Java - Invoking class methods

Question

I'm following a java tutorial, i find this piece of code:

//load the AppTest at runtime
Class cls = Class.forName("com.mkyong.reflection.AppTest");
Object obj = cls.newInstance();

//call the printIt method
Method method = cls.getDeclaredMethod("printIt", noparams);
method.invoke(obj, null);

My question is: if i don't know the class type, is not much easier (and faster) try to cast the object instead of invoking methods in that way?
Why (and when) i should use this way?

If u know all about the class then why not simply create the object using new instead of reflection — Juned Ahsan, Jun 18 '14 at 07:22
@JunedAhsan I think that's exactly why he is asking this, to determine if that is exactly equivalent - is it? — vikingsteve, Jun 18 '14 at 07:24
Exactly, this is my question. Why create an object using 'Class.forName'?? I can't find a real example to understand — bebeto123, Jun 18 '14 at 07:24
"if i don't know the class type, is not much easier (and faster) try to cast the object" - the question includes the answer. — pingw33n, Jun 18 '14 at 07:24
Duplicate of http://stackoverflow.com/questions/9222621/java-what-scenarios-call-for-the-use-of-reflection — Dinal, Jun 18 '14 at 07:24
@bebeto123 There are plenty of real world examples available. Just try to understand any framework such as hibernate/spring etc. — Juned Ahsan, Jun 18 '14 at 07:24

score 2 · Accepted Answer · edited May 23 '17 at 10:33

Read this question What is reflection and why is it useful?

It says:

"For example, say you have an object of an unknown type in Java, and you would like
to call a 'doSomething' method on it if one exists. Java's static typing system
isn't really designed to support this unless the object conforms to a known
interface, but using reflection, your code can look at the object and find out if
it has a method called 'doSomething' and then call it if you want to."

You can find a good tutotial here http://tutorials.jenkov.com/java-reflection/index.html

score 0 · Answer 2 · answered Jun 18 '14 at 07:24

One reason of doing it this way, not sure if in your case:

Method method = cls.getDeclaredMethod("printIt", noparams);
method.invoke(obj, null);

will allow u to call printIt even if it is private.

But if you cast to your class object then u will not be able to call private methods.

score 0 · Answer 3 · edited Jun 20 '20 at 09:12

If i don't know the class type, is not much easier (and faster) try to cast the object instead of invoking methods in that way?

Why (and when) i should use this way?

I assume you mean "If I know" not "If I don't know" above.

If you have com.mkyong.reflection.AppTest at compile-time, then you wouldn't need to use reflection to use it at all. Just use it:

// In your imports section (although you don't *have* to do this,
// it's normal practice to avoid typing com.mkyong.reflection.AppTest
// everywhere you use it)
import com.mkyong.reflection.AppTest;

// ...and then later in a method...

//load the AppTest at runtime
AppTest obj = new AppTest();

//call the printIt method
obj.printIt();

If you don't know the class at compile-time, you can't cast the result of newInstance to it, because...you don't have it at compile time.

A common middle ground is interfaces. If com.mkyong.reflection.AppTest implements an interface (say, TheInterface), then you could do this:

//load the AppTest at runtime
Class cls = Class.forName("com.mkyong.reflection.AppTest");
TheInterface obj = (TheInterface)cls.newInstance();

//call the printIt method
obj.printIt();

At compile-time, all you need is the interface, not the implementing class. You can load the implementing class at runtime. This is a fairly common pattern for plugins (like, say, JDBC implementations).

Java - Invoking class methods

3 Answers3