default constructor confusion

[Alangudi Balaji Navaneethan]

I studied in Head First Java that,

We need to provide a constructor with arguments to instantiate a class with initial values. If we do not provide any constructor one will be arranged by jvm.

I also read in a book that, if we give constructors with arguments then we must also give constructors without arguments also...

But when I tried the following program I am puzzled with the results what is the problem

    
class Test
{
    private int i;
 
    public Test(int j)
    {
         i = j;
    }
 
    /*public void Test()
    {
        i = 10;
    }*/
 
    public int getI()
    {
       return i;
    }
 
    public static void main(String[] args)
    {
       Test t = new Test(100);
       System.out.println(t.getI());
    }
}

I expected a compile time error. But it compiled and gave 100 as result.

[Burkhard Hassel]

Hi cowboys,


Alangudi Balaji Navaneethan wrote:

I also read in a book that, if we give constructors with arguments then we must also give constructors without arguments also...

No, that's wrong. You don't have to put a no-arg constructor in, as your example shows.

Only in cases of inheritance, where a subclass might call your superclass' no-arg constructor, and you have only constructors with args, you would had to add one.
Example, if I extend your class:

TestChild extends Test {
}

Then the compiler automatically adds the following lines (in italics):

TestChild extends Test {
    [I]TestChild(){
        super();
    }[/I]
}

...and then TestChild will not compile, because you out-commented the no-arg constructor in Test. And TestChild (automatically) calls it.



Yours,
Bu.