英文:
Why can I use interface as anonymous class?
问题
例如：
Runnable r = new Runnable() {
@Override
public void run() {
}
};
为什么没有错误？接口无法实例化。
英文:
For example:
Runnable r = new Runnable() {
@Override
public void run() {
}
};
Why there is no error? Interfaces cannot be instantiated.
答案1
得分: 1
那个语法不是用来实例化东西的。或者至少，不仅仅是用来实例化的。
那个语法是语法糖（简写），等同于：
class Whatever$DontLookAtThisName implements Runnable {
@Override public void run() {
}
}
Runnable r = new Whatever$DontLookAtThisName();
如果你在那里使用了一个类，它会等同于 class Whatever$DontLookAtThis extends TheThingieYouPutThere，除了 extends FOO 与 implements FOO之外，概念是相同的。
英文:
That syntax isn't for instantiating things. Or at least, not just for that.
That syntax is sugar (short hand) for:
class Whatever$DontLookAtThisName implements Runnable {
@Override public void run() {
}
}
Runnable r = new Whatever$DontLookAtThisName();
Had you used a class there, it'd have been shortfor for class Whatever$DontLookAtThis extends TheThingieYouPutThere, but other than 'extends FOOvsimplements FOO`, it's the same concept.
答案2
得分: 0
你正在创建一个实现Runnable接口的匿名类。注意：由于此接口只有一个抽象方法，在Java 8及更高版本中，你可以这样做：
Runnable r = () -> System.out.println("run");
r.run();
英文:
You are creating an anonymous class that implements the Runnable interface. Note: That since this interface has a Single Abstract Method, in Java 8+ you could just do
Runnable r = () -> System.out.println("run");
r.run();
答案3
得分: 0
以下是已翻译的内容：
有关这个主题，Oracle 提供了一份很好的文档：
> … 一个匿名类是一个表达式。匿名类表达式的语法类似于调用构造函数，不同之处在于代码块中包含了一个类定义。
匿名类表达式包括以下内容：
-
new运算符 -
要实现的接口名称或要继承的类名称。在此示例中，匿名类正在实现接口 [Runnable]。
-
包含构造函数参数的括号，就像普通类实例创建表达式一样。注意：当你实现一个接口时，没有构造函数，所以你使用一对空括号，就像这个示例中的情况一样。
-
一个类体，这是一个类声明体。更具体地说，在类体中，允许方法声明，但不允许语句。
英文:
There is a nice documentation from Oracle about this topic:
> ... an anonymous class is an expression. The syntax of an anonymous class expression is like the invocation of a constructor, except that there is a class definition contained in a block of code.
The anonymous class expression consists of the following:
-
The
newoperator -
The name of an interface to implement or a class to extend. In this example, the anonymous class is implementing the interface [Runnable].
-
Parentheses that contain the arguments to a constructor, just like a normal class instance creation expression. Note: When you implement an interface, there is no constructor, so you use an empty pair of parentheses, as in this example.
-
A body, which is a class declaration body. More specifically, in the body, method declarations are allowed but statements are not.
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