java Java泛型通配符：<? 扩展数> vs <T扩展数>
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Java Generics WildCard: <? extends Number> vs <T extends Number>
提问by vikky.rk
What is the difference between these 2 functions?
这两个函数有什么区别？
static void gPrint(List<? extends Number> l) {
for (Number n : l) {
System.out.println(n);
}
}
static <T extends Number> void gPrintA(List<T> l) {
for (Number n : l) {
System.out.println(n);
}
}
I see the same output.
我看到相同的输出。
采纳答案by Thilo
There is no difference in this case, because Tis never used again.
在这种情况下没有区别，因为T不再使用。
The reason for declaring a Tis so that you can refer to it again, thus binding two parameter types, or a return type together.
声明 a 的原因T是为了您可以再次引用它，从而将两个参数类型或一个返回类型绑定在一起。
回答by Bohemian
The difference is you can't refer to Twhen using a wildcard.
不同之处在于您T在使用通配符时不能引用。
You aren't right now, so there is "no difference", but here's how you could use Tto make a difference:
您现在不在，因此“没有区别”，但您可以使用以下方法T来有所作为：
static <T extends Number> T getElement(List<T> l) {
for (T t : l) {
if (some condition)
return t;
}
return null;
}
This will return the same typeas whatever is passed in. eg these will both compile:
这将返回与传入的任何类型相同的类型。例如，它们都将编译：
Integer x = getElement(integerList);
Float y = getElement(floatList);
回答by ns89
Tis a bounded type, i.e. whatever type you use, you have to stick to that particular type which extends Number, e.g. if you pass a Doubletype to a list, you cannot pass it a Shorttype as Tis of type Doubleand the list is already bounded by that type. In contrast, if you use ?(wildcard), you can use "any" type that extends Number(add both Shortand Doubleto that list).
T是有界类型，即无论您使用什么类型，您都必须坚持扩展的特定类型Number，例如，如果您将Double类型传递给列表，则不能将Short类型按原样传递给它T，Double并且列表已经受该类型限制类型。相反，如果使用?( wildcard)，则可以使用扩展的“任何”类型Number（将Short和都添加Double到该列表中）。
回答by Kanagaraj M
When you use T you can perform all type of actions on List. But when you use you can not perform add.
当您使用 T 时，您可以对 List 执行所有类型的操作。但是当你使用时你不能执行添加。
T - as same as object reference with full access
? - give partial access
T - 与具有完全访问权限的对象引用相同
？- 提供部分访问权限
static void gPrint(List<? extends Number> l) {
l.add(1); //Will give error
for (Number n : l) {
System.out.println(n);
}
static <T extends Number> void gPrintA(List<T> l) {
l.add((T)1); //We can add
for (Number n : l) {
System.out.println(n);
}
