Java Reflections error: Wrong number of arguments

Question

So I'm trying to invoke a classes constructor at runtime. I have the following code snippet:

String[] argArray = {...};
...
Class<?> tempClass = Class.forName(...);
Constructor c = tempClass.getConstructor(String[].class); 
c.newInstance(argArray);
...

Whenever I compile the code and pass it a class, I get an IllegalArgumentException: wrong number of arguments. The constructor of the class I'm calling takes in a String[] as the only argument. What's also weird is that if I change the constructor to take in an integer and use Integer.TYPE and call c.newInstance(4) or something, it works. Can someone explain to me what I'm doing wrong? Thank you.

Edit;; Complete error:

java.lang.IllegalArgumentException: wrong number of arguments
[Ljava.lang.String;@2be3d80c
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
at java.lang.reflect.Constructor.newInstance(Unknown Source)

Could you post the complete error?

Hophat Abc
– Hophat Abc

2013-01-24 00:08:42 +00:00
Commented Jan 24, 2013 at 0:08 — Hophat Abc
– Hophat Abc, Commented Jan 24, 2013 at 0:08

Community · Accepted Answer · 2017-05-23 11:45:34Z

12

This is happening because newInstance(Object...) takes varargs of Object, in other words Object[]. Since arrays are covariant, a String[] is also an Object[], and argArray is being interpretted as all arguments instead of first argument.

jdb's solution works because it prevents the compiler from misunderstanding. You could also write:

c.newInstance(new Object[] {argArray});

edited May 23, 2017 at 11:45

CommunityBot

11 silver badge

answered Jan 24, 2013 at 0:25

Paul Bellora

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Comments

jdb · Accepted Answer · 2013-01-24 00:19:07Z

10

I'm not sure if it is the best fix but this should work:

c.newInstance((Object)argArray);

answered Jan 24, 2013 at 0:19

jdb

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1 Comment

jdb Over a year ago

Without the Object[] wrapper newInstance tries to pass multiple String arguments to a single argument constructor. The exception message changes from (java.lang.IllegalArgumentException: argument type mismatch) to (java.lang.IllegalArgumentException: wrong number of arguments) depending on the size of the String array.

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