===
[ The Java FAQ Daily Tips, weekly publication ]
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| Issue
No: 166
3 June
2003 |
| 13103 subscribers
|
Foreword: Excuse me for
possible mistakes. English is not native language for me.
In this issue:
Hello dear friends!
Question: When do I to choose an abstract class over
an interface?
Answer: Choosing interfaces and abstract classes is
not an either/or proposition...
"Often in a design, you want
the base class to present only an interface for its derived classes. That is,
you don't want anyone to actually create an object of the base class, only to
upcast to it so that its interface can be used. This is accomplished by making
that class abstract using the abstract keyword. If anyone tries to make an
object of an abstract class, the compiler prevents them. This is a tool to
enforce a particular design.
You can also use the abstract
keyword to describe a method that hasn't been implemented yet-as a stub
indicating "here is an interface function for all types inherited from this
class, but at this point I don't have any implementation for it." An abstract
method may be created only inside an abstract class. When the class is
inherited, that method must be implemented, or the inheriting class becomes
abstract as well. Creating an abstract method allows you to put a method in an
interface without being forced to provide a possibly meaningless body of code
for that method.
The interface keyword takes
the concept of an abstract class one step further by preventing any function
definitions at all. The interface is a very handy and commonly used tool, as it
provides the perfect separation of interface and implementation. In addition,
you can combine many interfaces together, if you wish, whereas inheriting from
multiple regular classes or abstract classes is not possible."
from "Thinking in Java", http://javafaq.nu/java/book/Chapter01.shtml#Heading29
--
John
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Question: What is difference between Hashmap and
Hashtable?
Answer: there is almost no difference, since
"hashmap is Hash table based implementation of the Map interface" (citation from
API). The difference is that Hashtable is synchronized.
--
John
Question: My colleague, he is testing my code, got
"Headless Exception". He jokes that is thrown by code which is written by
headless people  He has shown me log file even. Is he joking with
me?
Answer: Probably not I mean that you are Ok
although this exception exists in reality.
It is thrown when code that
is dependent on hardware like keyboard, display, or mouse is called in an
environment that does not support a keyboard, display, or mouse. For example,
all modern servers have no display, keyboard and mouse.
--
John
Latest posts on our
message board |
| Threads
I have developed this little
program that illustrates the problem that is similar to a different one I am
making. I wonder how you make the first method run through completely then go
onto paint(g) which runs through the second. As of now, all three: down,first,
and second appear simultaneously.
// The "Threads" class.
import
java.applet.*;
import java.awt.*;
public class Threads extends
Applet
continues
here |
| |
| Class question
Hi,
My question is simple:
I have a Frame (Frame1) with a textfield component on it and a jbutton
and
another Frame (Frame2). When i push a jbutton on Frame1, Frame2 appears.
All
well so long. The problem is here: I have another textfield (editable)
on
Frame2 and a jbutton. I want to get the text of textfield on Frame2 when
i
push the button and have the text BACK ON FRAME 1.
I have a .java
file for each Frame: Frame1.java and Frame2.java. I 'm
creating an object
like this on Frame2.java:
Frame1 obj=new Frame1();
and i get Frame1's
methods with obj.
All seem good, i'm getting no error message, but it
doesn't work.
As you may have understood i'm a little bit rookie so any
help would be appreciatable. I' m working on Borland JBuilder 8
Thanks
in advance.
continues
here | | |
| GUI - refresh ..it will not end
Hi there
Since a very
long time I'm stuck in the same problem again & again...
Situation:
I'm using swings JFrame where I add an instance of a Class that extends
JLabel. I've overridden the paintComponent(Graphics g)-method where I do the
whole draw-stuff and finally call super.paintComponent(g).
Problem: The
content of the frame does not refresh, until the window is resized etc.
Code:
private JFrame myFrame = new JFrame();
continues
here | | |
| MS-Java emulation on Java 2?
Hi,
There is an old
avatar chat applet (example: http://eagles.chatserve.com/ip.html) called InstantPalace. The
applet works great, but it's pretty old, and only runs on the MS-Java and older
"Java 1" JREs. So users who have upgraded to Java 2 JREs can't use it. The
Applet was created by a company that is no longer in business, so chances of it
ever being updated for Java 2 compatibility are, unfortunately, not very good.
I was wondering if there was a way to emulate MS-Java inside of a Java 2
applet, so older Java2-incompatible applets such as this one could be used on
systems with Java 2.
continues
here | | |
| centering applet on screen
Hi,
I have an applet that I
want to position in the top center of my screen,
When I run the applet in
the applet viewer it shows up in the top left corner of my screen, Dose anyone
have any ideas on how I can fix this, this is the html code I am using. I also
tried using a center method in this code, but then my applet didn`t appear on
the screen. Any help wopuld be greatly appreciated.
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