Is it safe to extend Reference sub-classes?

I'd like to extend SoftReference to include an instance field for the referent's ID. I'll use this to remove the referent's reference from a Map keyed by the ID. Is this acceptable?

Yes, this can be a useful thing to do. That is why the Reference classes are not final.
If you are putting your References into a ReferenceQueue, then extra state information stored in the References can be used to determine what actions to take when they are removed from the queue.

Similar Messages

Extending and sub classing

i'm experimenting with extending and subclassing components in flex. i am trying to add an event listener to the datagridcolumn. i want to add a function to occur when i click the column (using a mouseclickevent).
each of my pieces of data in the datagrid have a custom style to look like a hyperlink (blue and underlined). the user will drill into details behind the numbers they see. i got this to work with itemClick and i found some good code to use for totaling the datagrid numbers by using the code from Alex's Flex closet http://blogs.adobe.com/aharui/2008/03/flex_3_datagrid_footers.html
my problem is i'm finding that i cannot use the styling or itemClick function to work with the footers and so am trying to subclass another footerdatagridcolumn to have a mouse click event to do something with. i'd like for the footer data to be blue and underlined as well if thats possible but not sure how to refer to my Flex css style in AS3. see the attached screenshot i'm trying to get the footer to work with itemClick and have the same style of blue and underlined for the columns that are highlighted below.

DataGridColumns do not dispatch mouse events. I would listen to the
DataGrid and use mouseEventToItemRenderer as needed.

How to reference the class-instance, created in parent class.

Hi, I have following scenario. During compilation I am gettting error message -- "cannot resolve symbol - symbol : Variable myZ"
CODE :
under package A.1;
     ClassZ
     Servlet1
          ClassZ myZ = new ClassZ;
under package A.2;
     Servlet2 extends Servlet1
          myZ.printHi();
How to reference the class-instance created in the parent class?

some corrections ...
under package A.1;
     ClassZ
     Servlet1
          init()
               ClassZ myZ = new ClassZ;
under package A.2;
     Servlet2 extends Servlet1
          myZ.printHi();

Is it possible to OVERLOAD a super-class method in a sub-class?

Hi all,
I have a query that
Is it possible to OVERLOAD a super-class method in a sub-class?
If it is possible, please give me an example.
Thanks,
Hari

Hi,
Is the method int Display(int a){} overloading
the super-class's void Display() method? If
possible, please clarify this and how it would be
method overloading?
hanks,
Hari
Hi Hari,
Yes, it is possible. Look at this piece of code:
class Senior
     void Display()
          System.out.println("Super class method");
class Junior extends Senior
     int Display(int a)
          System.out.println("Subclass method: "+a);
          return(a+10);
     }> }
class example
     public static void main(String args[])
          Junior j = new Junior();
          j.Display();
System.out.println("Subclass method
od "+j.Display(5));
Is this what you were asking? Hope this helped.Hi,
I guess you guys are confused here...
Overloading is achieved by methods in the same class...
Overriding is across a superclass subclass methds.

Java Sub-Classing Error. Please Help!

I am trying to understanding some classing sub-classing and the convertion exercise. The errors appear with the code at the bottom at compilation.
HelloWorld.java:7: cannot resolve symbol
symbol : method uniqueInSubSayHello()
location: class SayHello
sh2.uniqueInSubSayHello();
^
HelloWorld.java:11: cannot resolve symbol
symbol : method uniqueInSubSayHello()
location: class SayHello
sh2.uniqueInSubSayHello();
^
class HelloWorld {
     public static void main (String args[]) {
          subSayHello subsh=new subSayHello();
          SayHello sh2=subsh;
          sh2.haha();
          sh2.uniqueInSubSayHello();
     int callUnique() {
          sh2.uniqueInSubSayHello();
class SayHello {
     public SayHello() {
     public void haha() {
class subSayHello extends SayHello {
     public subSayHello() {
     void uniqueInSubSayHello() {

On line 7, sh2 points to an object of type SayHello (as you assigned two lines earlier) and therefore does not know about method UniqueUnSubSayHello.
On line 11, sh2 has not been declared, since sh2 is not a variable declared globally within class SayHello. Therefore, it is not found.
Best regards,
Cemil

Do sub classes get their own copy of base class static members?

Hi,
I am sub classing a certain classwhich has a static member boolean used for status updating. I have two sub classes, each needing its own static copy of the boolean. Do they get their own copy, or must I redeclare the member in each sub class?
AC

You must re-declare them (hiding the superclass member) if you wish the subclass members to be "independent".
class Coin {
    public static int FOO;
    public static int BAR;
class Coin2 extends Coin {
    public static int BAR = 2;
    public static void main(String[] args) {
        FOO = 1;
        System.out.println("Coin FOO = " + Coin.FOO); // 1
        System.out.println("Coin2 FOO = " + Coin2.FOO); // 1
        System.out.println("Coin BAR = " + Coin.BAR); // 0
        System.out.println("Coin2 BAR = " + Coin2.BAR); // 2
}HTH! :o)

Calling sub class method from superclass constructure

hi all
i have seen a program in which a super class constructure class methods of sub class before initialization of fields in subclass ,i want how this is posibble ?
thanks in advance

Hear is the code n other thing i have used final variable without initialization n compiler dosen't report error
abstract class Test
public Test()
System.out.println("In DemoClass Constructer");
this.show();
public void show()
System.out.println("In DemoClass Show() method");
class Sub1 extends Test
private final float number;
public Sub1(float n)
this.number=n;
System.out.println((new StringBuilder()).append("Number is==").append(number).toString());
int j;
public void show()
System.out.println("In Sub1 Class Show method ");
public class DemoClass
public static void main(String s[])
Sub1 obj1=new Sub1(5);
Sub1 obj2=new Sub1(6);
thanks for reply

Why are protected fields not-accessible from sub-classed inner class?

I ran across a situation at work where we have to sub-class an inner class from a third-party package. So, it looks something like this...
package SomePackage;
public class Outer {
   protected int x;
   public class Inner {
package OtherPackage;
public class NewOuter extends Outer {
   public class NewInner extends Outer.Inner {
}Now the NewInner class needs to access protected variables in the Outer class, just like the Inner class does. But because NewOut/NewInner are in a different package I get the following error message.
Variable x in class SomePackage.Outer not accessible from inner class NewOuter. NewInner.You can still access those protected variables from the NewOuter class though. So, I can write accessor methods in NewOuter for NewInner to use, but I am wondering why this is. I know that if NewOuter/NewInner are in the same package as Outer/Inner then everything works fine, but does not when they are in different packages.
I have to use JDK1.1.8 for the project so I don't know if there would be a difference in JDK1.2+, but I would think that nothing has changed. Anyway, if you know why Java disallows access as I have detailed please let me know.

Although I don't have the 1.1.8 JDK installed on my system, I was able to compile the following code with the 1.3.1_01 JDK and run it within a Java 1.1.4 environment (the JVM in the MSIE 5.5 browser). As long as you don't access any of the APIs that were introduced with 1.2+ or later, the classes generated by the JDK 1.2+ javac are compatible with the 1.1.4+ JVM.
//// D:\testing\SomePackage\Outer.java ////package SomePackage ;
public class Outer {
    protected int x ;
    public Outer(int xx) {
        x = xx ;
    public class Inner {
}//// D:\testing\OtherPackage\NewOuter.java ////package OtherPackage;
import SomePackage.* ;
public class NewOuter extends Outer {
    public NewOuter(int xx) {
        super(xx) ;
    public class NewInner extends Outer.Inner {
        public int getIt() {
            return x ;
}//// D:\testings\Test.java ////import OtherPackage.* ;
import java.awt.* ;
import java.applet.* ;
public class Test extends Applet {
    public void init () {
        initComponents ();
    private void initComponents() {
        add(new Label("x = ")) ;
        int myx = new NewOuter(3288).new NewInner().getIt() ;
        TextField xfld = new TextField() ;
        xfld.setEditable(false) ;
        xfld.setText(Integer.toString(myx)) ;
        add(xfld) ;
}//// d:\testing\build.cmd ////set classpath=.;D:\testing
cd \testing\SomePackage
javac Outer.java
cd ..\OtherPackage
javac NewOuter.java
cd ..
javac Test.java//// d:\testing\Test.html ////<HTML><HEAD></HEAD><BODY>
<APPLET CODE="Test.class" CODEBASE="." WIDTH=200 HEIGHT=100></APPLET>
</BODY></HTML>

Sub Class in applet

I have written an applet that does a mathematical operation on numbers then shows a diagram on the clicking of a button.
On the clicking of the button I removed the previous two panels and repainted the GUI with the diagram, created using a sub class/ The class header and constuctor is shown below for the sub class.
In netbeans, using applet viewer this works.
Yet for some reason when the applet code is used in a web page the main applet comes up fine and is operational but when clicking the diagram button nothing happens.
II would appreciate any help going?
Thanks
public class VennChart extends JPanel
      /**provides a venn diagram of fucntion
       *@param g is graphics component
      public void paintComponent(Graphics g)
         //frame initialisation
         super.paintComponent(g);
         //code to draw diagram
if(ae.getSource()==graph)
            pane.remove(holder);
            pane.remove(status);
            pane.repaint();
            pane.add(exit, "North");
            VennChart chart = new VennChart();
            temp = chart; //temporary JPanel variable to hold diagram
            pane.add(temp, "Center");
            pane.validate();
}

What error message do you get in the java console?

How do i Index on a Sub class

Hi,
Please let me know if there is a way to index on the vairable in a sub class.
Thanks & regards,
Ebe

Hi Cameron,
The design of the Object is as follows.
Object
--- getSubObjectList() - Returns SubObjectList
----- Contains the list of SubObject
---- SubObject Contains attributes which i query upon.
Please let me know a way how to index on these attributs. Please find the code am using below
Below is the extract method of my Extractor Class
public final class ObjectGraphNavigationValueExtractor extends
AbstractExtractor implements ValueExtractor, ExternalizableLite
public Object extract(Object object) {
if(this.path.equals("attr1") || this.path.equals("attr2")){
MyObject obj =
(MyObject)object;
SubObjectList subList = (SubObjectList) obj.getSubObjectList();
Object aoExtracted[] = subList.toArray();
for(int i=0;i<aoExtracted.length;i++){
SubObject objSubObject =
(SubObject) aoExtracted;
return new BeanWrapperImpl(SubObject).getPropertyValue(this.path);
return new BeanWrapperImpl(object).getPropertyValue(this.path);
I have applied index as follows.
cache.addIndex(new ObjectGraphNavigationValueExtractor("attr1"),true,null);
cache.addIndex(new ObjectGraphNavigationValueExtractor("attr2"),true,null);
Regards,
Ebe

Instantiation of similar object over a super class deciding the sub class

Hello all
First, sorry if I'm duplicating an already answered question. I didn't searched very deep.
Initial position:
I have 2 Object (A1 and A2) which share the most (about 90%) of their instance variables an the associated methods. The values of the instance variables are retrieved in the real implementation from a stream. Depending of the data of the stream, I have to instantiate either a A1 or A2 object.
A test implementation (using an int in case of the stream):
The super class A:
package aaa;
public class A
protected int version = -1;
protected String name = null;
protected AE ae = null;
protected A()
protected A(int v)
    // Pseudo code
    if (v > 7)
      return;
    if (v % 2 == 1)
      this.version = 1;
    else
      this.version = 2;
public final int getVersion()
    return this.version;
public final String getName()
    return this.name;
public final AE getAE()
    return this.ae;
}The first sub class A1:
package aaa;
public final class A1 extends A
protected A1(int v)
    this.version = v;
    this.name = "A" + v;
    this.ae = new AE(v);
}The second subclass A2:
package aaa;
import java.util.Date;
public final class A2 extends A
private long time = -1;
protected A2(int v)
    this.version = v;
    this.name = "A" + v;
    this.time = new Date().getTime();
    this.ae = new AE(v);
public final long getTime()
    return this.time;
}Another class AE:
package aaa;
public class AE
protected int type = -1;
protected AE(int v)
    // Pseudo code
    if (v % 2 == 1)
      this.type = 0;
    else
      this.type = 3;
public final int getType()
    return this.type;
}To get a specific object, I use this class:
package aaa;
public final class AFactory
public AFactory()
public final Object createA(int p)
    A a = new A(p);
    int v = a.getVersion();
    switch (v)
    case 1:
      return new A1(v);
    case 2:
      return new A2(v);
    default:
      return null;
}And at least, a class using this objects:
import aaa.*;
public final class R
public static void main(String[] args)
    AFactory f = new AFactory();
    Object o = null;
    for (int i = 0; i < 10; i++)
      System.out.println("===== Current Number is " + i + " =====");
      o = f.createA(i);
      if (o instanceof aaa.A)
        A a = (A) o;
        System.out.println("Class   : " + a.getClass().getName());
        System.out.println("Version : " + a.getVersion());
        System.out.println("Name    : " + a.getName());
        System.out.println("AE-Type : " + a.getAE().getType());
      if (o instanceof aaa.A2)
        A2 a = (A2) o;
        System.out.println("Time    : " + a.getTime());
      System.out.println();
Questions:
What would be a better way to encapsulate the logic into their respective objects ? Is there a way to let the super class A itself identify the type of the object and then extend from A to either A1 or A2 ?
Thanks in advance
Andreas

Hello jduprez
First, I would thank you very much for taking the time reviewing my problem.
Just for the record: have you considered regular serialization? If you control the software at both ends of the stream, you could rely on standard serialization mechanism to marshall the objects and unmarshall them automatically.In my case, I can't control the other site of the stream. At the end, the data comes from a FileInputStream and there aren't objects on the other site, only pur binary data.
- It seems you have one such factory class. Then you already have encapsulated the "determine class" logic, you don't need to add such logic in superclass A.I thought from an OO view, that the super class A is responsible of doing that, but that's where the problem starts. So at the end, it's better doing it in the factory class.
- A itself encapsulates the logic to load its own values from the stream.
- A1 and A2 can encapsulate the logic to load their own specific value from a stream (after the superclass' code has loaded the superclass' attributes values).
My advise would be along the lines of:
public class A {
.... // member variables
public void load(InputStream is) {
... // assign values to A's member variables
// from what is read from the stream.
public class A1 extends A {
... // A1-specific member variables
public void load(InputStream is) {
super.load(is);
// now read A1-specific values
public class AFactory {
public A createA(InputStream is) {
A instance;
switch (is.readFirstByte()) {
case A1_ID:
a = new A1();
break;
case A2_ID:
a = new A2();
break;
a.load(is);
}The example above assumes you have control over the layout of the data in the stream (here for a given A instance, the attributes defined in A are read first, then come the subclass-specific attributes.
The outcome is: you don't have to create a new A( ) to later create another instance, of a subclass.I like the idea. In the AFactory, is the "A instance;" read as "A a;" ?
Is there a way to let the super class A itself identify the type of the object and then extend from A to either A1 or A2 ?Note I initially read this question as "can an instance of a class mutate into another class", to which the answer is no (an object has one single, immutable class; it is an instance of this class, and of any superclass and superinterface, but won't change its class at runtime).Yes, I have been thinking about a way for mutating into a subclass to keep the already initialized values from the A class without copying or parsing again. But preventing an instance of an A class will be my way. So, in this aspect, Java didn't changed in the last 10 years... It's a long time ago I've used Java for some real projects.
You can, however, create an instance of another class, that copies the values off a priori A instance. Your example code was one way, another way could be to have a "copy constructor":
public class A {
public A(A model) {
this.att1 = model.att1;
this.att2 = model.att2;
public class A1 {
public A1(A model) {
super(model);
... // do whatever A1-specific business
)Still, I prefer my former solution less disturbing: I find the temporary A instance redundant and awkward.Nice to know. I prefer the first solution too.
Thank you again for the help and advices. My mind is searching sometimes for strange solutions, where the real is so close ;-)
Andreas

Is Overloading possible in Sub class

In every book I found that Overloading means method with the same name and with different parameter define in same class.
But is possible if I define method with the same name and with different parameter in Sub class?
Like example given below :
public class OverLadingTest {
     public void test(int a, int b) {
          System.out.println("Inside test(int a, int b)");
     public static void main(String args[]){
               OverloadedSubclass overloadedSubclass = new OverloadedSubclass();
               overloadedSubclass.test(10.20,10.20);
class OverloadedSubclass extends OverLadingTest {
     public void test(double a, double b) {
          System.out.println("Inside test(double a, double b)");
Thanks in advance.

The nays have it!
package forums;
class Singularity
public void test(int a) {
    System.out.println("Singularity.test(int a)");
class Universe extends Singularity
@Override
public void test(int a) {
    System.out.println("Universe.test(int a)");
public void test(int a, int b) {
    System.out.println("Universe.test(int a, int b)");
public class OverloadingTest
public static void main(String args[]) {
    Singularity s = new Universe();
    s.test(10);
    s.test(10, 20);
compile
C:\Java\home\src\forums>"C:\Program Files\Java\jdk1.6.0_12\bin\javac.exe" -Xlint -d C:\Java\home\classes -cp c:\java\home\src; C:\Java\home\src\forums\OverloadingTest.java
C:\Java\home\src\forums\OverloadingTest.java:27: test(int) in forums.Singularity cannot be applied to (int,int)
    s.test(10, 20);
     ^
1 errorI'd say that's pretty conclusive, wouldn't you? Go read up on Java's definition of a [method signature|http://java.sun.com/docs/books/tutorial/java/javaOO/methods.html]... It's not just the name, putz!
Cheers. Keith.

How can a sub class access exisiting stage item?

i have a button on the stage. the document class loads two
sub classes. how do you get the sub class to access/work with a
stage item that is already on the display list?
as3.0 is killing me!!! grrrr...
thanks!

The document class mapDC:
package com.main{
// this is the main document class for the rextag map
appllication
// import movie clip to extend to use on stage
import flash.display.MovieClip;
import flash.events.MouseEvent;
import com.main.tools.mapTools;
public class mapDC extends MovieClip {
// instance of class
public function mapDC() {
trace("MainMap initialized");
new mapTools(stage);
public function test() {
trace("here at test");
The mapTools Class:
package com.main.tools{
// import
import flash.events.MouseEvent;
import flash.display.*;
// this class will contain all the move methods and
functions
public class mapTools {
private var stage:Stage;
function mapTools(targetStage) {
trace("Map tools initialized");
stage = targetStage;
stage.root.mapSection.eastArrow.addEventListener(MouseEvent.ROLL_OVER,testOutput);
//---------- test output function
public function testOutput(evt:MouseEvent) {
trace("worked");
}

Method of JUErrorHandlerDlg sub class

Hi Forum
i am working jClint/Swing Application in jDevloper.And
i want to use own custom messageBox rather than exception
through from database BC4J.
I fund JUErrorHandlerDlg sub class for exception through
so how to method use for this.
please reply me some code.
javed

Thanks! for reply
But until i found problem ,how to catch exception
throw by bc4j(Jbo error meassage),exception has a
error number(like jbo-10426).
So using this error number how to use
this exception,In order to own message dilog.
please reply me some code,
use in our class as
public class subJUErrorHandlerDlg extends
JUErrorHandlerDlg
public subJUErrorHandlerDlg()
// i write bootstrap application
JUMetaObjectManager.setBaseErrorHandler(new subJUErrorHandlerDlg());

Stage reference from class

How do I reference the stage from a class? I need to add eventlisteners to the
stage for mouse movement. I also need to add custom cursors. I always get an error when I try to reference stage
. anything. Or event if I do this:
var stageRef:Stage;
then later: stageRef.stage.addEventList.....
What am I missing or not understanding?

I cannot get it to work. Here is my code:
package com {
// core
import flash.display.MovieClip;
import flash.display.Stage;
// import custom classes
import com.main.BuildStageClass;
import com.main.BootClass;
import com.utils.RotateClass;
import com.utils.ZoomClass;
import com.utils.PanClass;
import com.main.HotSpotClass;
// initialize class
public class DocClass extends MovieClip {
// custom classes
// variables
public static var build_stage_class:BuildStageClass;
public static var boot_class:BootClass;
public static var rotate_class:RotateClass;
public static var zoom_class:ZoomClass;
public static var pan_class:PanClass;
public static var hot_spot_class:HotSpotClass;
// stage containers
public static var main_stage:MovieClip;
public static var background_layer:MovieClip;
public static var content_layer:MovieClip;
public static var popup_layer:MovieClip;
// main constructor
public function DocClass() {
   // initialize classes
   build_stage_class = new BuildStageClass();
   boot_class = new BootClass();
   rotate_class = new RotateClass();
   zoom_class = new ZoomClass();
   pan_class = new PanClass(this.stage);
   hot_spot_class = new HotSpotClass();
   // add containers to stage
   main_stage = new MovieClip();
   addChild(main_stage);
   background_layer = new MovieClip();
   main_stage.addChild(background_layer);
   content_layer = new MovieClip();
   main_stage.addChild(content_layer);
   popup_layer = new MovieClip();
   main_stage.addChild(popup_layer);
==========================
Now inside the pan class:
package com.utils{
// import classes
import flash.display.MovieClip;
import flash.display.Stage;
import flash.events.MouseEvent;
import flash.geom.Point;
import flash.ui.Mouse;
// import main classes
import com.DocClass;
// initialize class
public class PanClass extends MovieClip {
// variables
public var do_pan:Boolean = true;
public var last_point:Number = 0;
public var cur_point:Number = 0;
public var hand_cursor:HandCursor;
public var grab_cursor:PanCursor;
private var myStage:Stage;
// main constructor
public function PanClass(docClassStage:Stage) {
   // constructor code
   myStage = docClassStage;
   myStage.addEventListener(MouseEvent.MOUSE_MOVE, panHandler);
Ideas?

Is it safe to extend Reference sub-classes?

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