Unbounded Wildcards无界通配符

The unbounded wildcard type is specified using the wildcard character (?), for example, List<?>. 使用通配符（?）指定无界通配符类型，例如，List<?>。This is called a list of unknown type. 这称为未知类型的列表。There are two scenarios where an unbounded wildcard is a useful approach:有两种情况下，无界通配符是一种有用的方法：

• If you are writing a method that can be implemented using functionality provided in the Object class.如果您正在编写一个可以使用Object类中提供的功能实现的方法。
• When the code is using methods in the generic class that don't depend on the type parameter. 当代码在泛型类中使用不依赖于类型参数的方法时。For example, List.size or List.clear. 例如，List.size或List.clear。In fact, Class<?> is so often used because most of the methods in Class<T> do not depend on T.事实上，通常使用Class<?>，因为Class<T>中的大多数方法不依赖T。

Consider the following method, printList:考虑以下方法，PrimtList:

public static void printList(List<Object> list) {
    for (Object elem : list)
        System.out.println(elem + " ");
    System.out.println();
}

The goal of printList is to print a list of any type, but it fails to achieve that goal — it prints only a list of Object instances; it cannot print List<Integer>, List<String>, List<Double>, and so on, because they are not subtypes of List<Object>. printList的目标是打印任何类型的列表，但它无法实现该目标—它只打印Object实例的列表；它无法打印List<Integer>、List<String>、List<Double>，等等，因为它们不是List<Object>的子类型。To write a generic printList method, use List<?>:要编写通用的printList方法，请使用List<?>：

public static void printList(List<?> list) {
    for (Object elem: list)
        System.out.print(elem + " ");
    System.out.println();
}

Because for any concrete type A, List<A> is a subtype of List<?>, you can use printList to print a list of any type:因为对于任何具体类型A，List<A>都是List<?>的子类型，您可以使用printList打印任何类型的列表：

List<Integer> li = Arrays.asList(1, 2, 3);
List<String> ls = Arrays.asList("one", "two", "three");
printList(li);
printList(ls);

It's important to note that List<Object> and List<?> are not the same. 需要注意的是，该List<Object>和List<?>不一样。You can insert an Object, or any subtype of Object, into a List<Object>. 你可以把一个Object或Object的任意子类插入到List<Object>。But you can only insert null into a List<?>. 但是null只能插入到List<?>。The Guidelines for Wildcard Use section has more information on how to determine what kind of wildcard, if any, should be used in a given situation.通配符使用指南部分提供了有关如何确定在给定情况下应使用哪种通配符（如果有）的更多信息。