instanceof problem

[Thomas Peter]

Hi all,

I'd like to test if a given Component is an instance of a given class.
Instead of having several methods like this ...

	public void checkJButton(Component co){
		if (co instanceof JButton)
		{
			System.out.println("Yes its a JButton");
		}
	}

I thought I could use an universal one ...

	public void checkComponent(Component co,Class cl){
		if (co instanceof cl)
		{
			// do something
		}
	}

But that doesn't compile. Any ideas how I could accomplish that task ?

cheers freefalling

[Maneesh Godbole]

Welcome to the Ranch.

In future, while posting code, please UseCodeTags

You should be invoking the instanceof operator like

if(component instanceof Component)

and not

if(component instanceof cl)

[Thomas Peter]

... thanks but I guess that's not what I'm looking for ;-)

I wanted to call a method like that

   
...
// co is any component and the second parameter is the Class I'd like to check for
checkComponent(co,JButton)
 

I want a method that doesnt check just one hardcoded class like

    
if(co instanceof JButton)

but a variable class

    
  void checkComponent(Component co,Class class)
  if(co instanceof class) ...
  

Thanks freefalling

[Abimaran Kugathasan]

I think, we can do, if we are able to modify the Class which you passed as argument to the method.

[Maneesh Godbole]

Thomas Peter wrote:... thanks but I guess that's not what I'm looking for ;-)
...I want a method that doesnt check just one hardcoded class like...
...but a variable class..

In that case check out Class#isInstance(Object obj)

[Abimaran Kugathasan]

I think,

public boolean method(Object ob, Class cl)  {
  return cl.isInstance(ob);
}

will work!

[Abimaran Kugathasan]

Oh, already answered!~

[Darryl Burke]

You need something like (typed here, may have typos or other bugs -- read the API for Class#isAssignableFrom)

void checkComponent(Component co,Class clazz) {
  if (co.getClass().isAssignableFrom(clazz)) {
    // do something
  }
}

Note that class is a keyword. You can't use it as a variable or parameter identifier.

edit Oops, was I really on the phonefor so long? isInstance is a better choice. Also, I got isAssignableFrom backwards should have been clazz.isAssignableFrom(co.getClass())

[Thomas Peter]

Thank you all !

The 'isInstance' helps a lot :-)

Cheers freefalling

[David Newton]

So, I'll ask what nobody else has: why do you want to do this? One of the advantages of OOP is that we really shouldn't need to do this kind of operation.

[Thomas Peter]

;-)

I almost expected that question. My problem is having programmed a long time in VB and now I want to learn how the things can be done in Java.

Explicitly I tried to change one JButtons label from another method. Or try to access a component directly by its name ...

Thanks freefalling