Calling another class

[Paul Keohan]

If you instantiate a class that has a main method and a constructor, what happens? I've tried this and both the constructor and the main() method seem to be getting ignored - and no error occurs.

[Cindy Glass]

class Test {
   Test(){
           System.out.println("In constructor");
   }
   public static void main(String args[]){
           System.out.println("In main");
           Test t = new Test();      
    }
}

if you run>java Test
the main method is executed which creates an object of type Test which executes the constructor. The output is:
In main
In constructor

[pascal betz]

hi,
since the main method is static, you do not instantiate an object by just calling it. only at the point where you call the constructor a object is created.
pascal

[Jason Kretzer]

Hmmmm. But can you call the main of the second class as member method of that class? Like this.
<pre>
class A
{
public A()
{
System.out.println("in constructor");
}
public static void main(String args[])
{
System.out.println("in class A in main");
}
}
class Test
{
public static void main(String args[])
{
A aclass = new A();
aclass.main(args);
}
}</pre>
-Jason

[Paul Keohan]

My question is :
What happens here when you execute class A? When I do this, nothing happens.

public class A
{
public static void main (String args[])
{
new B();
}
}
public class B
{
public B()
{
System.out.println("I'm in B constructor");
}
public static void main (String args[])
{
System.out.println("I'm in B.main()");
}
}
BTW, how do I get code to display in that small print? - the way everybody else does it.

[Cindy Glass]

Use the [ code] and [ /code] tags (without the spaces) and it will do that. If you just click the pad and pencil on my post above you can see how I did it. You won't be able to save any edits, but you can look at the typed post.
Your sample should print the "I'm in B constructor" message to the console.
Lots of classes have main methods that are never used to start an application. They are usually there for self testing purposes, and don't really get in the way if the class is not used to start an application.

[Jason Kretzer]

I used the HTML tags pre and /pre before and after my code respecitvely. Of course, you will want to enclose the tags in <>.
So, I guess you can use the main method as I described? Pauls, your code should display as Cindy mentioned. It is interesting trivia if nothing else.
-Jason

[Fabrizio Gianneschi]

Originally posted by Cindy Glass:
Lots of classes have main methods that are never used to start an application. They are usually there for self testing purposes, and don't really get in the way if the class is not used to start an application.

Yes, for example, I usually have a boolean final DEBUG property used to test the single classes:

public static void main(String[] args){
  if (DEBUG){
    //put code here
  }
}

------------------
Fabrizio Gianneschi
Sun Certified Programmer for Java2 Platform

[Stanley Tan]

main is an entry point into an application. If you create the class from another class, then main will never be called. The main method is only called when you start that class as an entry point (e.g. java ClassA). Constructors are used to instantiate and create objects.