About main() method

can any body tell me where the main method is declear.....is main () method written in java compiler or in a package.....????

bubun_java wrote:
main is a starting position of a program is written in jvmWhat?
..does it write..??More what?
Look I don't know what you're on about with the jvm business. Main is a method you write. Then when you invoke java it looks for a main method in the class you provide. Or if this is a jar with a manifest it tries to find a main method in the classname provided in the manifest.
There's no magic here. No secret methods or magic classes here.
If you have a specific doubt then please give some example code or an example situation that explains it. Saying things like "main is a starting position of a program is written in jvm" is not unhelpful to use in clearing your doubt because it doesn't make any sense.

Similar Messages

Bouncing Ball without Main Method

Hi. I needed to reserch on the Internet sample code for a blue bouncing ball which I did below. However, I try coding a main class to start the GUI applet and it's not working. How can I create the appropriate class that would contain the main method to start this particular application which the author did not provide? The actual applet works great and matches the objective of my research (http://www.terrence.com/java/ball.html). The DefaultCloseOperation issues an error so that's why is shown as remarks // below. Then the code in the Ball.java class issues some warning about components being deprecated as shown below. Thank you for your comments and suggestions.
Compiling 2 source files to C:\Documents and Settings\Fausto Rivera\My Documents\NetBeansProjects\Rivera_F_IT271_0803B_01_PH3_DB\build\classes
C:\Documents and Settings\Fausto Rivera\My Documents\NetBeansProjects\Rivera_F_IT271_0803B_01_PH3_DB\src\Ball.java:32: warning: [deprecation] size() in java.awt.Component has been deprecated
size = this.size();
C:\Documents and Settings\Fausto Rivera\My Documents\NetBeansProjects\Rivera_F_IT271_0803B_01_PH3_DB\src\Ball.java:93: warning: [deprecation] mouseDown(java.awt.Event,int,int) in java.awt.Component has been deprecated
public boolean mouseDown(Event e, int x, int y) {
2 warnings
import javax.swing.*;
public class BallMain {
* @param args the command line arguments
public static void main(String[] args) {
Ball ball = new Ball();
//ball.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
ball.setSize( 500, 175 ); // set frame size
ball.setVisible( true ); // display frame
import java.awt.*;*
*import java.applet.*;
import java.util.Vector;
// Java Bouncing Ball
// Terrence Ma
// Modified from Java Examples in a Nutshell
public class Ball extends Applet implements Runnable {
int x = 150, y = 100, r=50; // Position and radius of the circle
int dx = 8, dy = 5; // Trajectory of circle
Dimension size; // The size of the applet
Image buffer; // The off-screen image for double-buffering
Graphics bufferGraphics; // A Graphics object for the buffer
Thread animator; // Thread that performs the animation
boolean please_stop; // A flag asking animation thread to stop
/** Set up an off-screen Image for double-buffering */
public void init() {
size = this.size();
buffer = this.createImage(size.width, size.height);
bufferGraphics = buffer.getGraphics();
/** Draw the circle at its current position, using double-buffering */
public void paint(Graphics g) {
// Draw into the off-screen buffer.
// Note, we could do even better clipping by setting the clip rectangle
// of bufferGraphics to be the same as that of g.
// In Java 1.1: bufferGraphics.setClip(g.getClip());
bufferGraphics.setColor(Color.white);
bufferGraphics.fillRect(0, 0, size.width, size.height); // clear the buffer
bufferGraphics.setColor(Color.blue);
bufferGraphics.fillOval(x-r, y-r, r*2, r*2); // draw the circle
// Then copy the off-screen buffer onto the screen
g.drawImage(buffer, 0, 0, this);
/** Don't clear the screen; just call paint() immediately
* It is important to override this method like this for double-buffering */
public void update(Graphics g) { paint(g); }
/** The body of the animation thread */
public void run() {
while(!please_stop) {
// Bounce the circle if we've hit an edge.
if ((x - r + dx < 0) || (x + r + dx > size.width)) dx = -dx;
if ((y - r + dy < 0) || (y + r + dy > size.height)) dy = -dy;
// Move the circle.
x += dx; y += dy;
// Ask the browser to call our paint() method to redraw the circle
// at its new position. Tell repaint what portion of the applet needs
// be redrawn: the rectangle containing the old circle and the
// the rectangle containing the new circle. These two redraw requests
// will be merged into a single call to paint()
repaint(x-r-dx, y-r-dy, 2*r, 2*r); // repaint old position of circle
repaint(x-r, y-r, 2*r, 2*r); // repaint new position of circle
// Now pause 50 milliseconds before drawing the circle again.
try { Thread.sleep(50); } catch (InterruptedException e) { ; }
animator = null;
/** Start the animation thread */
public void start() {
if (animator == null) {
please_stop = false;
animator = new Thread(this);
animator.start();
/** Stop the animation thread */
public void stop() { please_stop = true; }
/** Allow the user to start and stop the animation by clicking */
public boolean mouseDown(Event e, int x, int y) {
if (animator != null) please_stop = true; // if running request a stop
else start(); // otherwise start it.
return true;
}

FRiveraJr wrote:
I believe that I stated that this not my code and it was code that I researched.and why the hll should this matter at all? If you want help here from volunteers, your code or not, don't you think that you should take the effort to format it properly?

Java programming language main method question?

Hello everyone I am quite new to the Java programming language and I have a question here concerning my main method. As you can see I am calling 4 others methods with my main method. What does the null mean after I call the method? I really don't understand is significance, what else could go there besides null?
public static void main(String[] args)
          int cansPerPack = 6;
          System.out.println(cansPerPack);
          int cansPerCrate = 4* cansPerPack;
          System.out.println(cansPerCrate);
          have_fun(null);
          user_input(null);
          more_java(null);
          string_work(null);
     }Edited by: phantomswordsmen on Jul 25, 2010 4:29 PM

phantomswordsmen wrote:
..As you can see I am calling 4 others methods with my main method. 'Your' main method? Your questions indicate that you did not write the code, who did?
..What does the null mean after I call the method?.. 'null' is being passed as an argument to the method, so there is no 'after the method' about it.
..I really don't understand is significance, what else could go there besides null? That would depend on the method signatures that are not shown in the code snippet posted. This is one of many reasons that I recommend people to post an SSCCE *(<- link).*
BTW - method names like have_fun() do not follow the common nomenclature, and are not good code for a newbie to study. The code should be put to the pointy end of your sword.

Instantiation outside main method

Why can't you do the following? I get an error message: Java programs\Exp.java:14: non-static variable scan cannot be referenced from a static context
boxes = scan.nextInt(); I don't understand this message since you can instantiate objects and initialize variables within a class and outside a method with no problem. What's so special about the main method that you cannot do this? By the way, I know how to fix this. I just want to know why you can't do this.
import java.util.Scanner;
public class Exp
     Scanner scan = new Scanner(System.in);
     public static void main(String[] args)
          int boxes;
          System.out.println("Enter a number: ");
          boxes = scan.nextInt();
          if(boxes > 5)
               System.out.println("So what!");
          else
               System.out.println("Less than!");
               System.out.println("I don't know.");
}

afried01 wrote:
Well, if the objects are instantiated outside the main method then none of them could be used inside that method. You would have to use them outside the main method or any other static method, that is, if I'm getting your point correctly.No. You could still use them within the main method, but first you'd have to instantiate an object of the class.
Keep in mind that your main method should be pretty lightweight. You should only use it to bootstrap your program (i.e., to start it up at the beginning). So generally you'd instantiate the primary classes of your application, maybe call an initialization or start method on one of them, etc. If your main method is more than 15 or so lines of code it's probably too heavy.
Or you could just make the field (outside the main class) static, so the (static) main method could use them, but as mentioned already that's generally not the right approach.

Packages and Main method.

What is the reason for the following. When you explicitly specify a package name for a class that contains the main method, why do you have to specify the fully qualified class name when you invoke the JVM?
For example:
package Exercises.Chp8.ExerciseTwo;
import Exercises.Chp8.Interfaces.*;
class Exercise2 implements myInterface {
     public void method1() {
          System.out.println("method 1 implemenmted");
     public void method2() {
               System.out.println("method 2 implemenmted");
     public void method3() {
               System.out.println("method 3 implemenmted");
public class testExercise2 {
     public static void main(String[] args) {
          Exercise2 ex = new Exercise2();
          ex.method1();
          ex.method2();
          ex.method3();
I would have to issue the following command:
java Exercises.Chp8.ExerciseTwo.testExercise2
for the java to run. If I specify java testExercise2, it displays a stack trace of the classLoader and an error messages reads regarding "wrong name". Can someone explain why this is?

Hi,
Please refer the following URL.
It is very well documented in the java site.
Please refer these URL's
http://java.sun.com/docs/books/tutorial/java/interpack/packages.html
http://java.sun.com/docs/books/tutorial/java/interpack/createpkgs.html
http://java.sun.com/docs/books/tutorial/java/interpack/usepkgs.html
http://java.sun.com/docs/books/tutorial/java/interpack/managingfiles.html
Also please refer this URL, which has a discussion about packages.
http://forum.java.sun.com/thread.jsp?forum=31&thread=151016
I hope this will help you.
Thanks
Bakrudeen

About calling method with arguments

Hi,
I have a problem about calling method using reflection. my method is like follows:
public myMethod(Integer var1, MyObject mobj) {
I've tried to call the method using the following code,
Class[] parameterTypes = new Class[] {Integer.class, MyObject.class};
Object[] arguments = new Object[] {new Integer(2), mobj};
Method met=cl.getMethod("myMethod", parameterTypes);
But the in the last line NoSuchMethodException is thrown.
How can I send the reference of MyObject to myMethod()?
Thanx
rony

Should work ok:
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
public class Test {
     static class MyObject {}
     public static void main(String[] args) throws Exception {
          Class c = Test.class;
          Class[] parameterTypes = new Class[] {Integer.class, MyObject.class};
          try {
               Object[] arguments = new Object[] {new Integer(2), new MyObject()};
               Method met = c.getMethod("myMethod", parameterTypes);
               met.invoke(new Test(), arguments);
          } catch (NoSuchMethodException e) {
               System.out.println(e);
          } catch (IllegalAccessException e) {
                 System.out.println(e);
          } catch (InvocationTargetException e) {
               System.out.println(e);
     public void myMethod(Integer var1, MyObject mobj) {
          System.out.println("myMethod");
}

Synchronizing methods and a static main method

I have a class with a static main method, calling 3 different jdbc updates.
I want to make sure this is thread safe when called for various clients.
It's goes:
args[0] = clientI
update 1:update field1 where clientid = client1
update 2:update field2 where clientid = client1
update 3:update field3 where clientid = client1I assume I am in jeopardy of mixing up my data?
I don't think I can synchronize a main method, nor would it do any good to synchronize the class?
Should I break the updates into methods, then synchronize them, or synchronize parts of the code?
you opinion would help greatly.

You haven't provided enough information.
Are those updates occurring sequentially? Then it's threadsafe.
Are they running in different threads? Then to make them threadsafe you'll need proper synchronization, or a database transaction.
Of course, looking at those particular updates, I don't see any reason why you'd need to worry about thread safety. They look to be independent of each other. Or perhaps you meant you need them to be atomic? That is, any query will see either the old values of all three fields, or the new values of all three fields. Nobody will be able to see the new values for fields 1 and 2, but the old value for field 3. If that's the case, then those three updates need to be in a database transaction.

How to run executable Java app from command line: No Main( ) method

This is the opposite question from what most users ask. I have the Java source code and a running Java executable version of the program in a windows environment. I don't want to run it in the normal way by double-clicking on it or using the executable. I need to be able to run the same application from the command line. If I can do this, I can then compile and run the code on my Mac, too, which is what I am ultimately trying to accomplish.
Normally, this would be easy. I would look for a Main method, normally public static void main (String[] args ) or some variation. I cannot find a main() anything, anywhere when searching the files. There must be some windows executable that links to the Java classes. It's very clever, no doubt, but that's not helping me.
I am trying to run what I can determine as the Main class, with a standard command line call, but that is not working either.
It is a Java graphics program, so mostly Swing container stuff.
I am not finding a manifest.txt file, telling me where a main method is, nor anything about the Main Class. There is no readme file for the program.
Any insight that would point me in the right direction? What should I be looking for?
Thanks in advance, Bloozman

It's possible there's a clever Windows executable that runs the class. But if it's a plain old Java class then there's a good chance there's a constructor. Look for that; it's possible you may be able to start by writing another Java class that creates an instance of your mystery class.
When you have that done, try running it. It's possible that just creating an instance might be sufficient to run it. Swing programs are sometimes written like that. If not, then start looking for methods with names like "run" or "go" or "start" that your little controller program can call.

Beginner - class & main method question

Hello, I am still really new to OOP and Java. I came across an example in the book I am using that threw me for a loop.
ex.
class MyClass {
public static void main(String [] args) {
int x = 42;
MyClass y = new MyClass();
int z = y.go(x);
int go(int arg) {
arg = arg * 2;
return arg;
what I am confused about is the creation of a new class that contains the main method (MyClass y = new MyClass()), wouldn't this create a new class that also has a main method? Or is this the kind of code I will never encounter outside an example in a book and shouldn't worry about it :-)
Thanks

Since the main method is static, it would be invoked through MyClass.main(...). Note that this is not actually creating an object, just calling a static method.
Creating an object is an entirely different story. MyClass mc = new MyClass(); instantiates a new object of the class MyClass. You couldn't do mc.main(...) because main is static. You can call things such as mc's go() method because they are not static and thus belong to objects of that class, rather than the class itself.
...Or something. That all made sense in my head, but if it didn't make sense to you just tell me.

Can we make executable jar file which dont have main method?

I have a problem.
With whatever R&D i have done so far my understanding is that in order to create executable jar files i need a main method.
my problem is i have certain MIS reports which are developed in java and deployed through JSP pages...so my java program has no Main Method and only objects of the classes are being called in the JSP pages.
for e.g: i have a java class file BulkSms.class and i make an object of it in the JSP page through which my purpose is achieved by calling various method in the BulkSms through it s objects and there is no main method.
Can i make an executable jar file without a main method and if so please give me a detailed procedure of how to go about it.
i want to make it executable so tht on the click of it the MIS report is displayed from the JSP page.

I am completely ignorant of JSPs, but you can not execute a jar or any java application unless you have a main method. It sounds like you need a way to launch the JSP - can't you do this with a browser?

Main method question in java, please help

Guys I have looked all over the internet, and cannot understand any of these questions: I would really appreciate any help on them. The first, and most important thing I would like to know is, what is static: what is a static int, or a static main method? Why is this necessary, and what does it do? Could you please provide an example? Also, why is String[] args necessary within the parameters of the main method? Is this so that it can execute it? Last thing is: objects. Where can I learn about them? I know they are made with the new operator, but don't understand their use? Could you explain why they are useful? Can you give them values? and how do they relate to the dot operator? After these are answered, I will be so happy because I will feel like I actually know a little bit. Right now, I'm just confused... Thank you so, so much in advance!

The first, and most important thing I would like to know is, what is static'static' means that the item concerned is associated with the class itself rather than an instance: so, it is accessible to all instances.
Could you please provide an example?
public static void main(String[] args)is an example, but you knew that. I don't really understand the question.
Also, why is String[] args necessary within the parameters of the main method?'args' receives the arguments you type on the command line after the class name or jar name, depending on how you invoke the application.
This is all covered in the documentation, which you should read before asking further questions.
Last thing is: objects. Where can I learn about them?If you don't know about objects you're not going to learn about them by asking questions on a forum. It's far too large a topic. Get yourself a book.

Main method in mulitple classes?

I'm new to java as some of you might know, and I've been working on a single class. Now I'm trying to add another class. I've tried many ways to do this, and I think I almost have it, but I'm still a little stuck. Here are a few questions:
1. I only can have one main method, which goes in the main class, right? Then the subclasses just have methods? So, how does the subclass know which methods to call first, and when? If there is a main method in the subclasses, t should be private and the main method in the main class should be public? So if I have multiple methods in a subclass, how do I tell my main method which to call? I've tried:
new Timer.test();
new Timer(test);Neither seem to work.
So, organized in a tree it should be like so:
Public class mainClass
     public main method
          new subClass
Private class subClass
      (no main method?)
          private subClassMethod1
          private subClassMethod2I've tried searching, but maybe I'm not searching for the right terms. I can find things on creating classes and methods, but not really on the way multiple classes are set up. Can someone explain so I understand exactly what I'm doing with multiple classes, or give me a link to a good explanation? Thanks
Also, the subclass ALWAYS extends mainClass right?
Edited by: LIVE3A17 on Apr 28, 2009 3:13 PM

LIVE3A17 wrote:
Ok, so I can use main methods in the other classes, and java knows which is the main class by which one is opened when the program is run. Got it. Don't even put a main method in the other classes, unless you intend on one of them to be an entry point. Otherwise it's just code bloat.
>
Now, I have a main method in both classes. When I have the main method in the main class set to public, I get an error in the second class saying "Cannot reduce the visiblity of the inherited method". I gives me the option to change visibilty of my main class to private. However, when I do this, and try to run the main class, I get an error saying main method not public.I don't know what you're talking about. Sounds like you are subclassing something and for whatever reason trying to 'override' the main method. The normal static main method isn't even inherited, being static. So this error you're speaking of doesn't make much sense. Of course without seeing your code, we're left to mindreading exercises which usually don't go so well.

Having trouble with "main" method

I have just installed j2re1.4.1 and I am having trouble running my applications. Every time I try to run even the smallest of apps I get an error message about the "Main" method.
In textpad i get this message:
"Exception in thread "main" java.lang.NoClassDefFoundError:"
and in Jcreator I get this message:
"The system couldnt find a suitable main method"
what is going wrong?How can I fix this?

It could be something as simple as Case :-/
If you have a file Foo.java with a class Foo with a main()
you can compile it with javac foo.javawhich will create Foo.classbut you cannot run it with java fooyou will get Exception in thread "main" java.lang.NoClassDefFoundError: roids (wrong name: Foo)whereas java Foowill work just fine.

Args within main() method

This might be a complex question. I hope that someone can give me a simple answer. What I want to know is what this code means?
public static void main(String[] args){
if(args > 2.length){
// ect...I understand that the main() method creates a String array.
How does the array get filled?
What does it get filled with?
Main is the starting point for the program and so where does this String array come from?
How can an array be created before the program even starts?

SquareBox wrote:
This might be a complex question. I hope that someone can give me a simple answer. What I want to know is what this code means?
public static void main(String[] args){
if(args > 2.length){
// ect...
Next time, copy/paste, rather than re-typing. That won't even compile. Presumably you meant
if (args.length > 2)
I understand that the main() method creates a String array. No.
It receives a reference to a String array as a parameter. Just like any other method receives its parameters. The array is created before the method is called. There's nothing special about main() as far as the Java language is concerned. It's only the JVM that attaches special meaning to it.
How does the array get filled? The JVM fills it from the args it receives from the OS/shell on the command line before invoking your main() method.
When you execute java ABC 123 MyClass the java executable is invoked, and the OS passes "ABC", "123", and "MyClass" to it from the shell. (Actually, I think it may also pass "java", but, whatever). The JVM then loads MyClass, creates a String array containing "ABC" and "123", and call's MyClass's main() method, passing a reference to that String[].
What does it get filled with? See above.
Main is the starting point for the program and so where does this String array come from?See above.
How can an array be created before the program even starts?main() is the starting point for your application which is run inside the JVM. It's not the starting point for the JVM, the shell, or the OS, all of which work together to create that array and deliver a reference to it to your main() method.

A question about a method with generic bounded type parameter

Hello everybody,
Sorry, if I ask a question which seems basic, but
I'm new to generic types. My problem is about a method
with a bounded type parameter. Consider the following
situation:
abstract class A{     }
class B extends A{     }
abstract class C
     public abstract <T extends A> T someMethod();
public class Test extends C
     public <T extends A> T someMethod()
          return new B();
}What I want to do inside the method someMethod in the class Test, is to
return an instance of the class B.
Normally, I'm supposed to be able to do that, because an instance of
B is also an instance of A (because B extends A).
However I cannot compile this program, and here is the error message:
Test.java:16: incompatible types
found   : B
required: T
                return new B();
                       ^
1 errorany idea?
many thanks,

Hello again,
First of all, thank you very much for all the answers. After I posted the comment, I worked on the program
and I understood that in fact, as spoon_ says the only returned value can be null.
I'm agree that I asked you a very strange (and a bit stupid) question. Actually, during recent months,
I have been working with cryptography API Core in Java. I understood that there are classes and
interfaces for defining keys and key factories specification, such as KeySpec (interface) and
KeyFactorySpi (abstract class). I wanted to have some experience with these classes in order to
understand them better. So I created a class implementing the interface KeySpec, following by a
corresponding Key subclass (with some XOR algorithm that I defined myself) and everything was
compiled (JDK 1.6) and worked perfectly. Except that, when I wanted to implement a factory spi
for my classes, I saw for the first time this strange method header:
protected abstract <T extends KeySpec> T engineGetKeySpec
(Key key, Class<T> keySpec) throws InvalidKeySpecExceptionThat's why yesterday, I gave you a similar example with the classes A, B, ...
in order to not to open a complicated security discussion but just to explain the ambiguous
part for me, that is, the use of T generic parameter.
The abstract class KeyFactorySpi was defined by Sun Microsystem, in order to give to security
providers, the possibility to implement cryptography services and algorithms according to a given
RFC (or whatever technical document). The methods in this class are invoked inside the
KeyFactory class (If you have installed the JDK sources provided by Sun, You can
verify this, by looking the source code of the KeyFactory class.) So here the T parameter is a
key specification, that is, a class that implements the interface KeySpec and this class is often
defined by the provider and not Sun.
stefan.schulz wrote:
>
If you define the method to return some bound T that extends A, you cannot
return a B, because T would be declared externally at invocation time.
The definition of T as is does not make sense at all.>
He is absolutely right about that, but the problem is, as I said, here we are
talking about the implementation and not the invocation. The implementation is done
by the provider whereas the invocation is done by Sun in the class KeyFactory.
So there are completely separated.
Therefore I wonder, how a provider can finally impelment this method??
Besides, dannyyates wrote
>
Find whoever wrote the signature and shoot them. Then rewrite their code.
Actually, before you shoot them, ask them what they were trying to achieve that
is different from my first suggestion!
>
As I said, I didn't choose this method header and I'm completely agree
with your suggestion, the following method header will do the job very well
protected abstract KeySpec engineGetKeySpec (Key key, KeySpec key_spec)
throws InvalidKeySpecException and personally I don't see any interest in using a generic bounded parameter T
in this method header definition.
Once agin, thanks a lot for the answers.

About main() method

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