Class constructor that implements an interface returns an "interface", why

Hi,
I am studying some code that I need to understand well. This code works, I just don't understand the following:
A class was defined extending an interface as so:
public class GeometricShape implements Area {
// constructor
public GeometricShape() {
System.out.println('bla);
In another file, GeometricShape class was instantiated as follows:
public class ExampleUse {
Area g = new GeometricShape();
My qustion is, why does the code above expects "new GeometricShape()" constructor to return an interface of type Area?
Can someone explain?
thanks

Can someone explain?When a class implements an interface, or extends another class, or when a interface extends another interface, it means that anywhere an instance of the parent class or interface is expected, the child can be used.
Wherever a Mammal is expected, you can provide a Dog or Cat or Whale or Human or NakedMoleRat. Each of those is a mammal.
If you say "give me some food," and you don't specify anything else, the person you're talking to can hand you a hamburger or an apple or a bowl of rice. Any of those will meet the requirements you put forth.
This is how the OO "is-a" relationship maps to Java.

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Example class that implements Serializable interface

Dear,
I have a class myData that I want to implement Serializable interface. class myData has only two fields: Integer iData1, String sData2.
Could anybody shown me how my myData class should be?
Thanks a lot!

Hey, if you have yet to obtain a remote reference from the app server ...then we are into pandora's box. I lost three whole heads of hair getting up on JBoss when I first started. You want to check out the JBoss forums on the JBoss website, and the enterprise javabeans forum here. Search some posts and read the free JBoss manual.
Unfortunately, there isn't a 'here, do this' solution to getting connected with JBoss. There are quite a few gotcha's. There are descriptors, descriptor syntax ...and this changes between releases so there seems to be alot of people saying 'here, do this' ...but you try and it doesn't work (wrong release). Here are some descriptors that I threw up recently for someone ...a place to start.
http://forum.java.sun.com/thread.jsp?forum=13&thread=414432
This drove me nuts until it all worked right. I was stuck for three weeks at one point ...ready to give up, but then I got it. Perservere ...its a nice container for learning in (its free!).
I will try and watch for you.
Oh, and put something in your head ...at least then you will keep your hair !
:)

Declaring Argument as Class of Enum That Implements Interface

I'm trying to declare a method parameter that is the class of an enum which implements a specified interface. I can't figure out the required syntax, or if this is even possible.
Here is the interface:
public interface Describer { String describe(); }_________________________________________________
Here' s an enum that implements the interface:
public enum DeviceProperty implements Describer {
TYPE("Device Type"),
USE("Device Use");
private String description;
private DeviceProperty(final String description) { this.description = description; }
@Override public String describe() { return description; }
Here's my unsuccessful attempt to declare the method parameter:
public <E extends Enum<E extends Describer>> void addChanges(final Class<E> properties) {
     final EnumSet<E> set = EnumSet.allOf(properties);
     for (final Enum<E> property : set) {
          // Do something with the property
The problem, of course, is that addChanges() doesn't compile. If I remove "extends Describer" it compiles, but then I can't detect compile time errors (i.e., an enum which doesn't implement Describer).
Is what I'm attempting even possible in Java? If so, I'd be very grateful if someone could explain the correct syntax.

doremifasollatido wrote:
TPD Opitz-Consulting com wrote:
aksarben wrote:
[...]Here's my unsuccessful attempt to declare the method parameter:
public <E extends Enum<E extends Describer>> void addChanges(final Class<E> properties)
Nothing is being returned at all. The method return type is "void".OK.
Then I understand that <tt><E extends Enum<E extends> & Describer></tt> is a more detailed definition for the parameter type <tt>Class<E></tt>, right?
And your method iterates over all the constants defined in the passes enum type.
Meight be usefull sometiems.. thanks!
bye
TPD

HELP?! a class that implements ..... : java.util.Vector

My instructions say this:
In the constructor, create the collection object (Since java.util.List is an interface, you will need to instantiate a class that implements this interface: java.util.Vector or java.util.LinkedList or java.util.ArrayList).
I know, this IS a homework assignment - I am not sure how to go about this. I DO have code so far up to this point, can anyone help?
import dLibrary.*;
import java.awt.Color;
* @author Rob
* @version 8/18/03
public class PhrasesII extends A3ButtonHandler
        private A3ButtonWindow win;
        private ATextField stringAcceptor;
        private ALabel listOstrings;
        private ALabel inputString;
        private ALabel status;
        private java.awt.List display;
        private java.util.List collection;
public PhrasesII()
        win = new A3ButtonWindow(this);
        stringAcceptor= new ATextField (50,420,200,25);
        stringAcceptor.place(win);
        listOstrings = new ALabel(100, 10, 100, 380);
        listOstrings.setFontSize(10);
        listOstrings.setText("List of Strings:");
        listOstrings.place(win);
        inputString = new ALabel(50,400,200,25);
        inputString.setFontSize(10);
        inputString.setText("Input String:");
        inputString.place(win);
        display = new java.awt.List();
        display.setLocation(200, 100);
        display.setSize(200, 250);
        display.setBackground(Color.lightGray);
        win.add(display, 0);
        win.setLeftText("Save");
        win.setMidText("Display");
        win.setRightText("Discard");
        win.repaint();
public void leftAction()
    public void midAction()
    public void rightAction()

I am getting a " can't resolve symbol" when I do thisYou have to either import java.util.ArrayList or specify it fully, e.g., "new java.util.ArrayList()".
Is that the line that is causing you problems? The error message should give the line number.
my instructions also say I have to use "interface
java.util.List when declaring your reference" so I am
confused about using "= new ArrayList();"What they're saying is that you want code like this:
private java.util.List frogs;    // this is the reference declaration
//... later on...
frogs = new java.util.ArrayList(); // this isn't a declarationWhat this means is that when you declare a field or variable, you should declare its type to be an interface.
But when you actually instantiate a value for that variable, then you should use a concrete class that implements that interface. (You have to; interfaces can't be instantiated.)
This is good programming style for reasons I don't have the space to explain here.

How to find out in program which all classes have implemented an interface

Hello,
I have created an interface and few classes are implementing the interface.
I want to know in a program which all class have implemented the interface.
Is it possible to find it out and how?
Regards,
Bikash.

Hi Bikash,
Read the Database view VSEOIMPLEM with where condition REFCLSNAME = Interface Name and version = 1.
This would give you all the classes which have implemented the interface and are active...
If you want to look at the values that the field version can have then see Type Group SEOC ans search for version....
Hope this help...
Regards,
Sitakant

Implementing Runnable interface Vs extending Thread class

hi,
i've come to know by some people says that Implementing Runnbale interface while creating a thread is better option rather than extending a Thread class. HOw and Why? Can anybody explain.?

Its the same amount of programming work...
Sometimes it is not possible to extend Thread, becuase your threaded class might have to extend something else.
The only difference between implementing Runnable and extending Thread is that by extending Thread, each of your threads has a unique object associated with it, whereas with Runnable, many threads share the same object instance.
http://developerlife.com/lessons/threadsintro/default.htm#Implementing

How can I know a class which implements Runnable interface has terminated?

Hello! I have a class which has implements Runnable interface, while I want to execute some operation when the thread has terminate in multithread enviroment.How can I know the thread has terminated?Does it give out some signal?Cant I just call my operation at the end of the run() method?

I want to execute some operation when
the thread has terminate in multithread enviroment....
Cant I just call my operation at the end
of the run() method?Sure. Before run() ends, invoke that other operation.
How
can I know the thread has terminated?Does it give out
some signal?Not that I'm aware of, but you can do what you described above, or I believe a different object can call isAlive on the thread.

How to get classes which implement the interface in program

Hi,
I created an interface and some classes are implementing it. I want to know in which classes the interface is implemented through program. I mean in which table the interface implemented details stores.
please helps regarding this.
Thanks,
Regards,
Priya

Hi.,
Read the database view VSEOIMPLEM with where condition., REFCLSNAME = <Interface Name> and Version = 1.
This gives the class names which implement the interface.,
hope this helps u.,
Thanks & Regards,
Kiran

Achiving serialization without implementing the interface serializable.

how to make a class serializable without implementing the interface the serializable.

... although the objects you're converting with XMLEncoder must be JavaBeans, not just any POJO.
For the latter, you may want to look at XStream, or JSX.

MARKER INTERFACE WHY?

hello i madhav new to this forum , i have doubt on marker interface . as it doesnot have any body why we r using that one instead of skipping.Is there any concept behind is there.More about my doubt is Take Example Cloneable Interface it Doesn't Have any Method In it,Clone() method is from Object class,then what is the necessity of implementing the interface .Why it should not be like that.
what is real technolgy behind MarkerInteface and Why we r going to be used in what situations it will needed.And Lastly is there any method related to Serailizable interface if not what it do .
marker interface can also called as Tag interface what is tag mean here just saying combing derived classes . i didn't get that one and how it will be .
if u could explain with example or real time project it would be better for me
thanks in advance

What is a "marker" interface?
Marker Interface pattern
c2.com - Marker Interface
experts.about.com - Marker Interfaces
The Purpose of the Marker Interface
Maximize your Design ROI with Marker Interfaces and JavaDoc

Implementing DOM Interface with existing Java classes

I had planned on using some tree-like Java classes as a Document Object Model, which would give me access to all sorts of XML and DOM tools like parsers and XSLT transformers. Initially, I thought all that would be neccessary is to implement all the DOM Interfaces in org.w3c.dom and then I would have a set of classes that conformed to DOM Level 1. It was my understanding that interfaces such as DOMImplementation and Document would interface with various XML tools, allowing creation of a class that implements Document and then Document would have its various factory methods that know how to create the various DOM nodes such as Element, Attr, Text, NamedNodeMap, NodeList, etc.
The problem I'm seeing now is that the JAXP specification (which is what the latest Xerces and Xalan tools conform to) has something called a DocumentBuilder and DocumentBuilderFactory that appear to be necessary to tell the framework what type of class to instantiate that implements the Document DOM interface. Those appear to have a lot of methods that deal with parsing of XML documents and I didn't really want to write or even subclass any existing Parsers in order to get the functionality of traversing and transforming a set of classes that implement the DOM interface.
Am I missing something here? Is it possible to plug in any (set of classes for) DOMImplementation and get them to work with the various DOM and XML tools out there?
Is there an easier way to allow parts of an application access to internal data structures but have the more generic tools or APIs, such as XSL transformers, access that same set of classes as a DOM with the generic DOM interface methods?
Can someone provide me with some guidance here? I'm in the process of finalizing some design on a system and need to know if this is possible or if I need to alter my design.
Thanks.

If I understand you correctly, I think I am working on a similar issue. I am unhappy with the methods given by the DOM for retrieving data from the XML file and for building a file. Our software has a bunch of code that uses these classes and it is extremely ugly. My solution was to create a facade on top of the DOM model. Essentially I have some simple classes that store all the pertinent info (for me) about the XML structure. Essentially that is the element or attribute name, its values and in the case of the element, it's children. This makes it easier for me to build and retreive the data. What I then built was a loader class and a builder class. The loader takes an XML file and parses it in using the DOM classes and builds a structure using my classes and returns the root element. The builder takes a root element and creates a DOM object out of it. This frees me of having to code around the DOM classes all over the place and makes it simple to upgrade our XML code if the DOM changes or a better DOM is released. I am using factories to facilitate this and allow me to have loaders for specific types of XML documents so that I can have a class for each which further simplifies the XML related tasks of other developers on my team.

Serializing a class that implements the Singleton pattern

Hello,
I am relatively new to Java and especially to serialization so the answer to this question might be obvious, but I could not make it work event though I have read the documentation and the article "Using XML Encoder" that was linked from the documentation.
I have a class that implements the singleton pattern. It's definition is as follows:
public class JCOption implements Serializable {
private int x = 1;
private static JCOption option = new JCOption();
private JCOption() {}
public static JCOption getOption() { return option; }
public int getX() { return x; }
public void setX(int x) { this.x = x; }
public static void main(String args[]) throws IOException {
    JCOption opt = JCOption.getOption();
    opt.setX(10);
    XMLEncoder encoder = new XMLEncoder(new BufferedOutputStream(new FileOutputStream("Test.xml")));
    encoder.setPersistenceDelegate(opt.getClass(), new JCOptionPersistenceDelegate());
    encoder.writeObject(opt);
    encoder.close();
}Since this class does not fully comply to the JavaBeans conventions by not having a public no-argument constructor, I have create a class JCOptionPersistenceDelegate that extends the PersistenceDelegate. The implementation of the instantiate method is as follows:
protected Expression instantiate(Object oldInstance, Encoder out) {
       Expression expression = new Expression(oldInstance, oldInstance.getClass(), "getOption", new Object[]{});
        return expression;
}The problem is that the resulting XML file only contains the following lines:
    <java version="1.5.0_06" class="java.beans.XMLDecoder">
        <object class="JCOption" property="option"/>
    </java> so there is no trace of the property x.
Thank you in advance for your answers.

How about this:
import java.beans.DefaultPersistenceDelegate;
import java.beans.Encoder;
import java.beans.Expression;
import java.beans.Statement;
import java.beans.XMLEncoder;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
public class JCOption {
    private int x = 1;
    private static JCOption option = new JCOption();
    private JCOption() {}
    public static JCOption getOption() { return option; }
    public int getX() { return x; }
    public void setX(int x) { this.x = x; }
    public static void main(String args[]) throws IOException {
      JCOption opt = JCOption.getOption();
      opt.setX(10);
      ByteArrayOutputStream os = new ByteArrayOutputStream();
      XMLEncoder encoder = new XMLEncoder( os );
      encoder.setPersistenceDelegate( opt.getClass(), new JCOptionPersistenceDelegate() );
      encoder.writeObject(opt);
      encoder.close();
      System.out.println( os.toString() );
class JCOptionPersistenceDelegate extends DefaultPersistenceDelegate {
    protected Expression instantiate(Object oldInstance, Encoder out) {
        return new Expression(
                oldInstance,
                oldInstance.getClass(),
                "getOption",
                new Object[]{} );
    protected void initialize( Class<?> type, Object oldInstance, Object newInstance, Encoder out ) {
        super.initialize( type, oldInstance, newInstance, out );
        JCOption q = (JCOption)oldInstance;
        out.writeStatement( new Statement( oldInstance, "setX", new Object[] { q.getX() } ) );
}   Output:
<?xml version="1.0" encoding="UTF-8"?>
<java version="1.5.0_06" class="java.beans.XMLDecoder">
<object class="JCOption" property="option">
<void property="x">
   <int>10</int>
</void>
</object>
</java>

Question about methods in a class that implements Runnable

I have a class that contains methods that are called by other classes. I wanted it to run in its own thread (to free up the SWT GUI thread because it appeared to be blocking the GUI thread). So, I had it implement Runnable, made a run method that just waits for the thread to be stopped:
while (StopTheThread == false)
try
Thread.sleep(10);
catch (InterruptedException e)
//System.out.println("here");
(the thread is started in the class constructor)
I assumed that the other methods in this class would be running in the thread and would thus not block when called, but it appears the SWT GUI thread is still blocked. Is my assumption wrong?

powerdroid wrote:
Oh, excellent. Thank you for this explanation. So, if the run method calls any other method in the class, those are run in the new thread, but any time a method is called from another class, it runs on the calling class' thread. Correct?Yes.
This will work fine, in that I can have the run method do all the necessary calling of the other methods, but how can I get return values back to the original (to know the results of the process run in the new thread)?Easy: use higher-level classes than thread. Specifically those found in java.util.concurrent:
public class MyCallable implements Callable<Foo> {
public Foo call() {
return SomeClass.doExpensiveCalculation();
ExecutorService executor = Executors.newFixedThreadPool();
Future<Foo> future = executor.submit(new MyCallable());
// do some other stuff
Foo result = future.get(); // get will wait until MyCallable is finished or return the value immediately when it is already done.

Stipulate that a parameterised type must implement an interface

Hi,
1000 excuses if this has been asked before:
I want to say "abstract class AbstractHolder<Y>" and also say "oh by the way, Y must implement interface Snodgrass"
In other words I want to limit the kind of subelements which are held by the AbstractHolder so that an attempt to establish an AbstractHolder subclass which holds subelements which do not implement Snodgrass results in a compiler error.
Of course you can put in an "instanceof" check in the constructor... but I think generics should enable you to say "abstract class AbstractHolder<Y implements Snodgrass>"... they don't appear to...
Anything to be done?
Thanks - MRodent

<Y extends Whatever>This works if Y is an interface extending interface Whatever.
It works if Y is a class extending class Whatever.
It works if Y is a class implementng interface Whatever.
I believe you can also say <Y extends A, B, C> or <Y extends A & B & C>
http://java.sun.com/j2se/1.5.0/docs/guide/language/generics.html
http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf

Creating a generic class with a constructor that takes an array of objects

I am relatively new to java and want to build a quick utility class that can generate a Run Length Encoding of any object. The idea is to take an array of objects of type "O" and then encode a string of objects that are "equal" to each other as an integer that counts the number of such equal instances and a single copy of the instance. This has the potential to very quickly reduce the size of some objects that I want to store. I would like to implement this class as a generic.
public class RunLengthEncoding<O> {
     private class RLEPair {
          private int length;
          private O object;
     public RunLengthEncoding(O[]) {
}As you can see, I need to make a constructor that takes an array of type "O". Is this possible to do? I can't seem to find the right syntax for it.
Thanks,
Sean

Sorry. Obvious answer:
public RunLengthEncoding(O[] oarray) {Again, sorry for the noise.

Class constructor that implements an interface returns an "interface", why

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