Question on constructors

Is there a rule on what constructors can accept? I am creating a class here
import java.io.Serializable;
import java.util.Date;
import java.util.ArrayList;
import java.util.List;
public class Meet implements Serializable
   private Long id;
   private Date date;
   private List<Event> events = new ArrayList<Event>();
   public Meet(Long id1, Date date1, List events1)
        this.id=id1;
        this.date=date1;
        this.events=events1;
}But i am after compilation i am getting Note: Recompile with -Xlint:unchecked for details. I know that this message is not neccessarily serious, just interested as to why i would recieve somthing like this, and whether it is related to the constructor. I know your problably wondering why i am not running -Xlint, I have forgotten what i have to write into the compiler settings (or relevant place that needs changing). I was just thinking that as the above class is so small and simple, someone would easily spot why this might be given.

The warning is not related directly to constructors. The last argument of the constructor should be a List of Events, not just a raw type.
public Meet(Long id1, Date date1, List<Event> events1) {
    this.id=id1;
    this.date=date1;
    this.events=events1;
}And the warning will disappear. Read the FAQ of generics for more information.

Similar Messages

Questions on Constructor

Hello all:
I have some doubts on constuctor. Can someone highlight to me?
1) Can I put a constructor in another class?
For example, I have a class, named Interact, which display a panel and some buttons. And another class is call Box. Third class is named Tools. My aim is that when I click the buttons of Interact class, it will invoke some actions accordingly, such as generate a new object of Box or Tools class, or call some methods of Box or Tool class.
Is it necessary to place the constructor of Box and Tools class inside the Interact class? So that can make a handler of these two classes by calling the constructors. Hence to use the handler to call the methods of Box and Tools classes.
I really hate to put a Box constructor inside a class other than Box class. Is there any alternative?
2)Say I have two methods in Box class. One is named "public static double getBoxSize()" in Box class. Another method is named "public doulbe getBoxNumber()". Now I want to call these two methods by click a button in Interact Class. Is it true that I must call the first method with an object name of Box Class. Say"
Box1.getBoxNumber()" ? And for the second method, I don't have to do so because the absence of "static" in the method header? Can I simply call this method by the statement " Box getBoxNumber" ?
3)Lastly, I am really blur about the concept of class handler. According to my teacher's word, class handler is an object of Box class which generated by Interact class. We can communicate between Box class and Interact class by the use of class handler, for example Box1. But as for my understanding, we can do without handler if we declare all methods in Box class in the same way as getBoxNumber() method. " public double getBoxNumber()". By doing so, we can call all the methods in Box class without using a Box class object.
Subconcious tell me that my concept is wrong somewhere. But I don't really see what is the point of using a handler. In fact, I don't quite understand what is a handler. Can someone clear my doubts?
Thanks.

Hello,
To answer all your questions, I have to write a small article :-)
1)You can not define a constructor of a class in another class.
2)You can call methods of an object in any order. But it's upto your application design how you want to invoke them. To call static methods you don't need an instance of a class. It's opposite of instance methods.
3)I think what you mean by handler is an object reference. Object reference is just a pointer(address) to the actual object.

Some question about constructor

The code below shows some of the text for a class called City. As you can see, a City has two instance variables, name and population. Write a complete public constructor (including header line) with no parameters, which sets the population to an initial value of -1, and the name to an initial value of "unknown".
this is the question:
this is what I wrote:
public class City {
private String name;
private int population;
public City(String n, int p){
name = n;
population = p;
public City(){
name = "unknown";
population = -1;
} y is it wrong?

Umass wrote:
The code below shows some of the text for a class called City. As you can see, a City has two instance variables, name and population. Write a complete public constructor (including header line) with no parameters, which sets the population to an initial value of -1, and the name to an initial value of "unknown".
this is the question:
this is what I wrote:
public class City {
private String name;
private int population;
public City(String n, int p){
name = n;
population = p;
public City(){
name = "unknown";
population = -1;
} y is it wrong?The comment about the missing closing brace appears to be correct. It would help if you'd post the compiler message instead of just telling us that it's "wrong". More info, please.
The code you posted isn't wrong, once you make that correction, but it's not the way I'd write it.
Here's what I would do:
public class City
    private static final String DEFAULT_NAME = "unknown";
    private static final int DEFAULT_POPULATION = 0;
    private String name;
    private int population;
    public City()
        this(DEFAULT_NAME, DEFAULT_POPULATION);
    public City(String name, int population)
        setName(name);
        setPopulation(population);
    private void setName(String name)
        if (name == null)
            throw new IllegalArgumentException("name cannot be null");
        this.name = name;
    private void setPopulation(int population)
        if (population < 0)
            throw new IllegalArgumentException("population cannot be negative");
        this.population = population;
}Here's why:
(1) No magic constants. I like having static final constants with a name that spells out what it's for. Better than comments, no ambiguity for users. A negative population makes no sense to me, but zero does.
(2) Objects should be fully initialized and ready to go. Write one constructor that fully initializes the object, every attribute. I like to do it with setters, because I can embed any rules for sensible values there. They protect both the constructor and any public setters that I might have without duplicating code. I made the setters private because I didn't want my City to be mutable.
(3) Have your default constructor call the full constructor. It makes clear what the default values are (in javadocs, preferrably).
%

A question about constructor

I got this piece of code from a book:
public class Bala{
int x, y;
Bala(int x1, int y1){
x=x1;
y=y1;
Bala(final Bala bala){ //here
x=bala.x;
y=bala.y;
What is the purpose of "final" here. I wrote a simple test programm to do the constructor with and without "final" and couldn't see any differences. Any help would be appreciated.

There is no need for final in your case, but look at this example:
public void init(final Bala bala) {
Button b = new Button();
b.addActionListener(new ActionListener() { // anonymous inner class
    public void actionPerformed(ActionEvent e) {
      // This part of the code is run when you press the button.
      // you can use bala here in this inner class,
      // but only because you made bala final.
     // (Doesn't matter if you do it from a method or constructor)
      bala.x = 5;
}I don't know other reasons where you would make it final. Making bala final, means that you can't modify it inside the init() method, i.e. you can't refer to another bala object, and you can't set it to null.

Question about constructors and inheritance

Hello:
I have these classes:
public class TIntP4
    protected int val=0;
    public TIntP4(){val=0;}
    public TIntP4(int var)throws Exception
        if(var<0){throw new Exception("TIntP4: value must be positive: "+var);}
        val=var;
    public TIntP4(String var)throws Exception
        this(Integer.parseInt(var));
    public String toString()
        return val+"";
public class TVF extends TIntP4
}Using those clases, if I try to compile this:
TVF inst=new TVF("12");I get a compiler error: TVF(String) constructor can not be found. I wonder why constructor is not inherited, or if there is a way to avoid this problem without having to add the constructor to the subclass:
public class TVF extends TIntP4
     public TVF (String var)throws Exception
          super(var);
}Thanks!!

I guess that it would not help adding the default constructor:
public class TVF extends TIntP4
     public TVF ()
           super();
     public TVF (String var)throws Exception
          super(var);
}The point is that if I had in my super class 4 constructors, should I implement them all in the subclass although I just call the super class constructors?
Thanks again!

How would i design the relationship between "question", "subquestion", and "answer"

Hi all. Consider the following scenario:
Scenario:
A Question has an Answer, but some Questions have Subquestions. For example:
1. Define the following terms: (Question)
a) Object (1 marks) (Subquestion)
An instance of a class. (Answer)
b) ...
2. Differentiate between a constructor and a destructor (2 marks)
(Question)
A constructor constructs while a destructor destroys.
(Answer)
Question:
I want to model Questions, Subquestion, and Answer as Entities with relationships/associations, preferably binary relationships as i feel ternary relationships will be problematic while programming. Any suggestion on how i would
go about this?
There is never infinite resources.
For the Question Entity, a question has the attributes "QuestionPhrase <String>", "Diagram<Binary>", and "Marks
<Decimal>".
For the SubQuestion Entity, a subquestion has the attributes "SubQuestionPhrase <String>", "Diagram<Binary>", and "Marks <Decimal>".
For the Answer Entity, an answer has attributes, "AnswerPhrase<String>", "Diagram <Binary>"

Yes. I am in .Net. I sure do hope i did not ask in the wrong forum. :-|
Hi KCWamuti,
If you need to design the relationship between Question table and Answer table in SQL Server, as Uri’s and Visakh’s posts, you can create the foreign key to establish relationship between tables, and use join in query to get your desired result. For more
information about JOIN in SQL Server, please review this article:
Different Types of SQL Joins.
However, if you need to model Questions, Subquestion, and Answer as Entities in .Net, then the issue regards data platform development. I suggest you post the question in the Data Platform Development forums at
http://social.msdn.microsoft.com/Forums/en-US/home?category=dataplatformdev . It is appropriate and more experts will assist you.
Thanks,
Lydia Zhang

Invoked super class constructor means create an object of it?

Hei,
i have create one class that extends another class. when i am creating an object of sub class it invoked the constructor of the super class. thats okay.
but my question is :
constructor is invoked when class is intitiated means when we create an
object of sub class it automatically create an object of super class. so means every time it create super class object.

Hi,
   An object has the fields of its own class plus all fields of its parent class, grandparent class, all the way up to the root class Object. It's necessary to initialize all fields, therefore all constructors must be called! The Java compiler automatically inserts the necessary constructor calls in the process of constructor chaining, or you can do it explicitly.
   The Java compiler inserts a call to the parent constructor (super) if you don't have a constructor call as the first statement of you constructor.
Normally, you won't need to call the constructor for your parent class because it's automatically generated, but there are two cases where this is necessary.
   1. You want to call a parent constructor which has parameters (the automatically generated super constructor call has no parameters).
   2. There is no parameterless parent constructor because only constructors with parameters are defined in the parent class.
   I hope your query is resolved. If not please let me know.
Thanks & Regards,
Charan.

BigVic Constructor? Constructors?

I'm taking an online Computer Science AP course and its all about Java. I've done pretty good so far, but I have a question about constructors. My instructor encouraged me to use Google to help me with my assignments and various Java questions.
I am supposed to write out the BigVic constructor that this needs. What exactly do I need to do? If possible, please don't give me the answer, just lead me in the right direction as to what I'm supposed to do. Also, maybe someone could give me a good explanation on constructors? I'm pretty confused over that.
Also, if its not too much trouble, could I get a simple definition for:
Classes
Constructors
Methods
Objects
I understand the general concept of those, but a specific definition would help to clarify some of my questions.
This is what is already given to me:
public class BigVic extends Looper
private int itsNumFilled; // count this's filled slots
private int itsNumEmpty; // count this's non-filled slots
public BigVic() // constructor
{    // left as an exercise (4.34)
} //======================
/** Tell how many of the executor's slots are filled. */
public int getNumFilled()
{    return itsNumFilled;
} //======================
/** Tell how many of the executor's slots are empty. */
public int getNumEmpty()
{    return itsNumEmpty;
} //======================
/** Do the original putCD(), but also update the counters. */
public void putCD()
{    if ( ! seesCD() && stackHasCD())
{    itsNumFilled++;
itsNumEmpty--;
super.putCD();
} //======================
/** Do the original takeCD(), but also update the counters.*/
public void takeCD()
{    if (seesCD())
{    itsNumFilled--;
itsNumEmpty++;
super.takeCD();
} //======================
}

Pluberus wrote:
I am supposed to write out the BigVic constructor that this needs. What exactly do I need to do? As you haven't told us about BigVic's requirements, your guess is as good or better than ours.
Also, if its not too much trouble, could I get a simple definition for:
Classes
Constructors
Methods
ObjectsSure, but do what your instructor stated: use Google for this. Why ask us to regurgitate that which is well documented online? I'd start at Wikipedia and at the Sun Java tutorials.

How to create a new class

Hi,
I'm trying to create a new class called Questions, which has
two properties (type and condition).
The action script is saved in the folder myComponents under
the project.
When I compile the Main app, I get the following error
message:
"1172 Definition myComponents:Question could not be found."
Why?
How do you call a constructor for a new class, that is not an
extension of an existing class?
Regards
/Acke
//-----------Class----------//
package myComponents
// Import all classes in the mx.events package
import mx.events.*;
public class Question {
// Public constructor.
public var Question(){
// Call the constructor in the superclass.
//super();
// Define properties and methods.
// Define public vars.
public var type:string;
public var condition:string;
//------Main----------//
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="
http://www.adobe.com/2006/mxml"
layout="absolute"
xmlns:MyComp="myComponents.*">
<mx:Script>
<![CDATA[
import mx.controls.Alert;
import mx.events.CloseEvent;
import myComponents.Question;
etc, etc....

quote:
Originally posted by:
MannenMytenLegenden
Hi,
I'm trying to create a new class called Questions, which has
two properties (type and condition).
The action script is saved in the folder myComponents under
the project.
When I compile the Main app, I get the following error
message:
"1172 Definition myComponents:Question could not be found."
Why?
How do you call a constructor for a new class, that is not an
extension of an existing class?
Regards
/Acke
//-----------Class----------//
package myComponents
// Import all classes in the mx.events package
import mx.events.*;
public class Question {
// Public constructor.
public var Question(){
// Call the constructor in the superclass.
//super();
// Define properties and methods.
// Define public vars.
public var type:string;
public var condition:string;
//------Main----------//
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="
http://www.adobe.com/2006/mxml"
layout="absolute"
xmlns:MyComp="myComponents.*">
<mx:Script>
<![CDATA[
import mx.controls.Alert;
import mx.events.CloseEvent;
import myComponents.Question;
etc, etc....
Here is a test class I created a while back, although this
one does extend Object it might help you out a bit with your
constructor issue:
package com.evilest
import mx.controls.Alert;
public class Testing_class extends Object
private var message:String = new String();
public function Testing_class(input:String) {
this.message = input;
public function showAlert():void {
Alert.show(this.message, "sup");
}

A doubt on overloading

it is said that functions with same name and signature can't be overloaded only by different return types.
but when i coded like as below,
class sub{
// declarations
sub(){
// initializations
int sub(){
return 100;
the above code did'nt rise errors, and works simply as some function in the class. when invoked using the object it returns the integer 100.
could anybody , please explain me the concept of java behind this.

its not a question whether constructors are method or
not. the question is overloading.No, the issue, in essence, is exactly that constructors are not methods. In the OP, we have a base constructor and a subclass method with the same name.
The subclass method is not an "overload with different return type" (the actual question) because:
- the base ctor is not a method
=> it is not inherited
=> therefore, it cannot be overloaded in the subclass.
Even within the base class, you cannot "overload a constructor by a method" because again, a constructor is not a method. You are allowed to give your class a method with a constructor name (although it is discouraged) but it doesn't qualify as an overload.
seocndly we can call constructors as special type of
functions but i think it is hardly used.Care to demonstrate that?

HELP i have a final tomorrow!

let's say we have public class Animal, it contains some constants
public static final int ANT;
public static final int CAT; etc..
there is a constructor in class Animal,
Animal a = new Animal (Animal.SHARK);
give JUST the signature for this constructor as it might appear in the book.....i know this is probably a dumb question, but could anyone explain the answer to this??? thanks

Hello Rocket,
To answer your question, the constructor signature would look like:
public Animal (int animalType)
//example:
public class Animal {
public static final int ANT=1;
public static final int CAT=2;
public static final int SHARK=3; //etc..
public Animal(int animalType) {
//..whatever code you need to process
public static void main (String [] args) {
Animal a=new Animal (Animal.SHARK);
The reason why its a constructor signature is that the computer knows which method to call because on the parameters passed in when instantiating a new object from the class (Animal in this case). Remember that a class is a blueprint for creating objects (instantiation).

Accordian panel "flashes"

I am using 1.5 files, accordian widget with images as
content. when changing panels, the content flashes strongly.
Distracting.
try this link:
http://www.jwcooper.com/NEWLOOK_edge_prod.html
How can i stop this? If i try "easing" or duration changes,
how do i do this? examples suggest "passing on" to constuctors and
things, but what does that mean?
Would i change the actual js file, or add code to the page?
If there is a process described for "passing on to constructors"
please direct me to this for my understanding.
thanks

You didn't say what browser you are using, so I'm guessing
that it is flashing in IE? Your pages seem to look fine in the
other browsers.
Try specifiying a full doctype to prevent IE from going into
"Quirks Mode". Your doctype header currently looks like this:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01
Transitional//EN">
<html>
Try using:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01
Transitional//EN" "
http://www.w3.org/TR/html4/loose.dtd">
<html>
or:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0
Transitional//EN" "
http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="
http://www.w3.org/1999/xhtml">
Regarding your questions about constructors, take a look at
this document, and hopefully it will answer some of your questions:
http://labs.adobe.com/technologies/spry/articles/accordion_overview/index.html
--== Kin ==--

Urgent constructor question..

I'm trying to call this Flower class in another class called Garden, in which I created a new Flower by using a statement
private Flower lastflower;
and it's saying it cannot find the symbol - constructor Flower.
Can anyone tell me why and help correct this problem?
Below is the code for my Flower class.
Any help is really appreciated, it's for my Java class!
import objectdraw.*;
import java.awt.*;
* Write a description of class Flower here.
* @author (your name)
* @version (a version number or a date)
public class Flower
    protected FilledOval dot;
    protected FilledRect stem;
    protected FilledOval petal1;
    protected FilledOval petal2;
    protected static final int boundary = 100;
    protected RandomIntGenerator colorGen =
            new RandomIntGenerator(0,255);
    protected Color petalColor;
    protected Boolean flowerContains=false;
    private DrawingCanvas canvas;
    public void changeColor(){
    dot = new FilledOval(150,150,15,15, canvas);
    dot.setColor(Color.YELLOW);
    petalColor = new Color(colorGen.nextValue(),
                                colorGen.nextValue(),
                                colorGen.nextValue());
    petal1.setColor(petalColor);
    petal2.setColor(petalColor);
    public void grow(Location point){
    stem = new FilledRect (dot.getX()+3, dot.getY()+10, 10, 10, canvas);
    stem.setColor(Color.GREEN);
    if (dot.getY()>boundary){
        dot.move(0,-4);
    else{
     petal1 = new FilledOval(dot.getX()-12, dot.getY()-25, 40,70,canvas);
     petal2 = new FilledOval(dot.getX()-25, dot.getY()-10, 70,40,canvas);
     dot.sendToFront();
     stem.sendToBack();
     petal1.setColor(petalColor);
     petal2.setColor(petalColor);
    public Boolean flowerContains(Location point){
        if (petal1.contains(point)){
            return true;
        else if (petal2.contains(point)){
            return true;
        else if (dot.contains(point)){
            return true;
        else{
            return false;
}

I don't care how fucking urgent you think it is, it isn't to us. We will answer your question when and how we feel llike it. Have some manners and if you must, then bump your original post. Don't create another time wasting piece of crap!

Constructor Inheritance Question

Here's a quote from the Java Tutorials at http://java.sun.com/docs/books/tutorial/java/javaOO/objectcreation.html :
"All classes have at least one constructor. If a class does not explicitly declare any, the Java compiler automatically provides a no-argument constructor, called the default constructor. This default constructor calls the class parent's no-argument constructor, or the Object constructor if the class has no other parent. If the parent has no constructor (Object does have one), the compiler will reject the program."
In order to fully understand this concept, I created two classes: a ClassParent and a ClassChild.
public class ClassParent
    public static void main(String[] args) {
       ClassParent tester = new ClassParent();
public class ClassChild extends ClassParent
    public static void main(String[] args) {
        ClassChild child = new ClassChild();
}Both classes compiled successfully, which raised the following question:
I understand that the ClassParent default constructor calls the Object's no-argument constructor.
Does the ClassChild also call the Object's constructor once it realizes that the ClassParent does not have a no-argument constructor?
And a somewhat non-related question:
Seeing how ClassParent calls Object's no-argument constructor if it does not have one of its own, does it also extend the Object class?
Edit: After running the following code, I realized that the answer to my last question is yes:
public class ClassParent
    public static void main(String[] args) {
       ClassParent tester = new ClassParent();
       boolean test = tester instanceof Object;
       System.out.println(test);
}Edited by: youmefriend722 on May 26, 2008 1:54 PM

youmefriend722 wrote:
I think I'm getting a basic grasp now but want to make sure that I'm not misunderstanding anything.
Constructor inheritance:
If a no-argument constructor is invoked but one isn't declared in that class, the superclass's no-argument constructor will be invoked. Well, sort of. If you invoke a constructor that doesn't exist, you get an error. Keep in mind that the invocation and the constructor may both be automatically supplied by the compiler, and that the compiler won't automatically create a no-arg constructor for a class if you define any constructors for that class.
So if you don't define any constructors in a class, then a no-arg one is created automatically (at compile time) and (at runtime) when you instantiate that class, that no-arg constructor will try to invoke the superclass's no-arg constructor.
But suppose you do define a constructor, one that takes an argument. Then if you try to invoke a no-arg constructor on that class, you'll get an error, and the superclass's no-arg constructor won't be invoked (because the error happens first).
If the superclass does not have a constructor, then the superclass's superclass's constructor will be invoked.No. That never happens. Every class has a constructor (although it might have permissions which make it inaccessible, which is a whole other issue we'll worry about later).
If the superclass does have a constructor but doesn't have a no-argument constructor, then there will be a compile-time error.Only if you try to invoke the no-arg constructor, which might happen implicitly. For example, if you write a superclass with a 2-arg constructor, and you write a subclass with a 1-arg constructor, and the 1-arg subclass's constructor invokes the superclass's 2-arg constructor, then there's no problem, even though in this example, the superclass doesn't have a no-arg constructor.
Constructors in general:
In every constructor, the superclass's no-argument constructor will be invoked if the none of the superclass's constructors are explicitly invoked.Yeah, I think that's right.

Anyone heard of static Constructor? Pure Concept Question

Hi All,
I guess this is a little obscure but let me ask this anyway.
Has anyone heard about a "static constructor"? No I am not talking about a static block in your code. I am talking about a static method which will be called every time you make a static call on a particular class. Just like the constructor gets called every time you create an Object. The difference will be that the static code will be executed only once while the static constructor will be called every time you call a static method.
My question: Do you see a need, from the experience you have got, about such a construct being a feature of a language? I was talking to some people of a very well known company - who were telling me about this special feature - as they put it - being part of their Proprietary language. They couldn't convince me as to why this "special" feature was required. I seek help from the Community to help me think about this. I tried seeking an answer to this question on the net but to no avail.
I for one, am not going to stop my quest after posting here - but I would be grateful if any of you could help me.
Thanks for all your support.
Best Regards,
Manish

Hi All,
I am overwhelmed by the response. Thanks for spending your time thinking about this.
Some of the questions you asked were...
teknologikl : Did the people say about how is it useful?
javax.pert : No. They tried to tell me things like initializing static objects which I could have as well done in a common method which I call in the start of every static method. Basically they could not convince me about the use of a static constructor. May be they did not know too well why it was put in the first place. Because I was talking to programmers rather than Designers. But I did not want to stop at that. I wanted to tickle my gray cells and yours to find out why would someone put this as part of a language? I agree with most of your posts but instead of telling me that this is not going to be very useful ( which I already thought of), I would appreciate if you can help me think Why would someone have put it at all. You see what I am saying?
rjwr: static factory methods
javax.pert: Yup. I am aware and use static factory methods. But this is a little different. This is called automatically by the VM - just like a normal constructor - whenever a static method is called.
dubwai: This sounds pretty lame to me...
javax.pert: Sorry if I wasted your time. But please look at my response to teknologikl above.
DrClap:
javax.pert: Yup, yup. I thought just the way you are saying. I am actually trying to get in touch with some one who knows more about this proprietary language - maybe one of the designers. I would be glad to share the findings when it comes - if it is worth sharing of course. Thanks for your time though. :)
rvflannery: Security framework.
javax.pert: True. I agree with you about this being a wrapper for security purposes. Maybe this is what they thought of. One reason I see is that because they expose the APIs to third party vendors. These APIs can talk, by extending certain objects, to the sensitive areas of the database. I guess this could be one of the reasons. I will share it once I know more about it.
trejkaz: For debugging.
javax.pert: See my answer to rvflannery above.
Thanks all of you for spending your time with me. I will keep you informed whenever I get something worth sharing.
Thanks again,
Best Regards,
Manish

Question on constructors

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