interfaces instantiated???

[Anshuman Acharya]

interfaces can't be instantiated - the first rule you're taought about them.
but why does this break down in these examples -
1.) Runnable r = new Runnable();
// some stuff;
Thread t = new Thread(r);
t.start();
2.) addActionListener( new ActionListener(){
public void actionPerformed(ActionEvent ae){
//
}
}
);
some clarification wanted.
also, what all interfaces can be instantiated then?
------------------

[Jane Griscti]

Hi Anshuman,
1. Runnable r = new Runnable() -- this will not compile; you'll see the following error:

java.lang.Runnable is abstract; cannot be instantiated
        Runnable r = new Runnable();
                     ^

2. In this case you are creating an Anonymous class based on the Interface ActionListener and you are implementing the one method the interface defines: actionPerformed().
What you first learned, interfaces cannot be instantiated is true for all interfaces.
Hope that helps.
------------------

Jane Griscti
Sun Certified Java 2 Programmer
"When ideas fail, words come in very handy" -- Goethe

[Anshuman Acharya]

thanks jen... but still a problem.
as for 2, no problems... i found a link to it last night. as you say, the compiler fills in the blank for the missing "implements".
but as for the first, i have seen this code appear and pass as a correct answer in 2 mock exams... i will just code and check! but if what you say is right, then i guess you should rely more on your fundamentals than these flashes in the pans of erratta!

[Randall Twede]

another thing you will find, which confused me at first, is sometimes a method will return an object of an interface type. for example Applet has a method
getAudioClip(URL url)
Returns the AudioClip object specified by the URL argument.
however AudioClip is an interface. What actually happens is the method creates a class which implements AudioClip, creates an object of this type, then upcasts it to the interface type(AudioClip) and returns it.

[Anshuman Acharya]

thanks jen... but still a problem.
as for 2, no problems... i found a link to it last night. as you say, the compiler fills in the blank for the missing "implements".
but as for the first, i have seen this code appear and pass as a correct answer in 2 mock exams... i will just code and check! but if what you say is right, then i guess you should rely more on your fundamentals than these flashes in the pans of erratta!

[Anshuman Acharya]

that was some interesting observation, randall! i did know that the return type of the getAudioClip was AudioClip object but it had slipped my mind that AudioClip was an interface.
All i can say is that Java's compiler seems to be pulling a lot of strings backstage than i have seen some other compilers do!
thanx

[Jane Griscti]

Hi Anshuman,
You can create a reference variable for an Interface type and assign any object that implements the Interface or one of it's superinterfaces.
For example,

class ARun implements Runnable {
    public void run() {
        // some code
    }
}
class TestRunnableAssignment {
    public static void main( String[] args )
    {
        Runnable r1 = new ARun();
    }    
}

The above compiles without error. Maybe this is what you have been running into on some of the mocks?


------------------

Jane Griscti
Sun Certified Java 2 Programmer
"When ideas fail, words come in very handy" -- Goethe

[Anshuman Acharya]

thanks jane,
but no.
the examples i saw instantiated the Runnable interface like this :
Runnable r = new Runnable();
Thread t = new Thread(r);
i just guess that whoever made this question had his/her fundamentals all warped up!