constructor

[Yuan Tseng]

Hi,
i was studying the book 'thinking in java'. and
is anyone can explain to me why have this result?

// constructor
 
class Tag   {
   Tag (int marker)   {
      System.out.println ("Tag (" + marker " )" ); 
   }
}
 
class Card   {
    Tag t1 = new Tag(1);
    Card()   {
        System.out.println ("Card() ");
        t3 = new Tag(33);
    }
 
    Tag t2 = new Tag(2);
    void f()   {
       System.out.println ("f() ");
    }
    Tag t3 = new Tag(3);
}
 
public class OrderOfInitialization   {
    public static void main (String []  args)   {
       Card t = new Card();
       t.f();
    }
} 

the output:
Tag (1 )
Tag (2 )
Tag (3 )
Card()
Tag (33 )
f ()

thanks for your help,
yuan

[ edited to preserve formatting using the [code] and [/code] UBB tags -ds ]
[ March 13, 2003: Message edited by: Dirk Schreckmann ]

[Dirk Schreckmann]

Why do you think the result is what it is? Would you have expected something to be different?

[karl koch]

hi

if changed the order of variables in the class Tag. The constructor is only called after the varibales are initialized.

// class Tag
class Tag   {   
  Tag (int marker)   {
    System.out.println (
       "Tag (" + marker " )" 
    );    
  }
} 

// class Card
class Card   {
  Tag t1 = new Tag(1); // 2     
  Tag t2 = new Tag(2); // 2
  Tag t3 = new Tag(3); // 2 
  
  Card() { // 3       
    System.out.println ("Card() ");
    t3 = new Tag(33); // 4
  }     
  void f() {   // 5     
    System.out.println ("f() ");    
  }    
}
// class OrderOfInitialization
public class OrderOfInitialization   {    
  public static void main (String []args) {
    Card t = new Card(); // 1
    t.f();
  }
} 

Here is what happens:

1) New Card is created

2) This first inits all variables of the class (t1, t2, t3) by calling the constructor of Tag (i guess you cant be shure that they are called in the order of appearance. im not sure about that)

3)After all variables are initialized, the Constructor of Card is executed

4) then t3 is reassigned to a new instance of Tag -> constructor of Tag is called again

5) and then the function f() is called

hope this helps.

k

[Yuan Tseng]

thanks, Karl.
yuan

[Yuan Tseng]

hi,
is java has any kind of tools that can show me step-by-step (like vb)?
thanks
yuan

[Barry Gaunt]

If you are new java then a nice little learning IDE can be found on the BlueJ site. It has a debugger and class inspector and will soon have JUnit capability.