Question

How do I instantiate an object using the Class class?

I would like to pass a Class object to a function and have that function return a new object of the class I pass in.

I will be doing casting after I get the object back. Also, are there any suggestions on design patterns for this sort of thing?

Here is some example pseudo code that shows what I want to do:

Class Animal {
    String name;
}

Class Cat extends Animal {
    boolean hasTail;
}

MAIN:

Class myClass = Cat.class;
Animal createAnimal(myClass) {
    return new myClass;
}

Edit:

Also, is there a way to cast within my function before returning the new instance or am I stuck with the default Object type?

return (myClass)myClass.newInstance();

Answer 1

I will be doing casting after I get the object back.

Also, is there a way to cast within my function before returning the new instance or am I stuck with the default Object type?

You don’t need to do casting if you take advantage of generics. Generics are described in relatively introductory detail here

Here is a java example derived from your psuedo code using generics and reflection:

public class ReflectionNewInstanceTest {

    static class Animal {
        String name;
    }

    static class Cat extends Animal { 
        boolean hasTail = true;
    }

    public static void main(String[] args) throws Exception {
        Animal a = createAnimal(Cat.class);
    }


    static <T extends Animal> T createAnimal(Class<T> clazz) throws Exception {
        return clazz.newInstance();
    }

}

As for patterns, any of the typical creational patterns should be looked at when you want to abstract away the creation of objects. The simplest is probably just using a factory method. The example I have above is just a general purpose factory method that doesn’t really gain you much. Here is a pretty short list of patterns you should look up:

• Abstract factory pattern
• Factory method pattern
• Builder pattern
• Prototype pattern