Why a base class cannot be a Private or Protected ?????
why a base class cannot be a Private or Protected ?????
why a base class cannot be a Private orProtected
?????Who would be able to access it? (Assuming "base"
means top-level class.)Private, none. Protected, all classes in same package
and subclasses (as with the methods).
Actually, its somewhat illogical that a class be
declared package-private, but not protected, since
protected is less restrictive.having a protected class would make no sense. subclasses are not permitted to lessen the protection of members of their superclass, so you'd end up with a class that nothing could use, except subclasses that in turn nothing else could use
Similar Messages
-
Override a private method in a Base class?
How can a subclass override a final method in its superclass.
This code compiles cleanly.
How is this possible?
Or Am I overlooking any point?
class MySuperClass{
private final String getMessage(){
return "hello";
protected String getAnotherMessage(){
return getMessage() + "world";
class MySubClass extends MySuperClass{
private final String getMessage(){
return "hi";
public String getAnotherMessage(){
return "anothermessage";
}getMessage is declared as private in the base class, and therefor cannot be overridden in any subclass.
Edit: I think I understand the question better now. You want to know why it compiles when you think it shouldn't. It's because the private method isn't really inherited at all. Your subclass created a brand new private method of the same signature, but it's completely unrelated. -
Why cannnot derived class pointer point to a base class object?
Hi,
Pleaseeee... explain me why cannot the derived class pointer point to a base class object.
I know that Base class pointer can point to a derived class object.
Thanks & Regards,
Vig....Example:
class Base
{ public: void foo(); } * pBase;
class Derived : public Base
{ public: void bar(); } * pDerived;
Now, what would happen if you assign pDerived = new Base() and then call pDerived->bar()?By forbidding such an assignment, compiler prevents you from writing error-prone code. -
Hi,
We've got an abstract base class (StringBase) which various types of strings inherit from. The copy constructor for this base class is private, since we don't want to allow copying when this class shouldn't be directly instantiated. A number of our methods take specify the base class as a reference, but take a derived class temporary as a default argument (see code appended).
This worked fine in 12.3, but in 12.4 beta, this now says:
Error: StringBase::StringBase(const StringBase&) is not accessible from __dflt_argA().
This works fine in clang and gcc, and indeed, this GNU document says it was a bug which was fixed in gcc 4.3.0:
Copy constructor access check while initializing a reference
which references http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#391
It only appears to error when the "-g" option is used, however, which doesn't seem right, and compiles fine if the "-g" option is removed, which makes me think it's a bug. Presumably the optimizer is eliding the copy when not using -g, but it's left in for debug mode, causing the compile error?
Many thanks,
Jonathan.
$ clang++ -std=c++11 defaultarg.cpp
$ g++ -std=c++11 defaultarg.cpp
$ /opt/SolarisStudio12.4-beta_mar14-solaris-x86/bin/CC -c defaultarg.cpp
$ /opt/SolarisStudio12.4-beta_mar14-solaris-x86/bin/CC -g -c defaultarg.cpp
"defaultarg.cpp", line 6: Error: StringBase::StringBase(const StringBase&) is not accessible from __dflt_argA().
1 Error(s) detected.
$ cat defaultarg.cpp
#include "stringbase.h"
#include "conststring.h"
static const ConstString S_DEFAULT("default value");
void SomeMethod( const StringBase& str = S_DEFAULT )
(void) str;
int main( void )
SomeMethod();
$ cat stringbase.h
#ifndef STRINGBASE_H
#define STRINGBASE_H
class StringBase
protected:
StringBase() {}
private:
StringBase( const StringBase& );
#endif
$ cat conststring.h
#ifndef CONSTSTRING_H
#define CONSTSTRING_H
#include "stringbase.h"
class ConstString : public StringBase
public:
ConstString() {}
ConstString( const char* ) {}
ConstString( const ConstString& );
#endifThanks for reporting the problem!
This looks like a compiler bug, I think an artifact of creating a helper function for the debugger for the default argument.
I have filed bug 18505648 for you. -
Cannot compile Library project in Flash builder 4: Unable to locate specified base class
I am migrating from Flex builder 3 to Flash builder 4, during that process I found that Flash builder could'nt compile Flex Library projects which contains components in the form of mxml files. Compiler throws "Unable to locate specified base class".
I have narrowed it down to by creating a flex library project named test library, below its screenshot with its package structure
ControlA is based on componentA
when i compile this project, mxml compiler throws this error
Unable to locate specified base class 'components.componentA' for component class 'ControlA'.
I cannot upload project code because file uploads are disabled on this forum so I am going to paste the code for both componentA and controlA
componentA
<?xml version="1.0" encoding="utf-8"?>
<mx:Canvas xmlns:mx="http://www.adobe.com/2006/mxml" xmlns:local="*" width="400" height="300">
</mx:Canvas>
ControlA
<?xml version="1.0" encoding="utf-8"?>
<components:componentA xmlns:mx="http://www.adobe.com/2006/mxml" xmlns:components="components.*" width="400" height="300">
</components:componentA>
To further test it, I have made another library project with same package structure using pure AS3, and it compiled without errorsIt might be this happens and other errors if you forget to Run Flash Builder 4.5 Buritto and startup the other version FB4.
It uses the same project listings and that can be confusing since you're not expecting the old version to be aware of new Buritto (MobileApplications) projects you added (thinking) only to the BETA version installed elseware no doubt.
Well it might not be your problem but worthy of noting for other's I'm sure.
Otherwise you've double checked you're using SDK 4.5 Hero with it too! right? -
Accessing private field of Derived object in Base class
Hi,
I have this piece of code I wrote a while ago to test something. The issue is accessing a private field of Base class in Base but of a Derived object.
Here is the code:
class Base
private int x;
public int getX()
return x;
public int getX(Derived d)
// return d.x;
return ((Base) d).x;
}The commented code does not work but casting d to Base does.
Can someone please explain the reasoning for this.
Forgot to mention that the compilation error is that x has private access in Base.
Thank you.
Edited by: 953012 on Apr 1, 2013 8:42 AM>
As I understand the explanation says that you can access any private member within the code of the class that encloses the private member. So in this case x is the private member and the line of code (return d.x) is in Base which encloses the private member. Does it have to do with the fact that the Derived class does not in fact inherit the private members of Base?
>
It has to do with the entire quote from the spec
>
A private class member or constructor is accessible only within the body of the top level class (§7.6) that encloses the declaration of the member or constructor. It is not inherited by subclasses
>
Your code is
public int getX(Derived d)
// return d.x;
return ((Base) d).x;
}The 'Derived' class is NOT 'the top level class that encloses the declaration of the member'. It does NOT inherit 'x' which is a private member of 'Base'. As far as the 'Derived' class is concerned 'x' does not exist.
>
If outside Base code I have Derived d = new Derived() and I call d.getX() then isn't that like calling d.x in myX()?
>
How is that the same? 'Base' owns 'x' and can do whatever it wants with it. 'Derived' has no knowledge of 'x' and CAN NOT access it. -
Why does the main class cannot find symbol symbol : constructor Car(double) location:
class Car .. ??
class Car
{ //variable declaration
double milesStart; double milesEnd; double gallons;
//constructors
public Car(double start, double end, double gall)
{ milesStart = start; milesEnd = end; gallons = gall; }
void fillUp(double milesE, double gall)
{ milesEnd = milesE; gallons = gall; }
//methods
double calculateMPG()
{ return (milesEnd - milesStart)/gallons; }
boolean gashog() { if(calculateMPG()<15) { return true; } else { return false; } }
boolean economycar() { if(calculateMPG()>30) { return true; } else { return false; } } }
import java.util.*; class MilesPerGallon
{ public static void main(String[] args)
double milesS, milesE, gallonsU;
Scanner scan = new Scanner(System.in);
System.out.println(\"New car odometer reading: 00000\");
Car car = new Car(milesS); car.fillUp(milesE, gallonsU);
System.out.println(\"New Miles: \" + milesE); milesE = scan.nextDouble();
System.out.println(\"Gallons used: \" + gallonsU);
gallonsU = scan.nextDouble();
System.out.println( \"MPG: \" + car.calculateMPG() );
if(car.gashog()==true) { System.out.println(\"Gas Hog!\");
if(car.economycar()==true) { System.out.println(\"Economy Car!\");
} System.out.println(\"\");
milesS = milesS + milesE;
System.out.println(\"Enter new miles\");
milesE = scan.nextDouble();
System.out.println(\"Enter gallons used: \");
gallonsU = scan.nextDouble();
car.fillUp(milesE, gallonsU);
System.out.println(\"Initial miles: \" + milesS);
System.out.println(\"New Miles: \" + milesE);
System.out.println(\"Gallons used: \" + gallonsU);
System.out.println( \"MPG: \" + car.calculateMPG() );
System.out.println(\"\"); } }Why does the main class cannot find symbol symbol : constructor Car(double) location:
class Car .. ??
Please tell us which line of code you posted shows 'Car (double)'.
The only constructor that I see is this one:
Car(double start, double end, double gall) -
Import a class cannot take effect until the restart of wls6.1,why?
Hi all,
I am using Weblogic6.1,I knew that JSP,Servlet,EJB can be auto depolyed
without restart weblogic,but when I
import a class written by myself in JSP or Servlet£¬the modification of this
class cannot take effect until
the restart of weblogic server¡£Why? How can I solve it£¿
thanks in advance!
Check that your classpath is not set up to this class when you start the
server and check the ProductionDeploymentMode to false in the starting
parameters!
Hope it helps.
Pat
"jxc" <[email protected]> wrote in message
news:3bef34c3$[email protected]..
> Hi all,
> I am using Weblogic6.1,I knew that JSP,Servlet,EJB can be auto
depolyed
> without restart weblogic,but when I
> import a class written by myself in JSP or Servlet£¬the modification of
this
> class cannot take effect until
> the restart of weblogic server¡£Why? How can I solve it£¿
>
> thanks in advance!
>
>
>
>
>
-
Why the constructor of the base class calls first when u run the java app.
why the constructor of the base class calls first when u run the java application
For the record the other very exciting questions are:
pls give the differences between application server and web server with examples
will u pls narrate the differences between servlet context and servlet config.....
where can we find servlet config...
is there any methods to access servlet config -
Why can't classes with private constructors be subclassed?
Why can't classes with private constructors be subclassed?
I know specifying a private nullary constructor means you dont want the class to be instantiated or the class is a factory or a singleton pattern. I know the workaround is to just wrap all the methods of the intended superclass, but that just seems less wizardly.
Example:
I really, really want to be able to subclass java.util.Arrays, like so:
package com.tassajara.util;
import java.util.LinkedList;
import java.util.List;
public class Arrays extends java.util.Arrays {
public static List asList(boolean[] array) {
List result = new LinkedList();
for (int i = 0; i < array.length; i++)
result.add(new Boolean(array));
return result;
public static List asList( char[] array) {
List result = new LinkedList();
for (int i = 0; i < array.length; i++)
result.add(new Character(array[i]));
return result;
public static List asList( byte[] array) {
List result = new LinkedList();
for (int i = 0; i < array.length; i++)
result.add(new Byte(array[i]));
return result;
public static List asList( short[] array) {
List result = new LinkedList();
for (int i = 0; i < array.length; i++)
result.add(new Short(array[i]));
return result;
public static List asList( int[] array) {
List result = new LinkedList();
for (int i = 0; i < array.length; i++)
result.add(new Integer(array[i]));
return result;
public static List asList( long[] array) {
List result = new LinkedList();
for (int i = 0; i < array.length; i++)
result.add(new Long(array[i]));
return result;
public static List asList( float[] array) {
List result = new LinkedList();
for (int i = 0; i < array.length; i++)
result.add(new Float(array[i]));
return result;
public static List asList( double[] array) {
List result = new LinkedList();
for (int i = 0; i < array.length; i++)
result.add(new Double(array[i]));
return result;
// Now that we extend java.util.Arrays this method is not needed.
// /**JCF already does this so just wrap their implementation
// public static List asList(Object[] array) {
// return java.util.Arrays.asList(array);
public static List asList(Object object) {
List result;
Class type = object.getClass().getComponentType();
if (type != null && type.isPrimitive()) {
if (type == Boolean.TYPE)
result = asList((boolean[])object);
else if (type == Character.TYPE)
result = asList(( char[])object);
else if (type == Byte.TYPE)
result = asList(( byte[])object);
else if (type == Short.TYPE)
result = asList(( short[])object);
else if (type == Integer.TYPE)
result = asList(( int[])object);
else if (type == Long.TYPE)
result = asList(( long[])object);
else if (type == Float.TYPE)
result = asList(( float[])object);
else if (type == Double.TYPE)
result = asList(( double[])object);
} else {
result = java.util.Arrays.asList((Object[])object);
return result;
I do not intend to instantiate com.tassajara.util.Arrays as all my methods are static just like java.util.Arrays. You can see where I started to wrap asList(Object[] o). I could continue and wrap all of java.util.Arrays methods, but thats annoying and much less elegant.Why can't classes with private constructors be
subclassed?Because the subclass can't access the superclass constructor.
I really, really want to be able to subclass
java.util.Arrays, like so:Why? It only contains static methods, so why don't you just create a separate class?
I do not intend to instantiate
com.tassajara.util.Arrays as all my methods are static
just like java.util.Arrays. You can see where I
started to wrap asList(Object[] o). I could continue
and wrap all of java.util.Arrays methods, but thats
annoying and much less elegant.There's no need to duplicate all the methods - just call them when you want to use them.
It really does sound like you're barking up the wrong tree here. I can see no good reason to want to subclass java.util.Arrays. Could you could explain why you want to do that? - perhaps you are misunderstanding static methods.
Precisely as you said, if they didn't want me to
subclass it they would have declared it final.Classes with no non-private constructors are implicitly final.
But they didn't. There has to be a way for an API
developer to indicate that a class is merely not to be
instantiated, and not both uninstantiable and
unextendable.There is - declare it abstract. Since that isn't what was done here, I would assume the writers don't want you to be able to subclass java.util.Arrays -
Cannot find Super class/Base class when extending VO
I am extending a SQL based VO and when I select the extends in the VO Wizard step 7. I am unable to find the base class for the object and rowimpl. I have copied them in the class path and still they don't show up (but can see other classes) ? Could you please tell where am I going wrong?
Thanks,
HariWell here is the deal - I have the whole of $JAVA_TOP/per directory in my class path and also under myprojects/oracle/apps/per directory. I have created a BC4J abc.oracle.apps.per.selfservice.appraisals.finalrate. Here is where I will be creating the new SQLVO. In the step 7 of VO Wizard there is extends button when I select and navigate through oracle.apps.per.selfservice.appraisals.server I don't see my FinalCompListVORowImpl class although I could see it in the navigator. Do I need to navigate to this level for step7 of VO Wizard or default it to oracle.jbo.server.ViewRowImpl? Also I have included the Java and xml files in my project. Your help is appreciated.
Thanks,
Hari -
Why inner classes cannot have static declarations ?
Hi Friends,
When i tried to make static declarations on a inner class which is non static, i am getting compilation error saying "inner classes cannot have static declarations". I want to know reason behind this implementation.
Code which i have tried:
public class TestOuter
class TestInner{
static int i =10;
public static boolean validate(int a){
if(a==0)
System.out.println("Invalid data");
return false;
return true;
public static void main(String a[]){
boolean result = new TestOuter.TestInner.validate(0);
System.out.println("Result="+result);
Thanks,
Shiju V.so I think if the
outer class is not static , then Inner class can't be
static as well. This is incorrect. An enclosed class can be indeed static while the outer is not, and vice versa.
The difference between static/non static in regards to enclosed classes is that the static ones are 'top-level' and cannot access the members of the enclosing class.
The effect of making an enclosed class static means there is only one instance of any variables, no matter how many instances of the outer class are created. In this situation how could the static inner class know which variables to access of its non static outer class. Of course the answer is that it could not know, and thus an static inner class cannot access instance variables of its enclosing class.
Now, regarding non-static inner classes, and trying to give a valid answer to the original post:
As with instance methods and variables, a non-static inner class is associated with an instance of its enclosing class and has direct access to that object's instance variables and methods.
TestOuter outer = new TestOuter();
TestOuter.TestInner inner = outer.new TestInner();Because an inner class is associated with an instance (inner class implicitly keeps a reference to the object of the enclosing class that created it), it cannot define any static members itself. Static members cannot access the this pointer.
So, in an ordinary (non-static) inner class, the link to the outer class object is achieved with a special this reference. A static inner class does not have that special this reference, nor would a static method/variable of an ordinary (non-static) inner class. -
Why does this class not read and write at same time???
I had another thread where i was trying to combine two class files together
http://forum.java.sun.com/thread.jspa?threadID=5146796
I managed to do it myself but when i run the file it does not work right. If i write a file then try and open the file it says there are no records in the file, but if i close the gui down and open the file everything get read in as normal can anybody tell me why?
import java.io.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import bank.BankUI;
import bank.*;
public class testing extends JFrame {
private ObjectOutputStream output;
private BankUI userInterface;
private JButton SaveToFile, SaveAs, Exit; //////////savetofile also saves to store need to split up and have 2 buttons
//private Store store; MIGHT BE SOMETHING TO DO WITH THIS AS I HAD TO COMMENT THIS STORE OUT TO GET IT WORKING AS STORE IS USED BELOW
private Employee record;
//////////////////////////////////////from read
private ObjectInputStream input;
private JButton nextButton, openButton, nextRecordButton ;
private Store store = new Store(100);
private Employee employeeList[] = new Employee[100];
private int count = 0, next = 0;
/////////////////////////////////////////////from read end
// set up GUI
public testing()
super( "Employee Data" ); // appears in top of gui
// create instance of reusable user interface
userInterface = new BankUI( 9 ); // nine textfields
getContentPane().add( userInterface, BorderLayout.CENTER );
// configure button doTask1 for use in this program
SaveAs = userInterface.getSaveAsButton();
SaveAs.setText( "Save as.." );
//////////////////from read
openButton = userInterface.getOpenFileButton();
openButton.setText( "Open File" );
openButton.addActionListener(
// anonymous inner class to handle openButton event
new ActionListener() {
// close file and terminate application
public void actionPerformed( ActionEvent event )
openFile();
} // end anonymous inner class
); // end call to addActionListener
// register window listener for window closing event
addWindowListener(
// anonymous inner class to handle windowClosing event
new WindowAdapter() {
// close file and terminate application
public void windowClosing( WindowEvent event )
if ( input != null )
closeFile();
System.exit( 0 );
} // end anonymous inner class
); // end call to addWindowListener
// get reference to generic task button doTask2 from BankUI
nextButton = userInterface.getDoTask2Button();
nextButton.setText( "Next Record" );
nextButton.setEnabled( false );
// register listener to call readRecord when button pressed
nextButton.addActionListener(
// anonymous inner class to handle nextRecord event
new ActionListener() {
// call readRecord when user clicks nextRecord
public void actionPerformed( ActionEvent event )
readRecord();
} // end anonymous inner class
); // end call to addActionListener
//get reference to generic task button do Task3 from BankUI
// get reference to generic task button doTask3 from BankUI
nextRecordButton = userInterface.getDoTask3Button();
nextRecordButton.setText( "Get Next Record" );
nextRecordButton.setEnabled( false );
// register listener to call readRecord when button pressed
nextRecordButton.addActionListener(
// anonymous inner class to handle nextRecord event
new ActionListener() {
// call readRecord when user clicks nextRecord
public void actionPerformed( ActionEvent event )
getNextRecord();
} // end anonymous inner class
); // end call to addActionListener
///////////from read end
// register listener to call openFile when button pressed
SaveAs.addActionListener(
// anonymous inner class to handle openButton event
new ActionListener() {
// call openFile when button pressed
public void actionPerformed( ActionEvent event )
SaveLocation();
} // end anonymous inner class
); // end call to addActionListener
// configure button doTask2 for use in this program
SaveToFile = userInterface.getSaveStoreToFileButton();
SaveToFile.setText( "Save to store and to file need to split this task up" );
SaveToFile.setEnabled( false ); // disable button
// register listener to call addRecord when button pressed
SaveToFile.addActionListener(
// anonymous inner class to handle enterButton event
new ActionListener() {
// call addRecord when button pressed
public void actionPerformed( ActionEvent event )
addRecord(); // NEED TO SPLIT UP SO DONT DO BOTH
} // end anonymous inner class
); // end call to addActionListener
Exit = userInterface.getExitAndSaveButton();
Exit.setText( "Exit " );
Exit.setEnabled( false ); // disable button
// register listener to call addRecord when button pressed
Exit.addActionListener(
// anonymous inner class to handle enterButton event
new ActionListener() {
// call addRecord when button pressed
public void actionPerformed( ActionEvent event )
addRecord(); // adds record to to file
closeFile(); // closes everything
} // end anonymous inner class
); // end call to addActionListener
// register window listener to handle window closing event
addWindowListener(
// anonymous inner class to handle windowClosing event
new WindowAdapter() {
// add current record in GUI to file, then close file
public void windowClosing( WindowEvent event )
if ( output != null )
addRecord();
closeFile();
} // end anonymous inner class
); // end call to addWindowListener
setSize( 600, 500 );
setVisible( true );
store = new Store(100);
} // end CreateSequentialFile constructor
// allow user to specify file name
////////////////from read
// enable user to select file to open
private void openFile()
// display file dialog so user can select file to open
JFileChooser fileChooser = new JFileChooser();
fileChooser.setFileSelectionMode( JFileChooser.FILES_ONLY );
int result = fileChooser.showOpenDialog( this );
// if user clicked Cancel button on dialog, return
if ( result == JFileChooser.CANCEL_OPTION )
return;
// obtain selected file
File fileName = fileChooser.getSelectedFile();
// display error if file name invalid
if ( fileName == null || fileName.getName().equals( "" ) )
JOptionPane.showMessageDialog( this, "Invalid File Name",
"Invalid File Name", JOptionPane.ERROR_MESSAGE );
else {
// open file
try {
input = new ObjectInputStream(
new FileInputStream( fileName ) );
openButton.setEnabled( false );
nextButton.setEnabled( true );
// process exceptions opening file
catch ( IOException ioException ) {
JOptionPane.showMessageDialog( this, "Error Opening File",
"Error", JOptionPane.ERROR_MESSAGE );
} // end else
} // end method openFile
public void readRecord() // need to merger with next record
Employee record;
// input the values from the file
try {
record = ( Employee ) input.readObject();
employeeList[count++]= record;
store.add(record);/////////ADDS record to Store
store.displayAll();
System.out.println("Count is " + store.getCount());
// create array of Strings to display in GUI
String values[] = {
String.valueOf(record.getName()),
String.valueOf(record.getGender()),
String.valueOf( record.getDateOfBirth()),
String.valueOf( record.getID()),
String.valueOf( record.getStartDate()),
String.valueOf( record.getSalary()),
String.valueOf( record.getAddress()),
String.valueOf( record.getNatInsNo()),
String.valueOf( record.getPhone())
// i added all those bits above split onto one line to look neater
// display record contents
userInterface.setFieldValues( values );
// display message when end-of-file reached
catch ( EOFException endOfFileException ) {
nextButton.setEnabled( false );
nextRecordButton.setEnabled( true );
JOptionPane.showMessageDialog( this, "No more records in file",
"End of File", JOptionPane.ERROR_MESSAGE );
// display error message if class is not found
catch ( ClassNotFoundException classNotFoundException ) {
JOptionPane.showMessageDialog( this, "Unable to create object",
"Class Not Found", JOptionPane.ERROR_MESSAGE );
// display error message if cannot read due to problem with file
catch ( IOException ioException ) {
JOptionPane.showMessageDialog( this,
"Error during read from file",
"Read Error", JOptionPane.ERROR_MESSAGE );
} // end method readRecord
private void getNextRecord()
Employee record = employeeList[next++%count];//cycles throught accounts
//create aray of string to display in GUI
String values[] = {String.valueOf(record.getName()),
String.valueOf(record.getGender()),
String.valueOf( record.getStartDate() ), String.valueOf( record.getAddress()),
String.valueOf( record.getNatInsNo()),
String.valueOf( record.getPhone()),
String.valueOf( record.getID() ),
String.valueOf( record.getDateOfBirth() ),
String.valueOf( record.getSalary() ) };
//display record contents
userInterface.setFieldValues(values);
//display record content
///////////////////////////////////from read end
private void SaveLocation()
// display file dialog, so user can choose file to open
JFileChooser fileChooser = new JFileChooser();
fileChooser.setFileSelectionMode( JFileChooser.FILES_ONLY );
int result = fileChooser.showSaveDialog( this );
// if user clicked Cancel button on dialog, return
if ( result == JFileChooser.CANCEL_OPTION )
return;
File fileName = fileChooser.getSelectedFile(); // get selected file
// display error if invalid
if ( fileName == null || fileName.getName().equals( "" ) )
JOptionPane.showMessageDialog( this, "Invalid File Name",
"Invalid File Name", JOptionPane.ERROR_MESSAGE );
else {
// open file
try {
output = new ObjectOutputStream(
new FileOutputStream( fileName ) );
SaveAs.setEnabled( false );
SaveToFile.setEnabled( true );
Exit.setEnabled( true );
// process exceptions from opening file
catch ( IOException ioException ) {
JOptionPane.showMessageDialog( this, "Error Opening File",
"Error", JOptionPane.ERROR_MESSAGE );
} // end else
} // end method openFile
// close file and terminate application
private void closeFile()
// close file
try {
//you want to cycle through each recordand add them to store here.
int storeSize = store.getCount();
for (int i = 0; i<storeSize;i++)
output.writeObject(store.elementAt(i));
output.close();
input.close();// from read!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
System.exit( 0 );
// process exceptions from closing file
catch( IOException ioException ) {
JOptionPane.showMessageDialog( this, "Error closing file",
"Error", JOptionPane.ERROR_MESSAGE );
System.exit( 1 );
} // end method closeFile
// add record to file
public void addRecord()
int employeeNumber = 0;
String fieldValues[] = userInterface.getFieldValues();
// if account field value is not empty
if ( ! fieldValues[ BankUI.IDNUMBER ].equals( "" ) ) {
// output values to file
try {
employeeNumber = Integer.parseInt(
fieldValues[ BankUI.IDNUMBER ] );
String dob = fieldValues[ BankUI.DOB ];
String[] dateofBirth = dob.split ("-"); // what used to put between number chnage to /
String sDate = fieldValues[ BankUI.START ];
String[] startDate = sDate.split ("-");
String sex = fieldValues[ BankUI.GENDER ];
char gender = (sex.charAt(0)); // check if m or f prob check in employee
if ( employeeNumber >= 0 ) {
/* create new record =String name, char gender, Date dob, String add, String nin, String phone, String id, Date start, float salary*/
record = new Employee(
fieldValues[ BankUI.NAME ],
gender,
new Date( Integer.parseInt(dateofBirth[0]),
Integer.parseInt(dateofBirth[1]),
Integer.parseInt(dateofBirth[2])),
fieldValues[ BankUI.ADDRESS ],
fieldValues[ BankUI.NATINTNO ],
fieldValues[ BankUI.PHONE ],
fieldValues[ BankUI.IDNUMBER ],
new Date( Integer.parseInt(startDate[0]),
Integer.parseInt(startDate[1]),
Integer.parseInt(startDate[2])),
Float.parseFloat( fieldValues[ BankUI.SALARY ] ));
if (!store.isFull())
store.add(record);
else
JOptionPane.showMessageDialog( this, "The Store is full you cannot add\n"+
"anymore employees. \nPlease Save Current File and Create a New File." );
System.out.println("Store full");
store.displayAll();
System.out.println("Count is " + store.getCount());
// output record and flush buffer
//output.writeObject( record );
output.flush();
else
JOptionPane.showMessageDialog( this,
"Account number must be greater than 0",
"Bad account number", JOptionPane.ERROR_MESSAGE );
// clear textfields
userInterface.clearFields();
} // end try
// process invalid account number or balance format
catch ( NumberFormatException formatException ) {
JOptionPane.showMessageDialog( this,
"Bad ID number, Date or Salary", "Invalid Number Format",
JOptionPane.ERROR_MESSAGE );
// process exceptions from file output
catch ( ArrayIndexOutOfBoundsException ArrayException ) {
JOptionPane.showMessageDialog( this, "Error with Start Date or Date of Birth\nPlease enter like: 01-01-2001",
"IO Exception", JOptionPane.ERROR_MESSAGE );
// process exceptions from file output
catch ( IOException ioException ) {
JOptionPane.showMessageDialog( this, "Error writing to file",
"IO Exception", JOptionPane.ERROR_MESSAGE );
closeFile();
} // end if
} // end method addRecord
public static void main( String args[] )
new testing();
} // end class CreateSequentialFileSure you can read and write at the same time. But normally you would be reading from one place and writing to another place.
I rather regret avoiding the OP's earlier post asking how to combine two classes. I looked at the two classes posted and realized the best thing to do was actually to break them into more classes. But I also realized it was going to be a big job explaining why and how, and I just don't have the patience for that sort of thing.
So now we have a Big Ball Of Tar™ and I feel partly responsible. -
Error with instance variable and constructor of base class
I am getting the error on the following statement "protected Point apoint" of by base class. What is causing this problem? See my code:
package Shapes;
public abstract class Shapes
protected Point apoint; / ****error at this line****/
public void move(double xdelta, double ydelta)
apoint.x += xdelta;
apoint.y += ydelta;
public abstract void distance();
public abstract void show();
public String toString()
return "\nThe starting point is at coordinates ("+x+","+y+")";
package Shapes;
public class Lines extends Shapes
protected Point start;
protected Point end;
protected double x;
protected double y;
double delta;
public Lines( final Point apoint, final Point bpoint)
start.x = apoint.x;
start.y = apoint.y;
end.x = bpoint.x;
end.y = bpoint.y;
public Lines(double xstart, double ystart, double xend, double yend)
this.xstart = xstart;
this.ystart = ystart;
this.xend = xend;
this.yend = yend;
public Lines(double x, double y)
this.x = x;
this.y = y;
public void distance(final Lines start, final Lines end)
delta = abs(Math.sqrt (super.x - start.x) * (super.x - end.x) +
(super.y - start.y) * (super.y - end.y));
Lines move(Point start, double delta)
end.x += delta;
end.y += delta;
return new Point(end);
public Lines show()
g.drawLine( start.x, start.y, end.x, end.y);
public String toString()
return super.toString()+ "\n moved distance is("
+ start + ") : (" + end + ")"
+" \n to position(" + start + " ) : (" + end + ")";
package Shapes;
public class Rectangle extends Lines
double delta;
public Rectangle (Point top_lft_pt, Point bottom_rght_pt)
super(top_lft_pt, bottom_rght_pt);
public Rectangle(double x_top_lft, double y_top_lft, double x_bottom_rght, double y_bottom_rght)
super(x_top_lft, y_top_lft, x_bottom_rght, y_bottom_rght);
public Rectantgle is_it_diagonal()
if(top_lft.x > bottom_rght.x && bottom_rght.x < top_lft.x)
System.out.println("Point are Not Diagonal");
public void distance(double delta)
delta = abs(Math.sqrt (top_lft.x - bottom_rght.x) * (top_lft.x - botton_rght.x) +
(top_lft.y - bottom_rght.y) * (top_lft.y - bottom_rght.y));
public Rectangle move(final Point top_lft_pt, double delta)
bottom_rght.x += delta;
bottom_rght.y += delta;
return new Point(bottom_rght_pt);
public void get_top_rght_coords()
Point top_rght = new Point();
top_rght.x = bottom_rght.x;
top_rght.y = top_lft.y;
public void get_bottom_left_coords()
Point bottom_lft = new Point();
bottom_lft.x = top_lft.x;
bottom_lft.y = bottom_rght.y;
public Rectangle show()
g.drawLine(top_lft.x, top_lft.y, top_rght_x, top_rght_y);
g.drawLine(bottom_lft.x, bottom_lft.y, bottom_rght.x, bottom_rght.y);
g.drawLine(top_lft.x, top_lft.y, bottom_lft.x, bottom_lft.y);
g.drawLine(top_rght.x, top_rght.y, bottom_rght.x, bottom_rght.y);
package Shapes;
public class Circles extends Lines
protected double radius;
public double delta;
protected Point center;
public Circles(double x, double y, double radians)
super(x, y);
radius = radians;
public Circles(Point acenter)
center.x = acenter.x;
center.y = acenter.y;
public void distance()
delta = abs(Math.sqrt(super.x - x) * (super.x - x) + (super.y - y) * (super.y -y));
public Circles move(final Point Circles, double delta)
return new Point(Circles);
public void show()
g.drawOval(Circles.x, Circles.y, radius, radius);
package Shapes;
import java .math .*;
import java.util .Random ;
public class ShapesDriver
Shapes[] aShape = new Shapes[10];
static double range = 101.0;
static Random r1 = new Random();
public static void main(String [] args)
double[][] coords = new double[10][10];
double[] radians = new double [10];
Shapes anyShape = {
new Circles(radians),
new Lines(coords),
new Rectangle(coords)
Shapes aShape; /***error at this line***/
for(int i = 1; i <= coords.length; i++)
for(int j = 1; j <= coords.length; j++)
coords[i][j] = r1.nextDouble () * range;
radians[j] = r1.nextDouble () * range;
aShape[i] = anyShape[r1.nextInt(aShape.length)];
System.exit(1);Thanks for your help with this problem. Now I have another problem that I am having a hard time figuring out. My program is using inheritance and polymorphism. I trying to use only one version of methods in the superclass points which extends the shapes class which is the abstract superclass vs. repeating the implementation of the methods (move, show, etc.) in each of the subclasses. The error I am getting is this "declare the class abstract, or implement abstract member 'Point Shapes.move()'. As you see, move is declared abstract and of type point in the Shapes superclass and it's behavior is implemented in the subclass Points extending Shapes. The other subclasses, circle, rectangle and lines will invoke the move method from Points and return a new point object. Why I am I getting the error to declare or implement move when the implementation is in Point?
Below is code, please help?
import java .awt .Point ;
public class ShapesDriver
public static void main(String args[])
int count = 0;
int choice;
Point start = null;
Point end = null;
System.out .println(" 1 i am here");
start = new Point((int)(Math.random()* 10.0), (int)(Math.random()* 10.0));
end = new Point((int)(Math.random()* 10.0), (int)(Math.random()* 10.0));
System.out .println(" 2 i am here");
System.out.println (start);
System.out.println(end);
for(int i = 0; i <= count; i++)
System.out .println(" 3 i am here");
choice = (int)(4.0 * Math.random());
System.out .println(" 4 i am here");
System.out.println (choice);
switch(choice)
case 0: //Lines
System.out .println(" 5 i am here");
Lines apoint = new Lines((int)(Math.random()* 10.0), (int)(Math.random()* 10.0),
(int)(Math.random()* 10.0), (int)(Math.random()* 10.0));
Point coords = new Point((int)(Math.random()* 10.0),
(int)(Math.random()* 10.0));
new Point((int)(Math.random()* 10.0),
(int)(Math.random()* 10.0));
break;
case 1: //Circles
System.out .println(" 6 i am here");
Circles center = new Circles((double)(Math.random()* 10.0),
(double)(Math.random()* 10.0), (double)(Math.random()* 10.0),
(double)(Math.random()* 10.0),(double)(Math.random()* 10.0));
Circles centerpt = new Circles((int)(Math.random()* 10.0),
(int)(Math.random()* 10.0), (int)(Math.random()* 10.0),
(int)(Math.random()* 10.0), (double)(Math.random()* 10.0));
break;
case 2: //Rectangles
System.out .println(" 7 i am here");
Rectangle top_lft_pt = new Rectangle ((double)(Math.random()* 10.0),
(double)(Math.random()* 10.0), (double)(Math.random()* 10.0),
(double)(Math.random()* 10.0), (double)(Math.random()* 10.0),
(double)(Math.random()* 10.0));
Rectangle bottom_rgt_pt = new Rectangle ((double)(Math.random()* 10.0),
(double)(Math.random()* 10.0), (int)(Math.random()* 10.0),
(int)(Math.random()* 10.0), (int)(Math.random()* 10.0),
(int)(Math.random()* 10.0));
break;
default:
System.out .println(" 9 i am here");
System.out.println("\nInvalid shape choice =" + choice);
System.exit(0);
break;
try
System.in.read();
catch(java.io.IOException ioe)
import java .awt .Point ;
public abstract class Shapes
private String shape;
public Shapes(String ashape)
shape = new String(ashape);
public abstract Point move();
public double distance()
return 0.0;
public abstract void show();
public String toString()
return "\nThis shape is a" + shape;
import java .awt .Point ;
public class Rectangle extends Points
protected Point top_lft;
protected Point top_rgt;
protected Point bottom_lft;
protected double top_lft_x;
protected double top_lft_y;
protected double bottom_rgt_x;
protected double bottom_rgt_y;
protected Point bottom_rgt;
protected double delta = 0;
protected String shape = "Rectangle";
public Rectangle(double x, double y, double top_lft_x,
double top_lft_y, double bottom_rgt_x,
double bottom_rgt_y)
super(x, y);
top_lft_x = top_lft_x;
top_lft_y = top_lft_y;
bottom_rgt_x = bottom_rgt_x;
bottom_rgt_y = bottom_rgt_y;
public Rectangle( double x, double y, Point bottom_rgt, Point top_lft)
super(x, y);
bottom_rgt_x = bottom_rgt.x;
bottom_rgt_y = bottom_rgt.y;
top_lft_x = top_lft.x;
top_lft_y = top_lft.y;
public String toString()
return super.toString() + " coordinates top left= " + top_lft_x + "," + top_lft_y + "and bottom right" +
bottom_rgt_x + "," + bottom_rgt_y + ")";
public void is_it_diagonal()
if(top_lft_x < bottom_rgt_x && bottom_rgt_x > top_lft_x)
System.out.println("Points are Not Diagonal");
public double distance()
distance ();
return delta;
public Point move(Point top_lft)
move();
bottom_rgt_x += delta;
bottom_rgt_y += delta;
return top_lft;
public void get_other_coords()
top_rgt.x = bottom_rgt.x;
top_rgt.y = top_lft.y;
bottom_lft.x = top_lft.x;
bottom_lft.y = bottom_rgt.y;
public void show()
super.show();
System.out.println("new coords are :");
System.out .println ("(" + top_lft_x + "," + top_lft_y + ")");
System.out .println ("top right(" + top_rgt + ")");
System.out .println ("(" + bottom_rgt_x + "," + bottom_rgt_y + ")");
System.out .println ("bottom right(" + bottom_rgt + ")");
import java .awt .Point ;
public class Points extends Shapes
protected double delta = 0.0;
protected double x;
protected double y;
protected String ashape = "Points";
public Points( double x, double y)
super("Points");
x = x;
y = y;
public Points( Point start)
super("Points");
x = start.x;
y = start.y;
public String toString()
return super.toString() + "References coordinates(" + x + y + ")";
public double distance(double x1, double y1 )
return delta =(Math.sqrt(x - x1) * (x - x1) + (y - y1) *
(y - y1));
public Point move(Point pts)
pts.x += delta;
pts.y += delta;
return pts;
public void show()
System.out.println("This shape is a" + ashape + "(" +
"(" + x+ "," + y + ")");
import java .awt .Point ;
public class Lines extends Points
double delta = 0;
protected double x;
protected double y;
protected String ashape = "Lines";
public Lines( double x1, double y1, double x, double y)
super(x1,y1);
x = x;
y = y;
public Lines(Point start, Point end)
super(start);
x = end.x;
y = end.y;
public String toString(Point end)
return super.toString() + "line points = (" + x + "," + y +
"(" + end + ")";
public double distance()
distance ();
return delta;
public Point move(Point lines)
move ();
return lines;
public void show()
System.out.println("This shape is a" + ashape + "(" +
"(" + x + "," + y + ")");
import java .awt .Point ;
public class Circles extends Points
protected double radius;
protected double xcenter;
protected double ycenter;
protected String ashape = "Circle";
public Circles( double x, double y, double cx, double cy, double radians)
super(x, y);
xcenter = cx;
ycenter = cy;
radius = radians;
public Circles( Point acenter, Point ref, double radians)
super(ref);
xcenter = acenter.x;
ycenter = acenter.y;
radius = radians;
public String toString(Point ref)
return super.toString() + " reference points (" +
ref + ")" +
"center coordinates (" + xcenter + "," + ycenter + ")";
public double distance(Point ref)
return (Math.sqrt(ref.x - xcenter) * (ref.x - xcenter)
+ (ref.y - ycenter) *
(ref.y - ycenter));
public Point move(Point center)
move();
return center;
public void show()
System.out.println("This shape is a" + ashape + "(" +
"(" + xcenter + "," + ycenter + ")"); -
Set fields of derived class in base class constructor via reflection?
Does the Java Language Specification explicitly allow setting of fields of a derived class from within the base class' constructor via reflection? The following test case runs green, but I would really like to know if this Java code is compatible among different VM implementations.
Many thanks for your feedback!
Norman
public class DerivedClassReflectionWorksInBaseClassConstructorTest extends TestCase {
abstract static class A {
A() {
try {
getClass().getDeclaredField("x").setInt(this, 42);
} catch (Exception e) {
throw new RuntimeException(e);
static class B extends A {
int x;
B() {
B(int x) {
this.x = x;
public void testThatItWorks() {
assertEquals(42, new B().x);
assertEquals(99, new B(99).x);
}why not just put a method in the superclass that the subclasses can call to initialize the subclass member variable?In derived classes (which are plug-ins), clients can use a field annotation which provides some parameter metadata such as validators and the default value. The framework must set the default value of fields, before the class' initializer or constructors are called. If the framework would do this after derived class' initializer or constructors are called, they would be overwritten:
Framework:
public abstract class Operator {
public abstract void initialize();
}Plug-In:
public class SomeOperator extends Operator {
@Parameter(defaultValue="42", interval="[0,100)")
double threshold;
@Parameter(defaultValue="C", valueSet="A,B,C")
String mode;
public void setThreshold(double threshold) {this.threshold = threshold;}
public void setMode(String mode) {this.mode = mode;}
// called by the framework after default values have been set
public void initialize() {
}On the other hand, the default values and other metadata are also used to create GUIs and XML I/O for the derived operator class, without having it instantiated. So we cannot use the initial instance field values for that, because we don't have an instance.
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