reflection InstantiationException

Thread view:

zero - 06 Nov 2005 13:28 GMT

I'm trying to figure out reflection, and I've run into a problem. In the
code below the JFrame and String get instantiated fine, but MyClass gives
an InstantiationException.

What am I overlooking here?

import java.util.ArrayList;

public class ReflectionTest
{
    public static void main(String[] args) throws Exception
    {
        ArrayList<String> typeNames = new ArrayList<String>();
        typeNames.add(String.class.getName());
        typeNames.add(javax.swing.JFrame.class.getName());
//        typeNames.add(MyClass.class.getName());

        ArrayList<Object> data = new ArrayList<Object>();
        for(String name : typeNames)
            data.add(Class.forName(name).newInstance());

        System.out.println(data.toString());
    }

    private class MyClass
    {
        public MyClass()
        {
            super();
        }

        public String toString()
        {
            return "MyClass";
        }
    }
}

Reply to this Message

Chris Uppal - 06 Nov 2005 13:53 GMT

> I'm trying to figure out reflection, and I've run into a problem. In the
> code below the JFrame and String get instantiated fine, but MyClass gives
> an InstantiationException.
>
> What am I overlooking here?

> public MyClass()
> {
> super();
> }

Despite appearances, MyClass (or rather ReflectionTest$MyClass, to give it its
proper name), does not have a no-args constructor. Since it is an inner class,
it contains a hidden reference to its outer object. That reference is passed
in as a hidden parameter to every constructor (the parameter is automatically
added by the compiler). So there is no no-args constructor, and so you cannot
create an instance with newInstance().

In this specific case you could declare MyClass to be a static member, in which
case its instances would not have outer objects, and so there would be a
no-args constructor just as it appears. That wouldn't solve the general
problem, which is that you cannot in general rely on classes having no-args
constructors. Any solution to that problem is necessarily
application-specific.

-- chris

Reply to this Message

Bjorn Abelli - 06 Nov 2005 13:59 GMT

"zero" wrote...

> I'm trying to figure out reflection, and I've run into
> a problem. In the code below the JFrame and String get
> instantiated fine, but MyClass gives
> an InstantiationException.
>
> What am I overlooking here?

You should consider an inner class as a part of the definition of the outer
class.

In this case you can't instantiate any MyClass at all outside of an instance
of class ReflectionTest, as it's defined as non-static.

To resolve it, you could either move the definition of MyClass outside
ReflectionTest, or making MyClass static.

// Bjorn A

Reply to this Message

Benji - 06 Nov 2005 18:27 GMT

> I'm trying to figure out reflection, and I've run into a problem. In the
> code below the JFrame and String get instantiated fine, but MyClass gives
> an InstantiationException.

> What am I overlooking here?
> <snip>

As the other posters said, since the class is internal, it actually has a
constructor that takes in an object of the parent type. You can actually
instantiate it, though. Here's an example I just wrote. You can modify
it to fit your needs:

import java.lang.reflect.Constructor;

public class Outer
{
public static void main(String[] args) throws Exception
{
Class innerClass = Inner.class;
Constructor cons = innerClass.getConstructor(new Class[]{Outer.class});
System.out.println(cons.newInstance(new Outer()));
}

public class Inner
{
public String toString() { return "hello, world!"; }
}
}

Signature

Of making better designs there is no end,
and much refactoring wearies the body.

Reply to this Message

zero - 06 Nov 2005 19:16 GMT

>> I'm trying to figure out reflection, and I've run into a problem. In
>> the code below the JFrame and String get instantiated fine, but
[quoted text clipped - 25 lines]
> }
> }

Yep that does it :-)

Thanks everyone for replying

Reply to this Message

reflection InstantiationException

Free Magazines