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(gè)或多個(gè)泛型變量定義，可以有一個(gè)或多個(gè)，泛型變量做參數(shù)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(gè)
list
，它的元素類型是
E
這個(gè)占位符表示的類型。這個(gè)類型有一個(gè)叫
add()
的方法，有一個(gè)參數(shù)類型為
E
，有一個(gè)名叫
get()
的方法，返回一個(gè)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(gè)實(shí)際的類型，產(chǎn)生一個(gè)參數(shù)化類型，就象
List<String>
。這樣做的原因是，為編譯器提供一個(gè)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(gè)
List
打算容納
String
類型的對(duì)象，這種類型安全檢查阻止你增加一個(gè)
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(gè)
String
類型對(duì)象，你不再需要把
Object
類型的返回值造型成一個(gè)
String
類型的對(duì)象。
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(jīng)被泛化了，你可能會(huì)在程序中頻繁的用到它們。類型安全的
Collection
是應(yīng)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(shí)這個(gè)新的，復(fù)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(fù)雜的細(xì)節(jié)。接下來，我們將覆蓋泛型參數(shù)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(shí)包括了
Java API
中關(guān)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(guān)于對(duì)象、
map--
關(guān)于關(guān)鍵字對(duì)象和值對(duì)象的對(duì)
)
。
Collection
將在第五章討論。這里我們只討論有關(guān)在
Java 5.0
的
Collection
類中，用泛型參數(shù)標(biāo)識(shí)
Collection
對(duì)象類型的內(nèi)容。這在
Java 1.4
或是更早的
Java
版本中，沒有這個(gè)用法。在沒有泛型應(yīng)用
Collection
的時(shí)候，需要程序員正確記得每個(gè)
Collection
的每個(gè)元素的類型。當(dāng)我們建立一個(gè)
Java 1.4
的
Collection
時(shí)，你知道打算把什么樣的類型對(duì)象放入這個(gè)
Collection
中，但是編譯器不知道這些。你增加元素時(shí)必須小心正確性。并且當(dāng)獲取一個(gè)
Collection
的元素時(shí)，你必須顯示的造型對(duì)象
Object
到它們實(shí)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(duì)不齊，算了，完整的還是發(fā)個(gè)pdf吧，要看自己下了，
不要忘了拍磚就行了！
pdf修訂了一下