Using reflection to create object with parameters in constructor

Question

I can not understand the following code:

Constructor<T>[] constructors = (Constructor<T>[]) clazz.getConstructors();  
for(int i = 0; i < constructors.length; i++){  
  Constructor<T> constructor = constructors[i];    
  if (constructor.getParameterTypes().length>0){    
    T instanceObject = constructor.newInstance(new Object[constructor.getParameterTypes().length]);  
        break;  
  }  

}

Have omitted try/catch and other stuff for clarity.
I can not understand how this works: T instanceObject = constructor.newInstance(new Object[constructor.getParameterTypes().length]);
It calls a constructor that has parameters, but passes as arguments Object?
How does this work? Passing Object independent of the actual formal parameters?

you need all arguments to be non-primitive and the c-tor to make sense w/ having all parameters null — bestsss
– bestsss, Commented Sep 12, 2012 at 7:51
@bestsss:Individual parameters are automatically unwrapped to match primitive formal parameters from Javadoc — Jim
– Jim, Commented Sep 12, 2012 at 7:58
Jim, except null is unwrapped to NPE. I know the spec and even the impl, really no need to quote. The unwrapping is simple: Integer.intValue(), Double.doubleValue() - hence the NPE. — bestsss
– bestsss, Commented Sep 12, 2012 at 9:05

Peter Lawrey · Accepted Answer · 2012-09-12 07:58:00Z

1

It attempts to pass dummy parameters which are all null. This can give you an Object but it doesn't mean it will be a useful one. ;)

I am not sure why it skips zero length constructors as this is the one constructor you are likely to be able to pass no arguments successfully.

edited Sep 12, 2012 at 7:58

answered Sep 12, 2012 at 7:50

Peter Lawrey

535k83 gold badges770 silver badges1.2k bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

bestsss Over a year ago

also can lead to NPE if any of the arguments are primitive

Peter Lawrey Over a year ago

it could lead to an NPE even if the arguments are not primitive but expect you to pass it something. ;) I assume the outer loop catches exceptions and keeps trying if it gets one.

bestsss Over a year ago

I mean it won't even get to the c-tor. Of course, any arbitrary data can lead to errors but at w/ primitives the c-tor won't even be invoked.

Boris Pavlović · Accepted Answer · 2012-09-12 07:51:47Z

1

An array of objects with number of elements equal to number of parameters in the constructor, hence:

new Object[constructor.getParameterTypes().length])

answered Sep 12, 2012 at 7:51

Boris Pavlović

64.9k29 gold badges127 silver badges150 bronze badges

Collectives™ on Stack Overflow

Using reflection to create object with parameters in constructor

2 Answers 2

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related