Overriding protected static instance variable???

Hi there,
I am trying to use the MVC-pattern (model-view-controler) pattern. I want a certain class to have a protected static variable:
class A{
protected static SomeADependingListener myListener = new SomeADependingListener();
}but as class B extends A, but should react differently to mouseevents, keyboardevents..... I want that B has another listener, but it should have the same variable-name:
class B extends A{
protected static SomeBDependingListener myListener = new SomeBDependingListener();
what happens if I do so? If I create an instance of B are there now two objects created (listeners), or only one?
Could someone explain this to me please?
Thx
Sincerely
Karlheinz Toni

You cannot override a field. You can only shadow it. That is, there will be no relationship between the two declarations. It is generally not recommended to use shadowing because it leads to confusing programs.
In your example methods inside class A will see only the field myListener defined in class A. Methods inside class B will (without qualification) see the field myListener defined in class B. But methods in class B can access the other myListener field in A by writing A.myListener.
You should re-think your design.

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Dear all,
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http://www.googlestepper.blogspot.com
http://www.scrollnroll.blogspot.com

JavaDriver wrote:
Anything TBD w.r.to pass by value/reference (without removing 'static' keyword)?Besides the use of acronyms in ways that don't make much sense there are two other large problems with this "sentence".
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Leslie VWhat you're asking is where to find the tutorials because you're lost. Okay. Here you go [http://java.sun.com/docs/books/tutorial/java/index.html]
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1) Do I really need to have a singleton to house the
cached data, or is it kosher to have the static
variable newsByWebsiteArea on the class
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2) Will my code require synchronized methods for
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findActiveByLocation()? I don't want to have
issues inside these methods while the
refreshCache() method is running.Yes.
3) If I have other non-synchronized static
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synchronized methods are finished executing?No. Only synchronized methods or blocks respect the locks held by other synchronized methods or blocks.
If so, would it be a better idea to go back to synchronizing
blocks of code instead of methods to ensure maximum
efficiency?Your earlier examples synchronized on the entire method's code. I don't understand why you would think that would work differently than just synchronizing the method itself. The lock is held for exactly the same code.
Now if the critical block was only part of the method, you might want to synchronize on that. But in your case it isn't.
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Run the following:
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Doesn't this cause you to have to change all the subclasses
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hi,
i'm new to XML.
here i'm trying to parse an XML and store their element data to the instance variable.
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thanks in advance.
praks
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I believe that the problem has to do with the value you assign to ctelement. You are assigning the value of localName to ctElement, however for the element: <name>...</name> the localname is empty string i.e. "", but qName equals "name". Because you are assigning empty string to ctElement, when you do the comparison in characters of ctElement to "name" it will always be false. So in startElement change it to ctElement = qName; Try it and see if it works. I have produced similar programs and it works for me.
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        Propagate the isSupported request to the chain of SupportObjects (based on 'include' values)
        until we get a 'true' result or we run out of SupportObjects. */
    public static boolean isSupported(String attribute, String classId, String cmdLevel, String platform) {
        if (attribute == null)
            return false;
        SupportObject obj = (SupportObject)objects.get(classId);
        // If the object type isn't even in the support matrix, we interpret that to
        // mean that it's a Broker or Agent, in which case all attributes are supported
        // at present.
        if (obj == null)
            return true;
        boolean result = false;
        while (result == false && obj != null) {
            result = obj.isSupported(attribute, cmdLevel, platform);
            if (result == false && obj.getInclude() != null) {
                obj = (SupportObject)objects.get(obj.getInclude());
            } else
                obj = null;
        return result;
    /** Locate an AttributeObject for a given attribute name, object type, and command level.
        This is a helper function for the getMaxLen and getType methods. */
    private static AttributeObject getAttributeObject(String attr, String classID, String cmdLevel) {
        AttributeObject result = null;
        SupportObject obj = (SupportObject)objects.get(classID);
        while (result == null && obj != null) {
            result = obj.getAttr(attr, cmdLevel);
            if (result == null && obj.getInclude() != null) {
                obj = (SupportObject)objects.get(obj.getInclude());
            } else
                obj = null;
        return result;
    /** Determine the maximum length for a given combintation of attribute name, object type,
        command level, and platform. If the AttributeObject can't be found, or if the length
        hasn't been set, return -1. */
    public static int getMaxLen(String attr, String classId, String cmdLevel, String platform) {
        AttributeObject a = getAttributeObject(attr, classId, cmdLevel);
        if (a == null)
            return -1;
        return a.getMaxLen(platform);
    /** Determine the PCF parm type for a given combintation of attribute name, object type,
        command level, and platform. If the AttributeObject can't be found, or if the length
        hasn't been set, return -1. */
    public static int getType(String attr, String classId, String cmdLevel, String platform) {
        AttributeObject a = getAttributeObject(attr, classId, cmdLevel);
        if (a == null)
            return -1;
        return a.getType(platform);
    /** Inner class to contain platform-specific info for an attribute. */
    class PlatformObject {
        /** This instance variable will contain the comma-delimited string of all
            platforms to which this object applies. */
        private String platform;
        private int maxlen;
        private String condition = null;
        private boolean include;
        private boolean readonly;
        private boolean lenSet;
        private int attrType;
        private boolean typeSet;
        public PlatformObject(String p, boolean readonly) {
            platform = p;
            this.readonly = readonly;
            lenSet = false;
            typeSet = false;
        public PlatformObject(String p) {
            this(p, false);
        public void setCondition(String condition, boolean include) {
            this.condition = condition;
            this.include = include;
        public boolean isReadonly() {
            return readonly;
        public boolean isExported(TopologyModelNode node) {
            if (condition != null) {
                boolean test = false;
                try {
                    test = Utilities.evaluateCondition(node, condition);
                } catch (Exception e) {}
                if (include ^ test)
                    return false;
            return !readonly;
        public void setLen(int len) {
            maxlen = len;
            lenSet = true;
        public void setType(int type) {
            attrType = type;
            typeSet = true;
        public String getPlatform() {
            return platform;
        public int getMaxLen() {
            if (!lenSet)
                return -1;
            return maxlen;
        public int getType() {
            if (!typeSet)
                return -1;
            return attrType;
    /** This class represents a specific MQSeries attribute. It can optionally contain
        instances of the PlatformObject class as needed. When the 'platforms' collection
        is empty, the attribute is supported on all platforms. It is also possible to
        include a PlatformObject for the 'all' platform, if specific attribute characteristics
        need representation. When the 'platforms' collection is non-empty, it will include one
        entry for each platform where the attribute is supported. */
    class AttributeObject {
        private String attribute;
        private String attrValue;
        private HashMap platforms;
        private String condition = null;
        private boolean include;
        private boolean readonly = false;
        public AttributeObject(String a) {
            this(a, false);
        public AttributeObject(String a, boolean readonly) {
            attribute = a;
            platforms = new HashMap();
            this.readonly = readonly;
        public void setCondition(String condition, boolean include) {
            this.condition = condition;
            this.include = include;
        public void setReadonly(boolean readonly) {
            this.readonly = readonly;
        public boolean isReadonly() {
            return readonly;
        public void addPlatform(PlatformObject p) {
            for (StringTokenizer st = new StringTokenizer(p.getPlatform(), ","); st.hasMoreTokens() ;) {
                platforms.put(st.nextToken(), p);
        public PlatformObject getPlatform(String platform) {
            PlatformObject p = (PlatformObject)platforms.get(platform);
            if (p == null)
                p = (PlatformObject)platforms.get("all");
            return p;
        public String getAttribute() {
            return attribute;
        public int getMaxLen(String platform) {
            PlatformObject p = getPlatform(platform);
            if (p == null)
                return -1;
            return p.getMaxLen();
        public int getType(String platform) {
            PlatformObject p = getPlatform(platform);
            if (p == null)
                return -1;
            return p.getType();
        public boolean isSupported(String platform) {
            if (platforms.isEmpty()) {
                return true;
            if (platforms.containsKey(platform)) {
                return true;
            if (platforms.containsKey("all")) {
                return true;
            return false;
        public boolean isExported(String platform, TopologyModelNode node) {
            if (readonly) {
                Log.log(Log.DEBUG, this, attribute + " is readonly, returning false");
                return false;
            if (condition != null) {
                Log.log(Log.DEBUG, this, "Testing condition = " + condition);
                boolean test = false;
                try {
                    test = Utilities.evaluateCondition(node, condition);
                    Log.log(Log.DEBUG, this, "Condition result is " + test);
                } catch (Exception e) {
                    Log.log(Log.ERROR, this, "Condition through an exception");
                if (include ^ test)
                    return false;
            if (platforms.isEmpty()) {
                return true;
            PlatformObject p = getPlatform(platform);
            if (p == null) {
                Log.log(Log.DEBUG, this, platform + " not found for " + attribute + ", returning false");
                return false;
            return p.isExported(node);
    /** This class represents a specific value of a queue manager's command level. A
        given instance of this class may 'include' a 'parent' instance through its
        include instance variable. The traversal of the parent/child hierarchy is
        delegated to the SupportObject class, since that is where the collection of
        VersionObjects lives. */
    class VersionObject {
        private String cmdLevel;
        private String include;
        private HashMap attributes;
        public VersionObject(String cmdLevel, String include) {
            this.cmdLevel = cmdLevel;
            this.include = include;
            attributes = new HashMap();
        public void addAttr(AttributeObject attr) {
            attributes.put(attr.getAttribute(), attr);
        public AttributeObject getAttr(String attr) {
            return (AttributeObject)attributes.get(attr);
        public String getCmdLevel() {
            return cmdLevel;
        public String getInclude() {
            return include;
        public boolean isSupported(String attr, String platform) {
            AttributeObject obj = (AttributeObject)attributes.get(attr);
            if (obj == null) {
                return false;
            } else
                return obj.isSupported(platform);
    /** This class represents an MQSeries object type, as identified by its classid, e.g.
        queue manager or local queue. This class implements an include facility similar to
        that described for the VersionObject. The traversal of that hierarchy is delegated
        to the static isSupported method, because the collection of SupportObject instances
        is a static variable of the SupportMatrix class. */
    class SupportObject {
        private String classId;
        private String include;
        private HashMap versions;
        public SupportObject(String classId, String include) {
            this.classId = classId;
            this.include = include;
            versions = new HashMap();
        public void addVersion(VersionObject obj) {
            String key = obj.getCmdLevel();
            versions.put(key, obj);
        public String getClassId() {
            return classId;
        public String getInclude() {
            return include;
        public boolean isSupported(String attr, String cmdLevel, String platform) {
            boolean result = false;
            VersionObject obj = (VersionObject)versions.get(cmdLevel);
            if (obj == null)
                obj = (VersionObject)versions.get("base");
            // I don't actually know what it means if obj is null at this point.
            // It probably can't happen.
            if (obj == null)
                return false;
            while (result == false && obj != null) {
                result = obj.isSupported(attr, platform);
                if (result == false && obj.getInclude() != null) {
                    obj = (VersionObject)versions.get(obj.getInclude());
                } else
                    obj = null;
            return result;
        public AttributeObject getAttr(String attr, String cmdLevel) {
            AttributeObject result = null;
            VersionObject obj = (VersionObject)versions.get(cmdLevel);
            if (obj == null)
                obj = (VersionObject)versions.get("base");
            // I don't actually know what it means if obj is null at this point.
            // It probably can't happen.
            if (obj == null)
                return null;
            while (result == null && obj != null) {
                result = obj.getAttr(attr);
                if (result == null && obj.getInclude() != null)
                    obj = (VersionObject)versions.get(obj.getInclude());
                else
                    obj = null;
            return result;
}

Are you the one who commented out the code you're looking for ?//String desc = StringFactory.getString(attributes.getValue("desc"));You just have to modify you AttributeObject class to hold a new field : String description. And then, it's up to you to create a new constructor or a new setter method.
Btw, this is not a Swing related question.

How to wash out an AM instance variable at the end of user session (logout)

We have a need to create AM instance variables to hold some session level data, which is too complex to be held in a transient VO.
What we observed is that the instance variable is not washed out at the end of the user session (i.e logout).
I think since the AM instance is not really destroyed but only release back to the pool the instance variable content persists and is carried over to the next session that access the AM instance.
What method in AM can be overridden to wash out this instance variable.
At the moment we are resetting it in prepareSession(), but this means that the information is still held till the next session access the AM.
It would be better to reset it when the AM is released back to the pool.
Any pointers would be helpful.

At the moment we are resetting it in prepareSession(), but this means that the information is still held till the next session access the AM.
Note that prepareSession() is also invoked after AM activation so this can(and probaly will) reset your variable inside existing session.
If you don't need to keep variable value between AM activations/passivations, you can use userData, for example:
this.getSession().getUserData().put("someKey", value);
UserData values are lost after AM passivation (by default).
If you need to keep UserData variable value for lifetime of user session, then you can override passivateState() and activateState() as in this sample:
Application State Management
Dario

Binding a JavaFX variable to a Java class instance variable

Hi,
I am pretty new to JavaFX but have been developing in Java for many years. I am trying to develop a JavaFX webservice client. What I am doing is creating a basic scene that displays the data values that I am polling with a Java class that extends Thread. The Java class is reading temperature and voltage from a remote server and storing the response in an instance variable. I would like to bind a JavaFx variable to the Java class instance variable so that I can display the values whenever they change.
var conn: WebserviceConnection; // Java class that extends Thread
var response: WebserviceResponse;
try {
conn = new WebserviceConnection("some_url");
conn.start();
Thread.sleep(10000);
} catch (e:Exception) {
e.printStackTrace();
def bindTemp = bind conn.getResponse().getTemperature();
def bindVolt = bind conn.getResponse().getVoltage();
The WebserviceConnection class is opening a socket connection and reading some data in a separate thread. A regular socket connection is used because the server is not using HTTP.
When I run the application, the bindTemp and bindVolt are not updated whenever new data values are received.
Am I missing something with how bind works? Can I do what I want to do with 'bind'. I basically want to run a separate thread to retrieve data and want my UI to be updated when the data changes.
Is there a better way to do this than the way I am trying to do it?
Thanks for any help in advance.
-Richard

Hi,
If you don't want to constantly poll for value change, you can use the observer design pattern, but you need to modify the classes that serve the values to javafx.
Heres a simple example:
The Thread which updates a value in every second:
// TimeServer.java
public class TimeServer extends Thread {
    private boolean interrupted = false;
    public ValueObject valueObject = new ValueObject();
    @Override
    public void run() {
        while (!interrupted) {
            try {
                valueObject.setValue(Long.toString(System.currentTimeMillis()));
                sleep(1000);
            } catch (InterruptedException ex) {
                interrupted = true;
}The ValueObject class which contains the values we want to bind in javafx:
// ValueObject.java
import java.util.Observable;
public class ValueObject extends Observable {
    private String value;
    public String getValue() {
        return this.value;
    public void setValue(String value) {
        this.value = value;
        fireNotify();
    private void fireNotify() {
        setChanged();
        notifyObservers();
}We also need an adapter class in JFX so we can use bind:
// ValueObjectAdapter.fx
import java.util.Observer;
import java.util.Observable;
public class ValueObjectAdapter extends Observer {
    public-read var value : String;
    public var valueObject : ValueObject
        on replace { valueObject.addObserver(this)}
    override function update(observable: Observable, arg: Object) {
         // We need to run every code in the JFX EDT
         // do not change if the update method can be called outside the Event Dispatch Thread!
         FX.deferAction(
             function(): Void {
                value = valueObject.getValue();
}And finally the main JFX code which displays the canging value:
// Main.fx
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.text.Text;
import javafx.scene.text.Font;
import threadbindfx.TimeServer;
var timeServer : TimeServer;
var valueObjectAdapter : ValueObjectAdapter = new ValueObjectAdapter();
timeServer = new TimeServer();
valueObjectAdapter.valueObject = timeServer.valueObject;
timeServer.start();
Stage {
    title: "Time Application"
    width: 250
    height: 80
    scene: Scene {
        content: Text {
            font : Font {
                size : 24
            x : 10, y : 30
            content: bind valueObjectAdapter.value;
}This approach uses less cpu time than constant polling, and changes aren't dependent on the polling interval.
However this cannot be applied to code which you cannot change obviously.
I hope this helps.

Design question: When to use instance variables

Looking for best practice/design advise.
I am working my way through an exercise where I convert a zip code to a postal barcode (and back), validate the format, and validate/generate a check digit. My code works fine. I posted comments and methods below for reference. In addition to the book I am using, I also read http://java.sun.com/docs/books/tutorial/java/javaOO/classvars.html.
My question is: In designing this class, should I have defined instance variables to hold the generated zipcode, barcode, and checkdigit and then used getter methods to get the zipcode or barcode?
I planned on creating a user interface to ask for a zipcode or barcode and present a conversion to the user. The only methods that need to be called are "convertBarcodeToZipcode" and "convertZipcodeToBarcode". All the other methods are utility methods. The strings that are returned by these two methods are presented to the user.
I could, easily enough, create and populate instance variables, but would I keep the return statements as well or remove them?
Thanks, Mike
ps: I am self-learning java, so I don't have an instructor to ask. :)
public class PostalBarCode {
     * Constructor.
     * Set up the barcode ArrayList
    public PostalBarCode() {
            barcodeUnits.add("||:::");
            barcodeUnits.add(":::||");
            barcodeUnits.add("::|:|");
            barcodeUnits.add("::||:");
            barcodeUnits.add(":|::|");
            barcodeUnits.add(":|:|:");
            barcodeUnits.add(":||::");
            barcodeUnits.add("|:::|");
            barcodeUnits.add("|::|:");
            barcodeUnits.add("|:|::");
     * Convert a barcode to a zipcode.
     * Assumes the input format is valid. Validates the check digit
     * @param theBarcode string to be converted
     * @return the zipcode as a String of empty string for error
    public String convertBarcodeToZipcode(String theBarcode) {}
     * Convert the Zipcode to a barcode
     * @param theZipcode string to be converted
     * @return the barcode as a string
    public String convertZipcodeToBarcode(String theZipcode) {}
     * Determines if theString is a barcode
     * @param theString
     * @return true if the input is a barcode, false otherwise.
    public boolean isBarcode (String theString) {}
     * Determines of theString is a zip code
     * @param theString the string to test for 5 digits
     * @return true for a zip code, false otherwise
    public boolean isZipcode (String theString) {}
     * Convert a barcode to a zipcode.
     * Assumes the input format is valid. Validates the check digit
     * @param theBarcode string to be converted
     * @return the zipcode as a String of empty string for error
    public String convertBarcodeToZipcode(String theBarcode) {}
     * Convert the Zipcode to a barcode
     * @param theZipcode string to be converted
     * @return the barcode as a string
    public String convertZipcodeToBarcode(String theZipcode) {}
     * Calculate the check digit from the zipcode
     * @param theZipcode as a String
     * @return the the check digit or -1 for error
    public int calculateCheckDigitFromZipcode(String theZipcode) {}
     * Calculate the check digit from the barcode
     * @param theZipcode as a String
     * @return the the check digit or -1 for error
    public int calculateCheckDigitFromBarcode(String theBarcode) {}
     * Validate the format of the barcode.
     * Should be 6 blocks of 5 symbols each.
     * Also check if frame bars are present and strip them.
     * @param theBarcode
     * @return true for proper format, false for improper format
    public boolean validateBarcodeFormat (String theBarcode){}
    private ArrayList<String> barcodeUnits = new ArrayList<String>();
}

These are just my two cents. The various methods that determine whether something is a zip or bar code can easily remain static. After all, you probably don't want to create an instance and then call something like isValid(), although you could do so. The various other methods could easily be instance methods. You could have a toZipCode() or toBarCode() method in each object to convert from one to the other. I would personally throw something like ValidationException rather than returning -1 for your various check-digit methods.
- Saish

Static private variable

Hello,
I was wondering, what is the use of static? I was doing a class that look like:
public class TheClass{
private static String a;
public TheClass() {
public Seta(String bb){
a = bb;
public String GetA(){
return a;
Then if in another class I do:
TheClass bb = new TheClass();
bb.SetA("BBBBB");
TheClass cc = new TheClass();
cc.SetA("CCCCC");
println(bb.GetA());
println(cc.GetA());both println output CCCCC. Does that mean that a static variable in a class will be share by all instace of that class?
So I could have a static variable status, that when updated or change could affect the beavior of all instatiated object in the JVM?
thanks,

If a variable is declared static, there exists exactly one incarnation of the variable, no matter how many instances (possibly zero) of the class may eventually be created. A static variable, sometimes called a class variable, is incarnated when the class is initialized.
A variable that is not declared static (sometimes called a non-static variable) is called an instance variable. Whenever a new instance of a class is created, a new variable associated with that instance is created for every instance variable declared in that class or any of its superclasses.
If a variable is declared private, then access is permitted only when it occurs from within the class in which it is declared.

Overriding protected static instance variable???

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