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和有什么不一样？和在 Java 泛型中？

发布于 2024-12-14 21:47:12 字数 121 浏览 0 评论 0 原文

我之前见过通配符用来表示任何对象 - 但最近看到了这样的用法：

<? extends Object>

由于所有对象都扩展对象，这两种用法是同义的吗？

原文

I've seen the wildcard used before to mean any object - but recently saw a use of:

<? extends Object>

Since all objects extend Object, are these two usages synonymous?

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っ〆星空下的拥抱 2024-12-21 21:47:13

和是同义词。

在某些情况下，泛型的 extends Object 实际上并不是多余的。例如，将导致 T 在擦除时变为 Object，而使用时，它将变为 Foo 正在被删除。（如果您试图保持与使用 Object 的预泛型 API 的兼容性，这可能很重要。）

来源：http://download.oracle.com/javase/tutorial/extra/generics/convert.html;它解释了为什么 JDK 的 java.util.Collections 类有一个具有以下签名的方法：

public static <T extends Object & Comparable<? super T>> T max(
    Collection<? extends T> coll
)

<?> and <? extends Object> are synonymous, as you'd expect.

There are a few cases with generics where extends Object is not actually redundant. For example, <T extends Object & Foo> will cause T to become Object under erasure, whereas with <T extends Foo> it will become Foo under erasure. (This can matter if you're trying to retain compatibility with a pre-generics API that used Object.)

Source: http://download.oracle.com/javase/tutorial/extra/generics/convert.html; it explains why the JDK's java.util.Collections class has a method with this signature:

public static <T extends Object & Comparable<? super T>> T max(
    Collection<? extends T> coll
)

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南城旧梦 2024-12-21 21:47:13

尽管应该是 的快捷方式扩展对象>，两者之间有微小的区别。
是可具体化的，而不是。他们这样做的原因是为了更容易区分可具体化的类型。有任何看起来像 < 的东西吗？ extends some>、、是不可具体化的。

例如，此代码可以工作

List aList = new ArrayList<>();
boolean instanceTest = aList instanceof List<?>;

，但会出现错误

List aList = new ArrayList<>();
boolean instancetest = aList instanceof List<? extends Object>;

，请阅读 Maurice Naftalin 的 Java generics and collections

Although <?> is supposed to be a shortcut for <? extend object>, there is a tiny difference between the two.
<?> is reifiable while <? extend object> is not. The reason they did this is to make it easier to distinguish reifiable type. Anything that looks like <? extends something>,<T>,<Integer> are nonreifiable.

For example, this code would work

List aList = new ArrayList<>();
boolean instanceTest = aList instanceof List<?>;

but this gives an error

List aList = new ArrayList<>();
boolean instancetest = aList instanceof List<? extends Object>;

for more info read Java generics and collections by Maurice Naftalin

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触ぅ动初心 2024-12-21 21:47:13

是。
您可以阅读下面的共享链接以了解更多详细信息。

<?>

“?”表示任何未知类型，它可以代表代码中的任何类型。如果您不确定类型，请使用此通配符。

ArrayList<?> unknownList = new ArrayList<Number>(); //can accept of type Number
unknownList = new ArrayList<Float>(); //Float is of type Number

注意： 表示任何内容。所以它可以接受不是从Object类继承的类型。

<? extends Object>

<代码> 表示您可以传递一个 Object 或扩展 Object 类的子类。

ArrayList<? extends Number> numberList = new ArrayList<Number>(); //Number of subclass
numberList = new ArrayList<Integer>(); //Integer extends Number
numberList = new ArrayList<Float>(); // Float extends Number

T – 用于表示类型
E – 用于表示元素
K – 键
V - 值
N – 数字
参考：

<?> is a shorthand for <? extends Object>.
You may read below shared link for more details.

<?>

"?" denotes any unknown type, It can represent any Type at in code for. Use this wildcard if you are not sure about Type.

ArrayList<?> unknownList = new ArrayList<Number>(); //can accept of type Number
unknownList = new ArrayList<Float>(); //Float is of type Number

Note: <?> means anythings. So It can accept of Type which are not inherited from Object class.

<? extends Object>

<? extends Object> means you can pass an Object or a sub-class that extends Object class.

ArrayList<? extends Number> numberList = new ArrayList<Number>(); //Number of subclass
numberList = new ArrayList<Integer>(); //Integer extends Number
numberList = new ArrayList<Float>(); // Float extends Number

T – used to denote type
E – used to denote element
K – keys
V - values
N – for numbers
Ref: