Question

I have a utility method and when irrelevant logic is removed from it, the simplified method would look like this:

public static <A extends Foo> List<A> getFooList(Class<A> clazz) {    List<A> returnValue = new ArrayList<A>();    for(int i=0; i < 5; i++) {         A object = clazz.newInstance();         returnValue.add(object);    }     return returnValue; } 

The problem is, that if clazz is an inner class such as Foo.Bar.class, then the newInstance() method will not work even if Bar would be public, as it will throw a java.lang.InstantiationException.

Is there a way to dynamically instantiate inner classes?

Answer 1

If it’s genuinely an inner class instead of a nested (static) class, there’s an implicit constructor parameter, which is the reference to the instance of the outer class. You can’t use Class.newInstance at that stage – you have to get the appropriate constructor. Here’s an example:

import java.lang.reflect.*;  class Test {     public static void main(String[] args) throws Exception     {         Class<Outer.Inner> clazz = Outer.Inner.class;          Constructor<Outer.Inner> ctor = clazz.getConstructor(Outer.class);          Outer outer = new Outer();         Outer.Inner instance = ctor.newInstance(outer);     } }  class Outer {     class Inner     {         // getConstructor only returns a public constructor. If you need         // non-public ones, use getDeclaredConstructors         public Inner() {}     } }