Is there any way to instantiate a class defined in anonymous inner class?

问题

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
     }
 };

回答1:

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.

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.

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);
  }
}

This prints:

B
inner foo
inner C

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.

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.

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

来源：https://stackoverflow.com/questions/42984297/is-there-any-way-to-instantiate-a-class-defined-in-anonymous-inner-class

标签

java

anonymous-inner-class