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Creating a class that extends more than one class.

Bill.Moo4-Oct-12 2:43

Bill.Moo

4-Oct-12 2:43

Hi, I'm from a C++ background and I need to create a class that extends both AbstractAction and Observable, but this of course can't be done 'out of the box' using Java.

So I would appreciate from those more experienced than me as to how I can achieve this, assuming it even can be.

--
Bill

Re: Creating a class that extends more than one class.

Nagy Vilmos4-Oct-12 3:00

Nagy Vilmos

4-Oct-12 3:00

You need to impliment interfaces rather then extend classes. A single class can only extend one super class, but you can implement as many interfaces as you like. The difficulty here is that you need to code every method:

Java

class Top {
   void doSomething() {
     // something is done
   }
}

interface First {
   void goFirst();
}

interface Last{
   void goLast();
}

class Bottom
      extend Top
      implement First,
            Second {

   void doSomething() {
     this.goFirst();
     super.doSomething();
     this.goLast();
   }

   void goFirst() {
      // the first bit;
   }
   void goLast() {
      // the last bit;
   }
   
}

Simples!

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Re: Creating a class that extends more than one class.

Bill.Moo4-Oct-12 3:48

Bill.Moo

4-Oct-12 3:48

Thanks for that, I'll do some reading on the matter.

--
Bill

Re: Creating a class that extends more than one class.

Peter_in_27804-Oct-12 3:03

Peter_in_2780

4-Oct-12 3:03

The "Java way" is to use interfaces to achieve (most of) multiple inheritance.
AFAIK, the "Observable/Observer" concept has been pretty much obsoleted by events and actions, so maybe you don't need to do what you can't.

Cheers,
Peter

Software rusts. Simon Stephenson, ca 1994. So does this signature. me, 2012

Re: Creating a class that extends more than one class.

Bill.Moo4-Oct-12 3:59

Bill.Moo

4-Oct-12 3:59

Where do you read this? I got the Observable/Observer from the "Java - The complete reference 8th Ed. Ch 18, p541" and there is no reference to it being obsolete or as a candidate for it either.

What I want to do is create a class that extends AbstractAction and implements runnable. The actionPerformed will display a file open dialog allowing the user to select N files, these files are then stored in a File[] and the thread started when the dialog is closed to process the files. But I wanted the observable so I could 'emit' messages to the Observer (my UI) so I could say "Processing filename.ext..." as the processing can take a bit of time, hence the use of the thread and 'status message'. But of course if there is a better way of doing this then I'm all for learning.

The Events / Actions sounds sensible, are you able to give me a
link to something?

--
Bill

Re: Creating a class that extends more than one class.

Peter_in_27804-Oct-12 11:36

Peter_in_2780

4-Oct-12 11:36

That's what I get for trying to make sense at midnight. I use "Java in a Nutshell, 5th ed". Observable/Observer are since Java 1.0 and the comment is "The Observable class and Observer interface are not commonly used. Most applications prefer the event-based notification model defined by the JavaBeans componenent framework and by the EventObject class and EventListener interface of this package. [java.util]" So I misread preference as superceding... I do remember the Event model being introduced (in Java 1.1) in response to a perceived hole in functionality.
Looks like you want to make your UI implement EventListener and have the worker thread spit out (your subclass of) EventObjects as required.
I'm sure there are plenty of examples floating around the web. (Java's like that Smile | :)

) I'd start with Mr G and something like "Java EventObject".

Good hunting,
Peter

Software rusts. Simon Stephenson, ca 1994. So does this signature. me, 2012

Re: Creating a class that extends more than one class.

pasztorpisti4-Oct-12 12:14

pasztorpisti

4-Oct-12 12:14

Before starting any serious java tutorials its worth reading through the official beginner java tutorial: http://docs.oracle.com/javase/tutorial/index.html[^]
Read at least the "Trails Covering the Basics" paragraph and all of its subpages. Its a novice coder tutorial so you will progress with that quickly and you can gain a lot of important java-specific info from it that makes your work much easier. It worth at least a quicky reading even if you are lazy to try the examples!

Re: Creating a class that extends more than one class.

Monster Maker21-Oct-12 21:08

Monster Maker

21-Oct-12 21:08

In java, multiple inheritance concept can only be accomplished through interfaces. I do it using the concept of adapter classes.

Java

interface Dog
{
	public void bark();
}

interface Cat
{
	public void meow();
}

class DogCat implements Dog,Cat
{
	public void bark()
	{
		System.out.println("Barking");
	}

	public void meow()
	{
		System.out.println("Meowww");
	}
}

public class Hybrid extends DogCat
{
	public static void main(String[] args)
	{
		Hybrid r =new Hybrid();
		r.bark();
		r.meow();
	}

}

Re: Creating a class that extends more than one class.

Gowtham Gutha15-Nov-12 6:59

Gowtham Gutha

15-Nov-12 6:59

Multiple inheritance for classes is however not supported by Java unfortunately. It is limited for interfaces only. Alternatively, you can use multi level inheritance that gets the same as the features of the multiple ones. Here is a simple trick that you can do to achieve this..

Let the super classes be S1,S2

class A extends S1
{

}
class B extends S2
{
}
class C
{
// write the code here by creating objects for A,B
// This could be good if S1,S2 are abstract and that
// you have implemented the abstract methods in the
// classes A,B. So you can create objects for A,B
// access methods in them (the implemented ones) and // also the normal ones.
}

Gowtham Gutha (http://java-demos.blogspot.com)

Last Visit: 31-Dec-99 18:00 Last Update: 9-May-24 2:39

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