Java Reflections

Question

I have an abstract class 'A' and class 'B' and 'C' extends A. I want to create instances of these during run time based on some variable. Something like below:

public abstract class A {
    public abstract int count(); 
}

public class B extends A {
    public int count () {
        //print 10;
    }
}

public class C extends A {
    public int count () {
        //print 20;
    }
}

I would use the below code to call the method count:

String token;
int i = 10;
if (i == 10)  //edit
    token = "B";
else 
    token = "C";

A a;
try {
     a = (A) (Class.forName("org.source."+token)).newInstance();
} catch (Exception e) {
     //print e
}

a.count();

Since I am new to java reflections, here are my 2 questions:

1. Is what I am doing above right (edit: in case of default constructors) ? (I am presuming yes)

2. The above works if the default constructor (without parameters) is called. How would I handle a situation where I have constructors that take in arguments. I am not very sure how I could use Constructor.newInstance() can be used in the above situation.

Any help appreciated,

Answer 1

What you are doing is right only if you want to call the no-arg constructor, which really, is the only thing you can do with the "newInstance" method in Class. See:

http://docs.oracle.com/javase/1.4.2/docs/api/java/lang/Class.html#newInstance()

to invoke constructors with n-args, use the Constructor class:

http://docs.oracle.com/javase/7/docs/api/java/lang/reflect/Constructor.html#newInstance(java.lang.Object...)

See the Reflection tutorial for examples on the usages of the above:

http://docs.oracle.com/javase/tutorial/reflect/member/ctorInstance.html

For more info.

Answer 2

Yes it will work.

If you want to use a constructor other than the default (no-args) one, read on.

Let's say for example you want to call the constructor with an int and String arguments. Here's the code that will do that:

Class<?> clazz = Class.forName("org.source."+ token );
Constructor<?> constructor = clazz.getConstructor(int.class, String.class);
Object object = constructor.newInstance(1, "hello");