[ZoneFire]

a questio here too...
what does it mean by "interfaces cannot contain concrete methods"?
but an abstract class can contain concrete methods?
what does it mean by concrete methods?
thank you

[Iron_Cross]

A concrete method is one where it actually has functionallity. For example this is concrete method:

public void printSomething()
{
    System.out.println("Printing a string of text characters!");
}

An abstract or non-concrete is one where the return type, name, and arugment list are known but what the method actually does is NOT known.

void printSomething();

In this case the return type is void, the name is printSomething and the aguement list contains nothing.

Now in an interface you're basically just describing a method. When you implement an interface you are creating that described method. That way you are guarunteed to be able to use it, no matter how it works.

An abstract class is something completely different. It's bascially just a place holder in a hierarchy tree of classes. An abstract class can not be instantiated, but can be inherited from.

For example, if I wanted to create a two classes NumberTextBox and LetterTextBox which only except numbers or letters, respectivly, I would first create a class called TextBox which has all the basic functionality that a text box needs, such as getting keyboard input, printing to the screen, stuff like that. But I don't want anyone to be able to use the class TextBox, just NumberTextBox and LetterTextBox.

Instead of writing the exact same basic functionality into both NumberTextBox adn LetterTextBox I would just inherit both of them from TextBox. But to ensure that people can't use TextBox I would mark that class as abstract.

The reason an abstract class has concrete methods is because you can't inherit from a method that doesn't exist yet (i.e. a non-concrete method) because it doesn't do anything.

I hope that helps clarify it a bit. If you have any more questions just ask

[ZoneFire]

So, when using the Abstract Class, i can inherit from the class and define the concrte methods in the class. However, i cant instantiate a new class from Abstract class.

While using interface, i just defined the method but without any implementation.When the interface is implemented in a client class, we need to specify the implementation and algorithm to do the work.

am i right?
thank you, Iron_Cross

[Alex81388]

Think of the interface as a guidelines or rules book. The interface simply says what methods are required to use it, whith their return typs and perameters.

You can only actually PUT TO USE one of the methods in another class, but you will have to have the others present (even if they do absolutely nothing) so the interface sees all of its required methods are there.

[C.Ingram]

Originally posted by ZoneFire
So, when using the Abstract Class, i can inherit from the class and define the concrte methods in the class. However, i cant instantiate a new class from Abstract class.

While using interface, i just defined the method but without any implementation.When the interface is implemented in a client class, we need to specify the implementation and algorithm to do the work.

am i right?
thank you, Iron_Cross

Pretty much, yes. The former (can) pretty much define an incomplete class. You inherit from the class, and fill in the blanks. The latter is similar, but to a lesser extent.

Use abstract classes when you want to provide some code. Use interfaces when you don't want to provide some code, and want the programmer to do *everything* him/herself.

http://java.sun.com/docs/books/tutor.../abstract.html
http://www.javaworld.com/javaworld/j...-abstract.html

[Iron_Cross]

Originally posted by ZoneFire
So, when using the Abstract Class, i can inherit from the class and define the concrte methods in the class. However, i cant instantiate a new class from Abstract class.

While using interface, i just defined the method but without any implementation.When the interface is implemented in a client class, we need to specify the implementation and algorithm to do the work.

am i right?
thank you, Iron_Cross

You're right.

[ZoneFire]

thanks a lot, Iron_Cross
but then i have some problem writing a class.
when i compiled the source code, the following error messages is shown.

D:\My Java\Rational.java:5: Rational is not abstract and does not override abstract method longValue() in java.lang.Number

public class Rational extends Number {
^
1 error

can someone tell me what is the problem?
besides, do we have any method to check whether the input is an integer?

thank you