Changing File Name To Name Of The Class

I am starting to learn java , If suppose we write a simple hello world program

class helloWorld
{
public static void main(String args[])
{
System.out.print("Hello World !");
}
}

And save this as test.java.Now after compiling it a helloWorld.class file is generated.But if we compile the same after adding "public" in front of 1st line, it throws error.

public class helloWorld
{
public static void main(String args[])
{
System.out.print("Hello World !");
}
}

but then changing the file name to the name of the class i.e. helloWorld.java, corrects the error.