这个Java语法是什么意思？ （`类 <? extends ContactAccessor> clazz`）

Question

I have been developing android applications for about 1 month now I am getting pretty familiar with the Java syntax but today I stumbled upon this piece of code :

try {
    Class<? extends ContactAccessor> clazz =
                      Class.forName(className).asSubclass(ContactAccessor.class);
    sInstance = clazz.newInstance();
} catch (Exception e) {
    throw new IllegalStateException(e);
}

Could somebody explain me what this Class<? extends ContactAccessor> clazz Class<? extends ContactAccessor> clazz does?

Answer 1

Class is used for reflection. <> means generic type. ? is a generic wildcard. Combined this means that clazz represents definition of a class which is a descendant of ContactAccessor . For further explanations, google for generics , wildcards and reflection .

Answer 2

That means that you use a class which extend a special base class. This is also called Generics in Java.

Answer 3

This means that the Class you need,it's type is unknown(hence the ? ). But you know one property of it - that it is a subclass of ContactAccessor . And you need to find that particular class - so you do : Class.forName(className).asSubclass(ContactAccessor.class); saying fetch me the class by it's class,which is a subclass of ContactAccessor .

Answer 4

That is an example of Java Generics (more here ). It implies that the clazz variable will be of a type which extends ContactAccessor .

Answer 5

It means you have a Class which is ContactAccessor class or a sub class of that class or interface.

Since you have ContactAccessor.class already, I would assume you have a sub class.

Answer 6

它定义了一个变量clazz ，它是扩展ContactAccessor类的泛型类的Class对象。

Answer 7

使用通配符（“？”），以便Class只接受ContactAccessor扩展的类，只接受它们中的任何一个。

Answer 8

任何类都是ContactAccessor类的subclass类。