When you program with generics, you will see many compiler warnings: unchecked cast warnings, unchecked method invocation warnings, unchecked parameterized vararg type warnings, and unchecked conversion warnings. The more experience you acquire with generics, the fewer warnings you’ll get, but don’t expect newly written code to compile cleanly.
當(dāng)你使用泛型編程時争舞，你將看到許多編譯器警告：unchecked 強(qiáng)制轉(zhuǎn)換警告版扩、unchecked 方法調(diào)用警告、unchecked 可變參數(shù)類型警告和 unchecked 自動轉(zhuǎn)換警告忠怖。使用泛型獲得的經(jīng)驗越多洞坑，得到的警告就越少臂拓，但是不要期望新編寫的代碼能夠完全正確地編譯。
Many unchecked warnings are easy to eliminate. For example, suppose you accidentally write this declaration:
許多 unchecked 警告很容易消除箫措。例如寂曹，假設(shè)你不小心寫了這個聲明：
Set<Lark> exaltation = new HashSet();
The compiler will gently remind you what you did wrong:
編譯器會精確地提醒你做錯了什么：
Venery.java:4: warning: [unchecked] unchecked conversion
Set<Lark> exaltation = new HashSet();
^ required: Set<Lark>
found: HashSet
You can then make the indicated（v. 表明哎迄；指出；顯示稀颁；adj. 表明的芬失；指示的） correction, causing the warning to disappear. Note that you don’t actually have to specify the type parameter, merely（adv. 僅僅，只不過匾灶；只是） to indicate（vt. 表明；指出租漂；預(yù)示阶女；象征） that it’s present with the diamond operator (<>), introduced in Java 7. The compiler will then infer the correct actual type parameter (in this case, Lark):
你可以在指定位置進(jìn)行更正，使警告消失哩治。注意秃踩，你實際上不必指定類型參數(shù)，只需給出由 Java 7 中引入的 diamond 操作符（<>）业筏。然后編譯器將推斷出正確的實際類型參數(shù)（在本例中為 Lark）：
Set<Lark> exaltation = new HashSet<>();
Some warnings will be much more difficult to eliminate. This chapter is filled with examples of such warnings. When you get warnings that require some thought, persevere! Eliminate every unchecked warning that you can. If you eliminate all warnings, you are assured that your code is typesafe, which is a very good thing. It means that you won’t get a ClassCastException at runtime, and it increases your confidence that your program will behave as you intended.
一些警告會更難消除憔杨。這一章充滿這類警告的例子。當(dāng)你收到需要認(rèn)真思考的警告時蒜胖，堅持下去消别！力求消除所有 unchecked 警告抛蚤。 如果你消除了所有警告，你就可以確信你的代碼是類型安全的寻狂，這是一件非常好的事情岁经。這意味著你在運(yùn)行時不會得到 ClassCastException，它增加了你的信心蛇券，你的程序?qū)凑疹A(yù)期的方式運(yùn)行缀壤。
If you can’t eliminate a warning, but you can prove that the code that provoked the warning is typesafe, then (and only then) suppress the warning with an @SuppressWarnings("unchecked") annotation. If you suppress warnings without first proving that the code is typesafe, you are giving yourself a false sense of security. The code may compile without emitting any warnings, but it can still throw a ClassCastException at runtime. If, however, you ignore unchecked warnings that you know to be safe (instead of suppressing them), you won’t notice when a new warning crops up that represents a real problem. The new warning will get lost amidst all the false alarms that you didn’t silence.
如果不能消除警告，但是可以證明引發(fā)警告的代碼是類型安全的纠亚，那么（并且只有在那時）使用 SuppressWarnings("unchecked") 注解來抑制警告塘慕。 如果你在沒有首先證明代碼是類型安全的情況下禁止警告，那么你是在給自己一種錯誤的安全感蒂胞。代碼可以在不發(fā)出任何警告的情況下編譯图呢，但它仍然可以在運(yùn)行時拋出 ClassCastException。但是啤誊，如果你忽略了你知道是安全的 unchecked 警告（而不是抑制它們）岳瞭，那么當(dāng)出現(xiàn)一個代表真正問題的新警告時，你將不會注意到蚊锹。新出現(xiàn)的警告就會淹設(shè)在所有的錯誤警告當(dāng)中瞳筏。
The SuppressWarnings annotation can be used on any declaration, from an individual local variable declaration to an entire class. Always use the SuppressWarnings annotation on the smallest scope possible. Typically this will be a variable declaration or a very short method or constructor. Never use SuppressWarnings on an entire class. Doing so could mask critical warnings.
SuppressWarnings 注解可以用于任何聲明中，從單個局部變量聲明到整個類牡昆。總是在盡可能小的范圍上使用 SuppressWarnings 注解姚炕。 通常用在一個變量聲明或一個非常短的方法或構(gòu)造函數(shù)。不要在整個類中使用 SuppressWarnings丢烘。這樣做可能會掩蓋關(guān)鍵警告柱宦。
If you find yourself using the SuppressWarnings annotation on a method or constructor that’s more than one line long, you may be able to move it onto a local variable declaration. You may have to declare a new local variable, but it’s worth it. For example, consider this toArray method, which comes from ArrayList:
如果你發(fā)現(xiàn)自己在一個超過一行的方法或構(gòu)造函數(shù)上使用 SuppressWarnings 注解，那么你可以將其移動到局部變量聲明中播瞳。你可能需要聲明一個新的局部變量掸刊，但這是值得的。例如赢乓，考慮這個 toArray 方法忧侧，它來自 ArrayList：
public <T> T[] toArray(T[] a) {
if (a.length < size)
return (T[]) Arrays.copyOf(elements, size, a.getClass());
System.arraycopy(elements, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
If you compile ArrayList, the method generates this warning:
如果你編譯 ArrayList，這個方法會產(chǎn)生這樣的警告：
ArrayList.java:305: warning: [unchecked] unchecked cast
return (T[]) Arrays.copyOf(elements, size, a.getClass());
^ required: T[]
found: Object[]
It is illegal to put a SuppressWarnings annotation on the return statement, because it isn’t a declaration [JLS, 9.7]. You might be tempted to put the annotation on the entire method, but don’t. Instead, declare a local variable to hold the return value and annotate its declaration, like so:
將 SuppressWarnings 注釋放在 return 語句上是非法的牌芋，因為它不是聲明 [JLS, 9.7]蚓炬。你可能想把注釋放在整個方法上，但是不要這樣做躺屁。相反肯夏，應(yīng)該聲明一個局部變量來保存返回值并添加注解，如下所示：
// Adding local variable to reduce scope of @SuppressWarnings
public <T> T[] toArray(T[] a) {
if (a.length < size) {
// This cast is correct because the array we're creating
// is of the same type as the one passed in, which is T[].
@SuppressWarnings("unchecked") T[] result = (T[]) Arrays.copyOf(elements, size, a.getClass());
return result;
}
System.arraycopy(elements, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
The resulting method compiles cleanly and minimizes the scope in which unchecked warnings are suppressed.
生成的方法編譯正確，并將抑制 unchecked 警告的范圍減到最小驯击。
Every time you use a @SuppressWarnings("unchecked") annotation, add a comment saying why it is safe to do so. This will help others understand the code, and more importantly, it will decrease the odds that someone will modify the code so as to make the computation unsafe. If you find it hard to write such a comment, keep thinking. You may end up figuring out that the unchecked operation isn’t safe after all.
每次使用 SuppressWarnings("unchecked") 注解時烁兰，要添加一條注釋，說明這樣做是安全的余耽。 這將幫助他人理解代碼缚柏，更重要的是，它將降低其他人修改代碼而產(chǎn)生不安全事件的幾率碟贾。如果你覺得寫這樣的注釋很難币喧，那就繼續(xù)思考合適的方式。你最終可能會發(fā)現(xiàn)袱耽，unchecked 操作畢竟是不安全的杀餐。
In summary, unchecked warnings are important. Don’t ignore them. Every unchecked warning represents the potential for a ClassCastException at runtime. Do your best to eliminate these warnings. If you can’t eliminate an unchecked warning and you can prove that the code that provoked it is typesafe, suppress the warning with a @SuppressWarnings("unchecked") annotation in the narrowest possible scope. Record the rationale for your decision to suppress the warning in a comment.
總之，unchecked 警告很重要朱巨。不要忽視他們史翘。每個 unchecked 警告都代表了在運(yùn)行時發(fā)生 ClassCastException 的可能性。盡最大努力消除這些警告冀续。如果不能消除 unchecked 警告琼讽，并且可以證明引發(fā)該警告的代碼是類型安全的，那么可以在盡可能狹窄的范圍內(nèi)使用 @SuppressWarnings("unchecked") 注釋來禁止警告洪唐。在注釋中記錄你決定隱藏警告的理由钻蹬。
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