Return object from passed class parameter

Question

In my program I have an abstract class A and some other classes which extends from A. Here is a little excerpt from my code:

public abstract class A {
    public A(int[] values) {
        // ...
    }
}

public class B extends A {
    public B(int[] values) {
        super(values);
        // setup other things
    }
}

// some more classes like B

Now I have a class which reads out some integer values from a file and writes them in an array. This class only have static methods and I want to return an object from a passed class. With this code I can give the class B to the method but how can I create and return an object of the class with I have called the method? I tried wi this:

B obj = Reader.readFromFile(path, B.class);

public class Reader {
    public static *** readFromFile(String path, Class<? extends A> c) {
        // read out the file and store values in an array
        // how can I create an object of the parameter c and return it from here?
    }
}

I do not want to make the whole class generic. Any ideas how to do this or is this not possible in Java?

Answer 1

You can create instance of provided class via reflection. In your case you need to get a proper constructor via c.getConstructor(int[].class) and then create new object calling newInstance(...) method of that constructor

Answer 2

Another solution would be that you instantiate the object before calling the method, and pass that instance to the method. Then, you could use a setter to pass the values:

public static void readFromFile(String path, A c) {
    c.setValues(...);
}

B should then provide a nullary constructor.

If you really need to instantiate the object within the method, you could still use reflection :

public static void readFromFile(String path, Class<? extends A> c) {
    int[] values = ...;
    Constructor<T> constructor = c.getConstructor(int[].class);
    T instance = constructor.newInstance(values);
}