Why Immediate and ultimate super class extends in one abstract class ?

What�s a need to extents the two super class in single class
public abstract class ProcessorServlet extends HttpServlet implements Servlet

What�s a need to extents the two super class in
single class
public abstract class ProcessorServlet extends
HttpServlet implements Servlet
Only one class is being extended. HttpServlet.
Servlet is an interface.
You may only extend ONE class. You may implement as many interfaces as you like (there is probably some limit but not worth worrying about).

Similar Messages

Why java does not force to declare atleast one abstract method

hi,
i can define an abstract class without declaring any abstract method in that class. But why wud i do this ? i mean when i have decided that a particular class should be inherited by other subclass and subclass should porvide implementation then there should be atleast one method in the abstract super class which requires implementation.
All i want to know is why java does not force to declare atleast one abstract method in abstract class.
there may be some situations where this restriction can create problem if it is like that then can anybody give some example.
manish

hi,
i didn't get u.
u r trying to say that i have an abstract class with
only static methods then my questions is why wud
declare such a class as 'abstract' class? because a
static method can't be abstract also. Even then if
somebody want to define such a class with only static
methods then compiler should force him to declare
atleast one abstract method which can be implemented
by subclass, because as i said before if sumbody
decide to define a class abstract then he wants that
it should be inhereted but as u r saying a class with
only static methods then it should not be an abstract
class it can be a simple class.there's no functional reason, really... actually, factory-like classes are often defined the way Ceci described
"abstract" only ensures that nobody can ever get an instance of that class (as a matter of fact, what would be the point of getting an instance, if no instance method exists ?)

Why Object is a super class in java?

hi all i have got one basic doubt in java. why Object class is a super class in java. C++ is also a object oriented language but there there is no concept of making object as a super class, but in java why we are having that. thanks

Personally, I find the fact that C++ (and Delphi) DOES NOT have a common base class something of an inconvenience at times. The reason is that Java is an (almost) pure Object Oriented language, while C++ and Delphi are partially-object-oriented additions to C and Pascal respectively.
RObin

Asset Super Class and Asset Super Number

Hi Gurus.
Please tell me how can we use Asset Super Number.
What is the use of Asset Suer Number and Asset Super Class
Thank You

Hi
Do a search on this forum to Asset Super Number. There is a message with the explanation and a direct link to the SAP help.
What is your requerement that you want to use this when you don't know what it is?

Super class methods

How to get the methods of current class and its super class using reflection. The retrieved methods should be only the methods of user-defined classes but not methods of Java APIs such as Object or any other class provided in JDK.
For ex, If Class B is inheriting Class A, i need the methods which are declared only in A & B but not from the Object Class. Similarly, If class C is extending any JDK API such as Thread, I need only the methods declared in class C.

thanku very much for the answer.
I am trying to execute the methods of another class using reflection. So I have to consider the inherited methods also. For that, I have to differentiate between standard classes & user-defined classes.Assume that u have one GUI screen where in once u select a class all its methods including inherited methods have to be displayed.
I can get the the superclass using the method getSuperClass(). But inorder to get the protected methods of superclass what is the method to be used ? If I use getMethods(), it returns only public methods, where as getDeclaredMethods() returns all the methods. Since I have to execute only public & protected methods, what is the best way to solve this ?

Accessing super class private variables from derived class

posted November 01, 2005 08:20 PM Profile for kenji mapes Email kenji mapes Send New Private Message Edit/Delete Post Reply With Quote Assume I have a default and a param constructor in both a subclass and a super class. The members are private.
So after validation logic in the sub class param. constructor, I want to access an instance variable of the super class's default constructor to set the subclass's matching variable to the default in the super class.
Is there anyway I can do this. Of course, I have inherited setters and getters.
Thanks.

posted November 01, 2005 08:20 PM Profile for
kenji mapes Email kenji mapes Send New Private
Message Edit/Delete Post Reply With QuoteI suppose this is the result of an attempted crossposting from another forum. :)

Weird one.. i can't return a variable from the extended to the super class

Hey everyone, i hope i'm not annoying you guys :)
So today's problem is to return a variable (int) from a method of the extended class and print it ont the super class.
I'm just testing the super class , if it works fine.
So the extended class ( FileIO) just read the file txt and return the integer or string ( from the txt file)
I already did a main method to that class and tested it, it works fine.
So now the problem is to print the integer ( that the extended class gets from the txt. ) inside the Super class. I mean , is the same thing but now im testing the Super class , just have to do the same thing, a super class method calls the extended class method and receive the integer from the txt file.
i think the problem is when i create the instance of the FileIO object , maybe its constructor ...i don't know.
The name of the txt file is passed from the super class to the extended class, but i think the error is not from there.
this.aero_le = new BufferedReader(new FileReader(super.ficheiroleitura_aero())); // super calls ficheiroleitura_aero() and receive the name of the txt file ( e.g "temp.txt") so i think that is correct.
here's the code of the Super class public class Aeroporto {
     private String filereader_voo = "temporary.txt";
     private String filereader_aero = "temp.txt";
     private String siglaAero = "";
     public FileIO file;
     public Aeroporto(){};
     public Aeroporto(String filereader_voo, String filereader_aero) throws IOException{
          this.filereader_voo = filereader_voo;
          this.filereader_aero =filereader_aero;
          file = new FileIO();
     public String siglaAero() {
          return siglaAero; }
     public String filereader_aero(){
          return filereader_aero;
public int nrLines() throws IOException{   // it was supose to retunr the number of lines ( integer) from the txt file .
          return Integer.parseInt(file.lerLinhaN(1,1));
// main() {
Aeroporto a = new Aeroporto();
          int v = a.nrLines();
          System.out.print(v);
// ***********************************************************+
// Extended Class
public class FileIO extends Aeroporto{
     private String ficheiroescrita;
     private PrintWriter vooescreve, aeroescreve ;
     private BufferedReader voo_le, aero_read;
     public FileIO(){}
     public FileIO(String filereader_voo, String filereader_aero, String ficheiroescrita) throws IOException {
          super(filereader_voo, filereader_aero);
          this.ficheiroescrita = ficheiroescrita;
          //If file does not exists , create one.
          try{
               this.aero_read = new BufferedReader(new FileReader(super.filereader_aero()));
               aero_read.close();
          catch(IOException ex){
               this.aeroescreve = new PrintWriter(new FileWriter(ficheiroescrita));
               aeroescreve.close();
public String lerLinhaN(int line, int column) throws IOException{ // this method works fine , i already tested this class.
          this.aero_read = new BufferedReader(new FileReader(super.filereader_aero()));
          for(int i = 0; i != line-1; ++i) aero_read.readLine();
          String linha = aero_read.readLine();
          String [] words = linha.split(" ");
          return words[column-1];
Maybe the error is that i use to test the Super class a default contructor on both classes... i don't know where the error is, i also did two small classes ( super and another that extends ) and get the string "Hello" from the super and print it inside the extended..and it works, that's why i think the error is when i call the extended class .. need help.
thanks.

Ok,
This one might actually work... atleast it compiles.import java.io.BufferedReader;
import java.io.FileReader;
import java.io.File;
import java.io.PrintWriter;
import java.io.FileWriter;
import java.io.IOException;
public abstract class FileIO {
     public static boolean CreateOutputFileIfNotExists(
          String outputFilename //the name of the file to ensure exists.
     ) throws IOException
          final String functionName = "FileIO.CreateOutputFileIfNotExists";
          boolean retVal = false;
          //If the output file does does not exist then create it.
          //post condition: output file exists or an IOException has been thrown.
          BufferedReader infile = null;
          try{
               infile = new BufferedReader(new FileReader(outputFilename));
               retVal = true;
          } catch(IOException ex) {
               PrintWriter outfile = null;
               try {
                    outfile = new PrintWriter(new FileWriter(outputFilename));
                    retVal = true;
               } catch(IOException ex2){
                    throw new IOException(functionName + ": cannot create output file " + outputFilename, ex2);
               } finally {
                    outfile.close();
                    if (outfile.checkError()) {
                         throw new IOException(functionName + ": error on output stream " + outputFilename);
          } finally {
               try {
                    infile.close();
               } catch(IOException ex){
                    throw new IOException(functionName + ": cannot close output file " + outputFilename, ex);
          return(retVal);
     public static String readLine(
               String inputFilename //the name of the file to read.
          , int     lineNumber    //1 based number of the line to read from.
     ) throws IOException
          final String functionName = "FileIO.readLine";
          String outputLine = null;
          // reads the numbered "lineNumber" from "inputFilename".
          BufferedReader infile = null;
          try {
               infile = new BufferedReader(new FileReader(new File(inputFilename)));
               for(int i=1; i<lineNumber; ++i) infile.readLine();
               outputLine = infile.readLine();
          } catch(IOException ex){
               throw new IOException(functionName + ": cannot read input file " + inputFilename, ex);
          } finally {
               try {
                    infile.close();
               } catch(IOException ex){
                    throw new IOException(functionName + ": cannot close input file " + inputFilename, ex);
          return(outputLine);
     public static String readWord(
               String inputFilename //the name of the file to read.
          , int lineNumber        //1 based number of the line to read from.
          , int wordNumber        //0 based number of the word to read.
     ) throws IOException
          final String functionName = "FileIO.readWord";
          String outputWord = null;
          // reads the numbered space-seperated "wordNumber" from the numbered "lineNumber" of "inputFilename"
          try {
               String[] words = FileIO.readLine(inputFilename, lineNumber).split(" ");
               if (wordNumber>0 && wordNumber<words.length) outputWord = words[wordNumber-1];
          } catch(IOException ex){
               throw new IOException(functionName + ": cannot read input file " + inputFilename, ex);
          return(outputWord);
}Design notes... FileIO is a generic helper class... there is nothing specific to Airports, flights, or any other "domain" specific stuff in it... so it's re-usable... you can keep it and reuse it on other projects, or even share it with your friends.
So... The airport class will just call the static methods on FileIO like this    ....
    int lineNumber=1;
    int wordNumber=1;
    String airportCode = FileIO.readWord(airportsFilename, lineNumber, wordNumber);
    ....How's that?
corlettk: my now mandatory edit.

Why isn't there a simpler way to initialize a subclass with its super class

Let me explain my doubt with an example...
public class Parent {
�    public String parent;
public class Child extends Parent{
�    public String child;
I've an instance of Parent p. I want to construct Child c, with the data in p.
The only way that is provided by Java language seems to be, having a constructor in Child like
public class Child extends Parent{
�    public String child;
�    public Child(Parent p){
�    �    parent = p.parent;
�    }
The problem with this is there is lot of redundant assignment code.
What I don't understand is why there is not a simpler way of doing this when a subclass is after all super class data + some other data(excuse me for not looking at the bahavior part of it)
I'm looking for something as simple as Child c = p, which I know is wrong and know the reasons too.
Yes, we can wrap Child over Parent to do this, but it necessitates repeating all the methods in Parent.
Why is the language writers didn't provide a simple way of doing this? I should be missing something here...I'm just searching for an explanation. May be I'm asking a dumb question, but this bugs me a lot...
Regards,
Kothapalli.

To answer DrClap, I'm demanding it now :-). Let me
not suggest something that Mr.Gosling didn't think of;
he should be having some reasons in not providing it.Because it's essentially impossible in the general case. One of the reasons you may be extending a class is to add new invariants to the superclass.
eg- extend java.awt.Rectangle by BoundedRectangle - extra invariant that none of its corner points may be less than 0 or more than 100 in any dimension. (The rectangle must be entirely contained in the area (0,0)-(100,100))
What would happen if you try to create a BoundedRectangle from a rectangle representing (50,50)-(150,150)? Complete invariant breakdown, with no generic way to handle it, or even recognise it.
Actually, BIJ and sgabie are asking for it. Provide an
automatic copy constructor. Every object should have
an implicit copy constructor defined. From any
subclass, I should be able to invoke
super(parentClass). From the subclass, we can invoke
super(parentClass) first and then go on to initialize the
subclass data.
I really don't know the implementation issues of this,
but what I'm looking for is something like this. I'm just
curious to know the implementation issues.Implementation issue #1: Classes that should not, under any circumstane, be copied.
* eg 1- Singleton objects. If there's an automatic copy constructor, then multiple singletons can be created, making them essentially useless. This by extension kills the Typesafe Enum pattern also.
* eg 2- Objects with extra information, such as java.sql.Connection - if you copied this, would the copy be using the same socket connection, or would another connection be required? What happens if opening another connection, and the JDBC driver requires the password to be entered for every new connection? If the wrong password is entered, what happens to the newly creating connection?
Implementation issue #2: Copying implementation?
* Already mentioned by RPWithey. The only guaranteed way to perform the copy would be with a shallow copy, which may end up breaking things.
Why can't you write the copy constructor yourself? If it's a special case, it only has to be done once. If it's a recurring case, you could write a code generation tool to write them for you, along with most of the rest of the class.

Problem with subclass and super class

here is the things i wanted to do
/*Write a method that takes the time as three integer arguments (hours, minutes and seconds),
and returns the number of seconds since the last time it was twelve o'clock.
Write a program that uses this method to calculate the amount of time in seconds between two times,
assuming both are within one twelve hour cycle of a clock.
here is a class to find the last time closes to 120'clock in sec.
import java.io.*;
public class Timer {
     int converter = 60;
     int secinTwelveHour = 43200;
     int converter2 = 12;
public int timerTime (int hour, int min, int sec){
          int totalSec = 0;
          //Finding the time
          if (hour > 0 && hour <= 24 && min > 0 && min <=60 && sec > 0 && sec <= 60 && hour <= 11){
               //find last 12 o'Clock
               hour = converter2 + hour;
               //change to sec time
               totalSec = (hour * converter * converter) + (min * converter) + sec;
          }else{
     if (hour > 0 && hour <= 24 && min > 0 && min <=60 && sec > 0 && sec <= 60 && hour >= 12){
               //find last 12 o'Clock in sec
               totalSec = ((hour * converter * converter) + (min * converter) + sec) - secinTwelveHour;
     }else{
          return -1;
}//End of return -1
          }//End of first else statment
     return totalSec;
     }//End of timerTimer
}//End of Program
and here is the super class which uses the class aboved
import java.io.*;
public class FindTime {
public int find2Time (int totalSec1, int totalSec2){
          int timeSec = 0;
          if(Timer.totalSec1 > Timer.totalSec2)
          timeSec = Timer.totalSec1 - Timer.totalSec2;
          else
          timeSec = Timer.totalSec2 - Timer.totalSec1;
     return timeSec;
     }//End of find2Time
public static void main( String [] arg){
     // Construct an instance of the Timer class
          Timer timerClass = new Timer();
          // Make a couple of calls of the method
          int totalSec1 = timerClass.timerTime(12, 3, 45);
          int totalSec2 = timerClass.timerTime(14, 23, 60);
          timeSec1 = find2Time (totalSec1, totalSec2)
          // Now print the values we got back
          System.out.println("Last closes Sec to 12 o'clock" + totalSec1);
          System.out.println("Last closes sec to 12 o'clock" + totalSec2);
          System.out.println("Last closes sec to 12 o'clock" + timeSec);
     }//End of main method
}//End of Program
Now i'm having program with the compliing can anyone help me out like tell me what i'm doing wrong and give me a bit of a code so that i can have a push start
thanks you

Does this do what you want? It is in two seperate classes.
import java.io.*;
public class FindTime {
public static void main( String [] arg){
int timeSec = 0;
// Construct an instance of the Timer class
     Timer timerClass = new Timer();
     // Make a couple of calls of the method
     int totalSec1 = timerClass.timerTime(12, 3, 45);
     int totalSec2 = timerClass.timerTime(14, 23, 60);
     timeSec = java.lang.Math.abs(totalSec1-totalSec2);
     // Now print the values we got back
     System.out.println("Last closes Sec to 12 o'clock " + totalSec1);
     System.out.println("Last closes sec to 12 o'clock " + totalSec2);
     System.out.println("Last closes sec to 12 o'clock " + timeSec);
     }//End of main method
}//End of Program
import java.io.*;
public class Timer {
int converter = 60;
int secinTwelveHour = 43200;
int converter2 = 12;
public int timerTime (int hour, int min, int sec){
     int totalSec = 0;
     //Finding the time
     if (hour > 0 && hour <= 24 && min > 0 && min <=60 && sec > 0 && sec <= 60 && hour <= 11){
     //find last 12 o'Clock
     hour = converter2 + hour;
     //change to sec time
     totalSec = (hour * converter * converter) + (min * converter) + sec;
     } else {
     if (hour > 0 && hour <= 24 && min > 0 && min <=60 && sec > 0 && sec <= 60 && hour >= 12){
     //find last 12 o'Clock in sec
     totalSec = ((hour * converter * converter) + (min * converter) + sec) - secinTwelveHour;
     } else {
          return -1;
     }//End of return -1
}//End of first else statment
return totalSec;
}//End of timerTimer
}//End of Program

In photoshop CS6 extended my 3D-extrusie does not work. Why? And can I fix this? I am a mac user. Pl

In photoshop CS6 extended my 3D-extrusie does not work. Why? And can I fix this? I am a mac user. Please help?

I have paid the full extended version, but it is not working. See my screenshots.

Why does after i i have unzipped and edited the classes...

Why after i have unzipped and edited the classes in the jar file and zipped them together again, it gives me the message "CAN NOT FIND MAIN CLASS"?
This is after i have edited any class in the .jar zip.I did not edit the Manifest text file,can someone who has experienced this problem show me how to overcome it.
Thank you.
Edited by: Blade_runner on Aug 6, 2008 3:04 PM

I hope you realize you're in the wrong forum.
This is not a forum for Java issues experienced when doing some programming on a desktop computer.
This is a forum for questions and issues about the graphical environment named Sun Java Desktop System (JDS) which runs on Solaris and Linux ...
http://en.wikipedia.org/wiki/Java_Desktop_System
Instead, you might consider:
http://forums.sun.com/index.jspa?tab=java

Why notify() and notifyall() is in Object class

pls help me
why notify() and notifyall() methods which are related to thread are define in Object class,instead of defining in Thread class

shouldn't be called on thread objects ever
at all (one of my pet peeves with Java)Why not? It would make a nice entry in the IOJJJ (International
Obfuscated Java Juggling Jamboree) if it existed ;-)First of all, sorry for my bad english. It's early here :)Nah, it's just a late a rainy Sunday afternoon here, so no problem ;-)
Second of all, the way that Thread is implemented, at least on
windows, it that calling Thread.join does a wait() on that thread object.
Thus, if you call notify() on a thread, you could actually be waking up
joined threads, which wouldn't be good. And if you called wait, you
could get woken up if the thread finishes which violates the contract of
wait and notify to some extent and also relies on an undocumented
feature.I take back my previous remark: it could not just be a nice little entry in
the IOJJJ, it would be a great entry in that contest! ;-)
kind regards,
Jos
ps. your example is the first example that clearly gives a reason for
spurious wakeups I've ever read; thanks; joined threads, I've got to
remember that example. <scribble, scribble, scribble/> done. ;-)

Why wait and notify kept in object class only

why wait and notify kept in object class only. is it maintain locks while doing wait and notify . please explain internals,

What do you mean in Object class "only"? If they're in Object, then they're in ALL classes.
They're in Object because any Object can serve as a lock.
For details of how to use them, see a Java threading tutorial such as http://java.sun.com/docs/books/tutorial/essential/threads/

Why addcomponent and removecomponent method are not in component class,

Hi,
why addcomponent and removecomponent method are not in component oand component class,
Thank you

java.awt.Container has the appropriate add and remove methods:
http://java.sun.com/javase/6/docs/api/java/awt/Container.html
If that seems odd to you, you need to think about the Composite Pattern:
http://en.wikipedia.org/wiki/Composite_pattern
[url #" style="display: block; background-image: url('http://upload.wikimedia.org/wikipedia/en/1/13/Compositepattern.png'); width: 406px; height: 271px]

Why do we use super when there is no superclass?

Hello,
I have a question about the word "super " and the constructor. I have read about super and the constructor but there is somethong that I do not understand.
In the example that I am studying there is a constructor calles " public MultiListener()" and there you can see " super" to use a constructor from a superclass. But there is no superclass. Why does super mean here?
import javax.swing.*;
import java.awt.GridBagLayout;
import java.awt.GridBagConstraints;
import java.awt.Insets;
import java.awt.Dimension;
import java.awt.Color;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
public class MultiListener extends JPanel
                           implements ActionListener {
    JTextArea topTextArea;
    JTextArea bottomTextArea;
    JButton button1, button2;
    final static String newline = "\n";
    public MultiListener() {
        super(new GridBagLayout());
        GridBagLayout gridbag = (GridBagLayout)getLayout();
        GridBagConstraints c = new GridBagConstraints();
        JLabel l = null;
        c.fill = GridBagConstraints.BOTH;
        c.gridwidth = GridBagConstraints.REMAINDER;
        l = new JLabel("What MultiListener hears:");
        gridbag.setConstraints(l, c);
        add(l);
        c.weighty = 1.0;
        topTextArea = new JTextArea();
        topTextArea.setEditable(false);
        JScrollPane topScrollPane = new JScrollPane(topTextArea);
        Dimension preferredSize = new Dimension(200, 75);
        topScrollPane.setPreferredSize(preferredSize);
        gridbag.setConstraints(topScrollPane, c);
        add(topScrollPane);
        c.weightx = 0.0;
        c.weighty = 0.0;
        l = new JLabel("What Eavesdropper hears:");
        gridbag.setConstraints(l, c);
        add(l);
        c.weighty = 1.0;
        bottomTextArea = new JTextArea();
        bottomTextArea.setEditable(false);
        JScrollPane bottomScrollPane = new JScrollPane(bottomTextArea);
        bottomScrollPane.setPreferredSize(preferredSize);
        gridbag.setConstraints(bottomScrollPane, c);
        add(bottomScrollPane);
        c.weightx = 1.0;
        c.weighty = 0.0;
        c.gridwidth = 1;
        c.insets = new Insets(10, 10, 0, 10);
        button1 = new JButton("Blah blah blah");
        gridbag.setConstraints(button1, c);
        add(button1);
        c.gridwidth = GridBagConstraints.REMAINDER;
        button2 = new JButton("You don't say!");
        gridbag.setConstraints(button2, c);
        add(button2);
        button1.addActionListener(this);
        button2.addActionListener(this);
        button2.addActionListener(new Eavesdropper(bottomTextArea));
        setPreferredSize(new Dimension(450, 450));
        setBorder(BorderFactory.createCompoundBorder(
                                BorderFactory.createMatteBorder(
                                                1,1,2,2,Color.black),
                                BorderFactory.createEmptyBorder(5,5,5,5)));
    public void actionPerformed(ActionEvent e) {
        topTextArea.append(e.getActionCommand() + newline);
        topTextArea.setCaretPosition(topTextArea.getDocument().getLength());
     * Create the GUI and show it. For thread safety,
     * this method should be invoked from the
     * event-dispatching thread.
    private static void createAndShowGUI() {
        //Create and set up the window.
        JFrame frame = new JFrame("MultiListener");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        //Create and set up the content pane.
        JComponent newContentPane = new MultiListener();
        newContentPane.setOpaque(true); //content panes must be opaque
        frame.setContentPane(newContentPane);
        //Display the window.
        frame.pack();
        frame.setVisible(true);
    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
}

OP wrote:
>
Isn't that simplier to use the constructor instead of using "super"? I mean using the constructor jpanel () ?
>
You can't call the super class constructor directly if you are trying to construct the sub-class. When you call it directly it will construct an instance of the super class but that instance will NOT be an integral part of the 'MultiListener' sub class you are trying to create.
So since 'MultiListener' extends JPanel if you call the JPanel constructor directly (not using super) your code constructing a new instance of JPanel but that instance will not be an ancestor of your class that extends JPanel. In fact that constructor call will not execute until AFTER the default call to 'super' that will be made without you even knowing it.
A 'super' call is ALWAYS made to the super class even if your code doesn't have an explicit call. If your code did not include the line:
super(new GridBagLayout()); Java would automatically call the public default super class constructor just as if you had written:
super();If the sub class cannot access a constructor in the super class (e.g. the super class constructor is 'private') your code won't compile.

Why Immediate and ultimate super class extends in one abstract class ?

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