有没有办法实例化匿名内部类中定义的类？ [英] Is there any way to instantiate a class defined in anonymous inner class?

问题描述

我随机写了一个代码&遇到了一个问题：如何实例化在匿名内部类中定义的E类（如下所示）;喜欢：

I was randomly writing a code & encountered a problem: how to instantiate class E (shown below) which is defined within an anonymous inner class; like:

 A c = new A() {
   class E{ //Statements
     }
 };

推荐答案

你不能编写一个使用普通程序的程序调用 new 来执行此操作：为了实例化类，它必须具有名称。匿名内部类，如该术语所暗示的，不具有名称。

You can't write a program that uses an ordinary call to new to do that: in order for a class to be instantiated, it must have a name. Anonymous inner classes, as that term implies, do not have a name.

因此，在该匿名内部类中存在的类也没有名称;因此无法在该匿名内部类的外部实例化。

Thus a class that exists within that anonymous inner class also has no name; thus it can not be instantiated outside of that anonymous inner class.

但您可以使用反射即可。请参阅我的Test.java：

But you can use reflection. See my Test.java:

import java.util.*;
import java.lang.reflect.*;

class B { 
  B() { System.out.println("B"); }
  void foo() { System.out.println("B.foo"); }
}

public class Test{
  B b;
  void bar() {
    b = new B() {
          class C { C() { System.out.println("inner C"); } }
          void foo() { System.out.println("inner foo"); }
    };
    b.foo();
}
public static void main(String[] args) throws Exception {
    Test test = new Test();
    test.bar();

    Class<?> enclosingClass = Class.forName("Test$1");
    Class<?> innerClass = Class.forName("Test$1$C");

    Constructor<?> ctor = innerClass.getDeclaredConstructor(enclosingClass);
    Object innerInstance = ctor.newInstance(test.b);
  }
}

打印：

B
inner foo
inner C

所以，是的，鉴于我们可以在运行时使用 mangled 类名 Test $ 1 $ C 并且该反射允许我们在运行时实例化对象（参见在这里了解详情），最终答案是：是的，它是
可能。

So, yes, given the fact that we can use the mangled class name Test$1$C at runtime, and that reflection allows us to instantiate objects at runtime, too (see here for details), the final answer is: yes, it is possible.

但仅限于记录：这并不意味着在实际代码中应该做这样的事情。这是培养创造力的一个很好的小谜题;但是不适合现实世界中的任何东西。

But just for the record: that doesn't mean at all that one should ever do something like this in real code. This is a nice little puzzle to train creativity; but not suited for anything in the real world.

在现实世界中，匿名内部类中的内部类是设计错误。故事结束。

In the real world, an inner class within an anonymous inner class is a design bug. End of story.

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