Java, making constructor call using list of parameters?

Question

So, I'm reading data from a file and depending on the text read, the program will use a constructor of the class that matches the parameters on the line. So the parameters given by the line of the text are stored into an ArrayList:

List<Object> parameters = new ArrayList<Object>();

Then I should somehow be able to create an object from those parameters, something like this:

constructor.newInstance(objects);

but I'm not quite sure how I could achieve that?

try {
            Class<?> objectClass = Class.forName("com.editor.object." +line.substring(4, from+4));
            Constructor[] allConstructors = objectClass.getDeclaredConstructors();
            for(Constructor constructor : allConstructors){
                Class<?>[] parameters = constructor.getParameterTypes();
                if(objects.size() == parameters.length){
                    for(int i = 0; i < parameters.length; i++){
                        if(objects.get(i).getClass().equals(parameters[i])){
                            if(i + 1 == parameters.length){
                                return constructor.newInstance(objects); //<-- This doesen't work, I have no idea how should I call the "random" constructor?
                            }
                        }
                    }
                }
            }
        } catch (ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (InstantiationException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

Edit 1: An example: I have the following line

new Platform(1, 1, 1, 1, 1);

-> Would create a new Platform object with parameters given. The referred class and constructor might be pretty much anything, so I can't rely on stuff like that. Of course I could just run that in code but I would like to learn more and that's why I won't do it the easiest way.

I think that you'd want to use parameters[i].isAssignableFrom(objects.get(i).getClass()) instead of equals, since you don't necessarily need the exact same class. — Andy Turner
– Andy Turner, Commented Jul 29, 2015 at 22:09
@AndyTurner Turner Added a comment in code to point the part that I don't know how to deal with. — nautilus_rs
– nautilus_rs, Commented Jul 29, 2015 at 22:15
@nautilus_rs - call constructor.newInstance(objects.toArray()). You are currently effectively invoking ctor.newInstance(new Object[] { objects }). — Andy Turner
– Andy Turner, Commented Jul 29, 2015 at 22:20

Andy Turner · Accepted Answer · 2015-07-29 22:24:01Z

1

Constructor.newInstance is a variadic method: its type signature is Constructor.newInstance(Object... args). If you invoke it with the ArrayList as a single argument, that will be interpreted the same as:

Constructor.newInstance(new Object[] { objects })

because ArrayList is not an array type. This will fail unless that constructor happens to accept a single List parameter, and even then it could well fail because the elements in the list are not of the required type for that constructor.

Instead, you can invoke it as:

Constructor.newInstance(objects.toArray())

which "explodes" the list into the separate objects, from the point of view of invoking that constructor.

answered Jul 29, 2015 at 22:24

Andy Turner

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