learning polymorphism and construction

[Jesse Crockett]

Instead of just doing exercises from the textbook, I've decided to write some of my own classes from scratch to get a better feel for how everything is working. I wrote the following code, which compiles but gives a runtime exception, "no such method: main." I'm trying not to use any superfluous stuff, hoping it will lead to a more solid understanding of what's going on. So, plz tell me, what's going on?

 
class PolyTest {
 
PolyTest() {
 
System.out.println("constructing PolyTest - no args");
}
 
PolyTest(int i) {
 
System.out.println("constructing PolyTest - arg i: " + i);
}
}
 
class PolyTest2 extends PolyTest {
 
PolyTest2() {
 
System.out.println("constructing PolyTest2 - no args");
}
 
PolyTest2(int j) {
 
super(j++);
System.out.println("constructing PolyTest2 - arg: j = " + j + ", after super(j++)");
}
}
 
class PolyTest3 extends PolyTest2 {
 
PolyTest3() {
 
System.out.println("constructing PolyTest3 - no args");
}
 
PolyTest3(int k) {
 
super(k++);
System.out.println("constructing PolyTest3 - arg k: " + k + " after super(k++)");
}
 
public static void main(String[] args) {
 
PolyTest pt3 = new PolyTest3();
}
}   

[Keith Lynn]

What did you type when you interpreted it?

[Jesse Crockett]

Could you rephrase the question? I don't understand what you are asking.
[ March 13, 2006: Message edited by: Jesse Crockett ]

[Keith Lynn]

What I mean is, when you did java ..., what was the exact filename you used?

[Jesse Crockett]

I changed PolyTest3 and its constructors to public, then renamed the file PolyTest3.java and it works now. Thanks.

Tho it's still unclear what I should learn from this...
[ March 13, 2006: Message edited by: Jesse Crockett ]

[Keith Lynn]

The rule is that you're allowed only one top-level public class in a source file, and the name of the source file must match the name of the public class. Remember Java is case-sensitive.

[Ilja Preuss]

Originally posted by Jesse Crockett:
I changed PolyTest3 and its constructors to public, then renamed the file PolyTest3.java and it works now. Thanks.

Tho it's still unclear what I should learn from this...

- with the class not being public, the main method wasn't accessible to the JVM. That is, a main method not only needs to be public, it also needs to be in a public class,

- a public class needs to be in a file with the same name, and

- making the constructor public wasn't necessary, I think. It should still work if you change that back.

[Keith Lynn]

- with the class not being public, the main method wasn't accessible to the JVM. That is, a main method not only needs to be public, it also needs to be in a public class,

I'm not sure about this statement. I have experimented with the following source file, and it compiles and runs correctly even though the class itself is not declared public. I do agree that the main method needs to be public.

class Test10 {
   public static void main(String[] args) {
      System.out.println("Hello");
   }
}

[Jesse Crockett]

thanks guys.

[Ilja Preuss]

Originally posted by Keith Lynn:
I'm not sure about this statement. I have experimented with the following source file, and it compiles and runs correctly even though the class itself is not declared public. I do agree that the main method needs to be public.

Frankly, I'm not fully sure either.

What JDK did you try this with? As far as I know, JVMs before Java 5 had some bugs regarding this - in Java 1.2, the main method even didn't have to be public, if I remember correctly.

[Garrett Rowe]

Works for me too...

class Test10
{
   public static void main(String[] args)
   {
      System.out.println(System.getProperty("java.version"));
      System.out.println(System.getProperty("java.vm.version"));
      System.out.println("Hello");
   }
}

O/P:

1.5.0_06
1.5.0_06-b05
Hello

[ March 13, 2006: Message edited by: Garrett Rowe ]