New instance of class based on argument

Question

I\'m trying to make a function that takes one of many classes that extends Foo, and return a new instance of that object in its Class, not a new instance of F

Answer 1

You can use Java's reflection here.

If you want to get a Class by just its classname, you use Class.forName:

Class type = Class.forName('package.SomeClass');

You can check if this class is Foo or a subclass of it:

boolean isFoo = type.isAssignableFrom(Foo.class);

You can then create a new instance very easily (assuming the constructor takes no arguments):

Object instance = type.newInstance();

Answer 2

Are you passing a Class object as the parameter, or in instance of a subclass of Foo?

The solution is almost the same in either case, you use the newInstance method on the Class object.

/**
 * Return a new subclass of the Foo class.
 */
public Foo fooFactory(Class<? extends Foo> c)
{
    Foo instance = null;
    try {
        instance = c.newInstance();
    }
    catch (InstantiationException e) {
        // ...
    }
    catch (IllegalAccessException e) {
        // ...
    }
    return instance; // which might be null if exception occurred,
                     // or you might want to throw your own exception
}

If you need constructor args you can use the Class getConstructor method and from there the Constructor newInstance(...) method.

Answer 3

Look at Abstract Factory pattern.

Answer 4

Your function could be like this

public class NewInstanceTest {
    public static Object getNewInstance(Foo fooObject){
        java.lang.Class fooObjectClass = fooObject.getClass();
        try {
            return fooObjectClass.newInstance();
        } catch (InstantiationException e) {
            e.printStackTrace();
            return null;
        } catch (IllegalAccessException e) {
            e.printStackTrace();
            return null;
        }
 }
 public static void main(java.lang.String[] args){
        // This prints Foo
        java.lang.System.out.println(getNewInstance(new Foo()).getClass().getName());
        // This prints Foo1
        java.lang.System.out.println(getNewInstance(new Foo1()).getClass().getName());
 }
}
class Foo{
}
class Foo1 extends Foo{
}

Hope this helps.