道打算把什么樣的類型對象放入這個 Collection 中，但是編譯器不知道這些。你增加元素時必須小心正確性。并且當獲取一個 Collection 的元素時，你必須顯示的造型對象 Object 到它們實際的類型。思考在 Java 1.4 中的如下代碼：?
???
太郁悶了，代碼總是對不齊，算了，完整的還是發個pdf吧，要看自己下了，
不要忘了拍磚就行了！
pdf修訂了一下