Passing Variables into Methods concept

[dimple bav]

Hi everyone

The code is from K&B chp 3 Quiz _ 4

class Mixer
{

Mixer() ---------------------------------> constructor with no-args
{

}

Mixer(Mixer m)--------------------------> constructor with ref var m
{
m1 = m; -------------------------> ref var m1 = m
}

Mixer m1; ----------------------------> class Mixer has an ref var m1 with no reference to an object (in other word no object is created)

public static void main( String args[])
{


Mixer m2 = new Mixer(); ----------------> fine

Mixer m3 = new Mixer(m2); ---------------------> ref var m2 is passed to Mixer class constructor Mixer(Mixer m) where we have m1 = m2

m3.go(); -----------------------------> Hi is printed

Mixer m4 = m3.m1; ------------------------> m4 ref var is equal to (ref var m3. ref var m1)-----------------> ?

m4.go(); --------------------------> Hi is printed

Mixer m5 = m2.m1; [b]--------------------------> m5 ref var is equal to (ref var m2. ref var m1)-----------------> ?


m5.go() -----------------> why Hi is not printed >

}

void go()
{
System.out.println("hi ");
}
}

[Raju Champaklal]

because m5 is null....this will give you null pointer exception

[dimple bav]

Raju Champaklal wrote:because m5 is null....this will give you null pointer exception

If i m not wrong it goes this way ----

Mixer m5 = m2.m1

Since Mixer m2 = new Mixer() has no arg to passed so m5 is null and hi will be not printed .

Right ?

Thank you .
Dimple

[Raju Champaklal]

m5 is equal to m2.m1....and m2.m1 is null...do you see anywhere m2.m1 getting initalised aanywhere?

[dimple bav]

I have a doubt on this

Mixer m2 = new Mixer(); --------------- > inside void main

Mixer m1; -------------> ouside main

Mixer m5 = m2.m1 -------------------------------> ref var m2. ref var m1

m2 ref var has an object whereas ref var m1 has no object.

So third ref var m5 = m2.m1 ----------------- what doe it mean ?

Thank you .
Dimple

[Raju Champaklal]

what do you think how is m1 related to m2?

[dimple bav]

So

Mixer m4 = m3.m1 -----------------> where intialized ?

Just because m3 ref var is intialized by passing ref var m2 i.e Mixer m3 = new Mixer(m2) where m1 = m2 in Mixer(Mixer m) constructor

Does it mean Mixer m4 = m3.m2 where m3 and m2 are both intialized.

Let me know my concept is right ?

In other words

Mixer m5 = m2.m1 ---------------- where m1 is not intialized .


Thank you.

[Raju Champaklal]

what do you think is the difference between these two codes?

Mixer m2 = new Mixer();

Mixer m3 = new Mixer(m2);

[Raju Champaklal]

did you understand why m2.m1 is null?

[dimple bav]

m2 ref var has a constructor with no args

and


m3 ref var has a constructor with a ref var m2 passed into it .


Thank you.

[dimple bav]

m2.m1 is null

Since m1 is a thus a ref var created with no object.

in other words

Mixer m1 -----------> declare

m1 = new Mixer() ----------> intialized

So m1 is not intialized.


Thank you.

[Raju Champaklal]

m2 ref var has a constructor with no args
what is the difference between calling a no arg constructor and arg constrcuotr? in which case is the instance variable m1 getting initialised?

[dimple bav]

m1 will be intialized with arg constructor .

Thank you.

[Raju Champaklal]

Mixer m2=new Mixer(); in this case m1 is null since m1 gets its default value which is null....nothing is passed to the constructor of m2...

Mixer m3=new new Mixer(m2) means m1 of m3(the instance variable of m3) now refers to the object referring to m2.......