If I have a String "Hello" and I know it is a name of a class, how to use it to create a object?

Thread view:

Enable EMail Alerts

Start New Thread

fAnSKyer - 25 Sep 2006 21:32 GMT

If I have a String "Hello" and I know it is a name of a class, how to
use the string to create a object?

Thanks, fAn

Reply to this Message

Andrew Thompson - 25 Sep 2006 21:36 GMT

> If I have a String "Hello" and I know it is a name of a class, how to
> use the string to create a object?

Reflection. But what actual general problem are
you attempting to solve?

Using reflection in Java is generally considered a
poor hack (though some types of applications
need to use it extensively - like servers).

Andrew T.

Reply to this Message

Thomas Schodt - 25 Sep 2006 21:45 GMT

> If I have a String "Hello" and I know it is a name of a class, how to
> use the string to create a object?

You mean like

String classname = "Hello";
Object o = Class.forName(classname).newInstance();

<http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Class.html#forName(java.lang.String)>
<http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Class.html#newInstance()>

Reply to this Message

fAnSKyer - 25 Sep 2006 21:58 GMT

Thanks a lot, also to Andrew T. :P

> > If I have a String "Hello" and I know it is a name of a class, how to
> > use the string to create a object?
[quoted text clipped - 6 lines]
> <http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Class.html#forName(java.lang.String)>
> <http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Class.html#newInstance()>

Reply to this Message

fAnSKyer - 26 Sep 2006 03:42 GMT

A following question,
It can run, but IF the class "Hello" have a method get();

using o.get() is wrong. Because it is a Object, not a Class. How to
solve then?

Thanks a lot
Fans.

Thomas Schodt 写道：

Reply to this Message

Dijon Yu - 26 Sep 2006 06:59 GMT

> A following question,
> It can run, but IF the class "Hello" have a method get();
>
> using o.get() is wrong. Because it is a Object, not a Class. How to
> solve then?

If method get() is not a static method, you must create a instance of
Hello class, you use Object o =
Class.forName(classname).newInstance();
like Thomas Schodt write
Dijon Yu

> Thomas Schodt 写道：
>
[quoted text clipped - 8 lines]
> > <http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Class.html#forName(java.lang.String)>
> > <http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Class.html#newInstance()>

Reply to this Message

Thomas Schodt - 26 Sep 2006 07:32 GMT

> A following question,
> It can run, but IF the class "Hello" have a method get();
>
> using o.get() is wrong. Because it is a Object, not a Class. How to
> solve then?

You either need to know about the class (which defeats the purpose)
or it needs to implement a known interface

interface KnownInterface {
Object get();
}

class UnknownClass implements KnownInterface {
Object get() {
// ...
}
}

String classname = "UnknownHello";
Object o = Class.forName(classname).newInstance();
KnownInterface ki = (KnownInterface)o;
Object ret = ki.get();

or you need to get serious with reflection - read
<http://java.sun.com/j2se/1.4.2/docs/api/java/lang/reflect/package-summary.html>

Reply to this Message

fAnSKyer - 27 Sep 2006 22:47 GMT

Thanks a lot.

What if KnownInterface is also a String and I need use this String to
do the following steps?
Still need help.

Thanks, fAns.
Thomas Schodt 写道：

> > A following question,
> > It can run, but IF the class "Hello" have a method get();
[quoted text clipped - 22 lines]
> or you need to get serious with reflection - read
> <http://java.sun.com/j2se/1.4.2/docs/api/java/lang/reflect/package-summary.html>

Reply to this Message

Thomas Schodt - 28 Sep 2006 10:06 GMT

> What if KnownInterface is also a String

Then it is not a known interface. o_O

> Still need help.

you need to get serious with reflection - read
<http://java.sun.com/j2se/1.4.2/docs/api/java/lang/reflect/package-summary.html>

There seems to be a number of threads around on this.
Read them.

Reply to this Message

su_dang@hotmail.com - 28 Sep 2006 16:22 GMT

> A following question,
> It can run, but IF the class "Hello" have a method get();
[quoted text clipped - 17 lines]
> > <http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Class.html#forName(java.lang.String)>
> > <http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Class.html#newInstance()>

Using reflection, you can obtain an object of the class Method which
can be used to invoke the method you want.

Reply to this Message

fAnSKyer - 02 Oct 2006 04:51 GMT

thanks and those who helped me :P

> > A following question,
> > It can run, but IF the class "Hello" have a method get();
[quoted text clipped - 20 lines]
> Using reflection, you can obtain an object of the class Method which
> can be used to invoke the method you want.

Reply to this Message

If I have a String "Hello" and I know it is a name of a class, how to use it to create a object?

Free Magazines