用反射创建内部类对象

Question

How do you create an inner class object with reflection? Both Inner and Outer classes have default constructors that take no parameters

Outer class {
    Inner class{
   }
    public void createO() {
        Outer.Inner ob = new Inner ();//that works
        Inner.class.newInstance(); //<--why does this not compile?
   }
}

Answer 1

"If the constructor's declaring class is an inner class in a non-static context, the first argument to the constructor needs to be the enclosing instance; see section 15.9.3 of The Java™ Language Specification ."

That means you can never construct an inner class using Class.newInstance ; instead, you must use the constructor that takes a single Outer instance. Here's some example code that demonstrates its use:

class Outer {
    class Inner {
        @Override
        public String toString() {
            return String.format("#<Inner[%h] outer=%s>", this, Outer.this);
        }
    }

    @Override
    public String toString() {
        return String.format("#<Outer[%h]>", this);
    }

    public Inner newInner() {
        return new Inner();
    }

    public Inner newInnerReflect() throws Exception {
        return Inner.class.getDeclaredConstructor(Outer.class).newInstance(this);
    }

    public static void main(String[] args) throws Exception {
        Outer outer = new Outer();
        System.out.println(outer);
        System.out.println(outer.newInner());
        System.out.println(outer.newInnerReflect());
        System.out.println(outer.new Inner());
        System.out.println(Inner.class.getDeclaredConstructor(Outer.class).newInstance(outer));
    }
}

(Note that in standard Java terminology, an inner class is always non-static. A static member class is called a nested class .)