Java InstantiationException when trying to create a new instance

Question

I am trying to load class files from a folder so that way users can interact with my program. Users are supposed to be able to just extend the abstract class I made and drop it in the folder, but whenever I try and load a class (that I made, so I know its correct), it gives me an InstantiationException. Here is a stack trace snippet and a snippet of where the error is occuring

[9:09:22 PM] Jav:         23:03:40 [SEVERE] java.lang.InstantiationException: UchihaClan
23:03:40 [SEVERE]       at java.lang.Class.newInstance0(Unknown Source)
23:03:40 [SEVERE]       at java.lang.Class.newInstance(Unknown Source)
23:03:40 [SEVERE]       at com.zavcoding.narutorpg.Classes.ClassLoader.load(ClassLoader.java:52)
23:03:40 [SEVERE]       at com.zavcoding.narutorpg.NarutoRPG.onEnable(NarutoRPG.java:38)



Class<?> clazz = loader.loadClass(name);
Object object = clazz.newInstance();
if (!(object instanceof UserMadeClass)) {
    System.out.println("Not a valid user class.");
    continue;
}
UserMadeClass umc = (UserMadeClass) object;

Do you have more of that stack trace? Sometimes there is a "caused by" section. — Thilo, Aug 28 '12 at 04:29
There's a typo `UserMadeclass` for `UserMadeClass` but I'm guessing that's not the real problem. :) — Ray Toal, Aug 28 '12 at 04:30
Have you tried a `UserMadeClass test = new UserMadeClass();`? Does it work? — Andreas Dolk, Aug 28 '12 at 04:32
Thought so... Can you do `javap UserClassExample` ? It will show whether it truly is a subclass of `UserMadeClass`. — Ray Toal, Aug 28 '12 at 04:33
UserMadeClass shouldn't be instantized as it is an abstract class. — Zach Sugano, Aug 28 '12 at 04:33
Java is open source, so you can look at [what newInstance() is actually doing](http://www.docjar.com/html/api/java/lang/Class.java.html). In particular, it appears that the only case in which it will throw an `InstantiationException` is if the given class has no accessible zero-argument constructor. — aroth, Aug 28 '12 at 04:39
The abstract class only has a constructor with arguments is that the issue? — Zach Sugano, Aug 28 '12 at 04:40
@ZachSugano - I'd say so, unless your subclass provides its own zero-argument constructor. — aroth, Aug 28 '12 at 04:42
The class you are *instantiating* must have a zero-arg constructor. The abstract class it is extending can have anything, as long as it is properly invoked. — Jon Newmuis, Aug 28 '12 at 04:44
If you get the Constructors using `getDeclaredConstructors()` you can invoke a constructor with an argument [as detailed here](http://docs.oracle.com/javase/tutorial/reflect/member/ctorInstance.html). — Roddy of the Frozen Peas, Aug 28 '12 at 04:44

Java InstantiationException when trying to create a new instance

0 Answers0