Making an object for a class based on a variable (is it even possible?)

Newbie & Debugging Questions
#1

Hey, sorry if this doesnt make much sense, but I cannot explain it very well.

so is it possible, and if so how, to make an object based on a variable. so for example. I have a string and the string is currently “Foo” is there a way to then make “new Foo foo;” based on that?

but of course I cant just do “new Foo foo;” The class has to be whatever the string is.

Sorry if this makes absolutely no sense, or if I am going about this the wrong way.

#2 
Object fooInstance = Class.forName(fooString).newInstance();
#3

I kinda get it, but obviously the question would be “why ?”
apart from that: if the fields of the new object have names, according to some string values… you couldn’t even use the new object / its fields, because at compile time there would be no fields / you wouldnt know its names yet - so you couldnt use it

sounds like dynamically named variables… I don’t know

but obviously what you really could do is to have a class with a hashmap, and then use that

#4

Reflection will do it. Keep in mind that invoking methods with Reflection is slow. But if you create all your class in a place where speed doesn’t matter then call the methods from a variable or stored collection the speed is normal.


public interface Shape {
  public void draw();
}
public class Triangle implements Shape {
  public void draw() {
    ...
  }
}
public class Square implements Shape {
  public void draw() {
    ...
  }
}
public class Program {
  ArrayList<Shape> shapes = new ArrayList<Shape>();
  public static void main(String args[]) {
    ArrayList<String> names = //load all your strings here
    for(String name:names) {
      //exceptions left out for brevity
      //invoke constructor method is slow
      shapes.add(Class.forName(name).newInstance());
    }
    for(Shape shape:shapes) {
      //since this part invokes the draw method without reflection the speed is normal
      shape.draw();
    }
  }
}
#5

I think that rivens method is exactly what I am looking for, thanks!

as for the why, it is because I need to make a new object to be added to a list, and which class it is is based on that string.

#6

It’s very useful for loading implementations of interfaces

#7

Scripting interfacing and/or hot deployment are a couple of examples.

#8

Hey, I tried rivens way, and I got an error saying that the class did not exist, when I am certain it does, so ist here any special things I have to have in the string like .class or soemthing?

#9

google FQCN

#10

You need the full class name (ie. with the package as well). So something like “java.util.ArrayList”, not just “ArrayList”.

#11

I tried that too, and it didnt work right. maybe i messed something up, I’ll give it another look.

EDIT:

Turns out I had the class name right, but I was just getting a different error, java.lang.InstantiationException.

a quick google says that it is caused when the class is an interface or an abstract, which my class is not :P. any thoughts?

thanks

#12

If your class has a non-default constructor, you need:

Class.forName(fqcn).getConstructor(...).newInstance(...)
#13

oh, so if it extends something like mine does?

I’ll try that, thanks :slight_smile:

#14

well, whether it extends something has nothing to do with it. just read the javadoc, and google a bit, if it’s not clear.