What's the proper way to use reflection to instantiate objects of unknown classes at runtime?

Question

I am working on a Configuration Loader class so that I can change the parameters of my program via an external text file (config.txt) rather than having to recompile my code with every change I make. It has been suggested that I use Java's Reflection to do this, but I'm a little confused as to how I might actually implement this. I have been able to successfully extract the class name and the arguments for its constructor from my text file, but how do I go from this to an instantiated object? here's what I have of my method so far: ``` public void loadObject(String classString, HashMap hm) { String className = props.getProperty(classString); Class c = Class.forName(className); } ``` classString is a string containing the name of the class, and hm is a hashmap where the class' constructor parameters map to their intended values. I.e., for `class Foo (int xPos, float yPos)`, "xPos" would map to a string of the intended int, and "yPos" maps to a string of the intended float. I want to be…

Accepted Answer

You would use `Class.getConstructor(Class<?>... parameterTypes)` to get a reference to the constructor followed by `Constructor.newInstance(Object... initargs)`.

However I would suggest taking a look at a dependency injection framework such as Spring or Guice as it sounds like what you are creating is a basic version of what they do.

Upon request for expanding this answer:

```
Class c = Class.forName(name);
Constructor ctor = c.getConstructor(Integer.class, Integer.class);
Integer param1 = hm.get("xPos") ...;
Integer param2 = hm.get("yPos") ...;
Object instanceOfTheClass = ctor.newInstance(param1, param2);
```

Of course instead of `param1` and `param2` you would create an array of arguments based upon what was in the input file (the same goes for the arguments to `getConstructor()`), etc.

What's the proper way to use reflection to instantiate objects of unknown classes at runtime?

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