constructor

[Shiva Mohan]

    
public class Three {
    float f;
    Three(){
       this(5);
         System.out.println(f);
    }
    Three(float f){
       System.out.println(this.f);//line 1
    }
    public static void main(String args[]) {
       Three t = new Three();
    }
}

i though line 1 would give error like this.f cannot reference before this(f) exceution.but it executed fine and printed f=0.0 .how?

[megha joshi]

Hi

Remember the cycle...
First the static code blocks run.
Second the instance variables are given default values always
After that the constructor and super constructor runs and then the instance variables are initialized to any values provided in construtors or initialization blocks.

The default value of instance variable f which is float is 0.0.
So its given that value and when the constructors executes...it prints the default value.

[Shiva Mohan]

Thanks meg.

Just now only i have known that

Second the instance variables are given default values always.

Thanks again for the help.

[Shiva Mohan]

One more doubt.

    
public class Three{
    float f;
    Three(){
       this(f);//line 1
       f = 3;
    }
    Three(float f){
       System.out.println(f);
    }
    public static void main(String args[]) {
       Three t = new Three();
    }
}

.why the 0.0 default value is not passing here?line 1 is showing compile error.

[Anton Uwe]

"f" does not exist at the moment you try to use. First, the (implicit) call (chain up to Object) to super() must be executed. Then f exists initialized with the default value.

[megha joshi]

Sorry I need to modify what I said earlier...

First static code blocks

At this point only static variables can be used.

Second the default call to this() or super() or the programmer inserted call to this()(args optional)

super() keeps on calling super() until the supermost object() is called.
At this point instance variables get their default values.

At this point instance variables can be used
Then the rest of the code in the constructor runs
And then initialization blocks

You can find the above in K&B book.

[Shiva Mohan]

    
public class Three {
float f;
Three(){
this(5);
System.out.println(f);
}
Three(float f){
System.out.println(this.f);//line 1
}
public static void main(String args[]) {
Three t = new Three();
}
}

As anton said,i was expecting this.f cannot reference before this(f) exceution.But this.f is executed fine and gave resul 0.0 before finished this(5) execution.is the above program flow of execution different from the below one.

public class Three{
float f;
Three(){
this(f);//line 1
f = 3;
}
Three(float f){
System.out.println(f);
}
public static void main(String args[]) {
Three t = new Three();
}
}

this program gave me the compile error at line 1 as expected.But i am asking how the first program is executing without an error.Please help me.

[Anton Uwe]

The sniplet from your first code

Three(float f){
System.out.println(this.f);//line 1
}

is the same as

Three(float f){
super();
System.out.println(this.f);//line 1
}

So, because super() was already executed before "line 1", "f" already exists with its default value.

[anil kumar]

hi

good question.

See
in the first question you are this(5).Here you are epassing a literal. To an overloaded constructor.From there the super class constructor is beging called.After it will come to

System.out.println(this.f);//line 1
this statement.Here replace that statement to this statement.
now we can assign any value to instance variable .
so try to assign this.f=10;

after execution of this statement the control will go to default constructor.Here if you want you can print.


ONE THING KEEP IN MIND THAT YOU CAN ASSIGN OR YOU CAN A USE(LIKE YOU DID IN THE LAST EXAMPLE OF THE PAGE) INSTANCE VARIABLE AFTER SUPER CLASS CONSTRUCTOR IS EXECUTED .
OK

in last example code you did like this .this(f).At this point the super class constructor is not called.That is why you are getting error.
[ February 27, 2007: Message edited by: anil kumar ]

[Shiva Mohan]

Thank you very much Anton and Anil.I got it.