HFJ doubt

[srikanth mycherla]

Dear Ranchers


In head first java of pg 238 and 239 i have a doubt ..

in pg 238 there is a statement ->" no matter where the object reference variable is declared (inside a method vs as as instance variable of a class ) the object always always always goes on the heap ..

Here i understood that the object always goes on heap even when object reference variable is declared inside a method ..(main method usually )

Iam unable to understand this line --Object reference variable is declared as an instance variable of a class ..

1)I have seen like object reference variable is always declared inside main method
please let me know such situations where object reference variable is declared as an instance member of a class or even in any normal method (not in a main method ) If possible please write small code ranchers ..


2)In 239 pg also i am unable to understand this line --what if the instance variable are objects?
and when the new object has instance variables that are object references rather than primitives ?

there is one example on this pg

public class cellphone {
private Antenna ant = new Antenna ();}

my opinion is this example should look like this i suppose

public class cellphone
{
public static void main(String args[])
{
private Antenna ant =new Antenna();}



If iam right then since ant reference variable is declared inside main method then it should be on stack pointing towards heaps object

correct me ranchers




3) IF the instance variable is a reference to an object both the reference and the object it refers are on the heap ...source from HFJ 247 BULLET POINT(2)

How come an instance variable be a reference to an object

I suppose we usually create references inside main method ..correct me

[Vijitha Kumara]

srikanth mycherla wrote:

Iam unable to understand this line --Object reference variable is declared as an instance variable of a class ..

1)I have seen like object reference variable is always declared inside main method
please let me know such situations where object reference variable is declared as an instance member of a class or even in any normal method (not in a main method ) If possible please write small code ranchers ..

It's just as you would do in a main method, no difference.

  
class InstanceReference{

Object object; //#1

void initializeObject(){
this.object  = new Object();
}

}
 

You can use an object of any type you want here instead of Object. Or even you may initialize the object reference in the same line as your declaration.

Object object  = new Object();//#1

[Vijitha Kumara]

2)In 239 pg also i am unable to understand this line --what if the instance variable are objects?
and when the new object has instance variables that are object references rather than primitives ?

there is one example on this pg

public class cellphone {
private Antenna ant = new Antenna ();}

Your statement is not clear to me but this represents a class with an instance reference variable which is pointing to an object of the class Antenna.

I suppose we usually create references inside main method ..correct me

That's just a one place where references can go in. But it's perfectly legal (and main is just another method with a few exceptions from other instance/class methods) to have that in any other method or as a instance variable as in my previous reply.

[Punit Singh]

In head first java of pg 238 and 239 i have a doubt ..

in pg 238 there is a statement ->" no matter where the object reference variable is declared (inside a method vs as as instance variable of a class ) the object always always always goes on the heap ..

Here i understood that the object always goes on heap even when object reference variable is declared inside a method ..(main method usually )

Here you are absolutely right.

Iam unable to understand this line --Object reference variable is declared as an instance variable of a class ..

1)I have seen like object reference variable is always declared inside main method
please let me know such situations where object reference variable is declared as an instance member of a class or even in any normal method (not in a main method ) If possible please write small code ranchers ..

  class Person{
	String name;//this is reference variable to String Object 
	//defined as Class instance variable and will be created on the heap
	//with Person Object in the heap
	String address;////this is reference .... same as above
	int age;//this is not reference variable, it is called primitive variable, but
	//will be created on the heap with Person Object
	
	public String getName()//this is instance method of Person Class, and will
	//not be created on the heap, 
	//it will be created code or test segment in the memory
	{
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public String getAddress() {
		return address;
	}
	public void setAddress(String address) {
		this.address = address;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
		
}
public class ObjectReferenceClear {
public static void main(String[] args) {
	Person p1;//this is reference variable to Person Object in main()
	//and this reference variable p1 will be created on stack, when
	//main() method is loaded in stack from the code segment when this 
	//main() will be run by JVM.
	String name="Srikanth";//reference variable to String object
	//name will go in stack with main(), but "Srikanth" will be created
	//on the heap
	int age=21;//int is primitive, so it will not be created on the heap,
	//it will be created on the stack with main() only. 21 will also get 
	//space on heap.
	p1=new Person();//new Person is an actual Object, will be created
	//on heap and its reference means its address will be given to p1 that
	//is on the stack with main().
	
	String address="14-B, California, US.";// Now its your turn to say what is
	//going here, I have covered all the things.
	
	p1.setName(name);
	p1.setAddress(address);
	p1.setAge(age);
	
	System.out.println(p1.getName());
	System.out.println(p1.getAddress());
	System.out.println(p1.getAge());
	}
}  

2)In 239 pg also i am unable to understand this line --what if the instance variable are objects?
and when the new object has instance variables that are object references rather than primitives ?

I have shown in my code, Person object has an instance variable "name", that is a String object.
And another instance variable is "age", that is primitive.

there is one example on this pg

public class cellphone {
private Antenna ant = new Antenna ();}

my opinion is this example should look like this i suppose

public class cellphone
{
public static void main(String args[])
{
private Antenna ant =new Antenna();}

No you are wrong here.

  public class cellphone {
private Antenna ant = new Antenna ();}
  

This is the right way.

3) IF the instance variable is a reference to an object both the reference and the object it refers are on the heap ...source from HFJ 247 BULLET POINT(2)

How come an instance variable be a reference to an object
I suppose we usually create references inside main method ..correct me

I think this doubt must have been cleared now by given code.

[Campbell Ritchie]

Punit Singh wrote:

. . .

  public class cellphone {
private Antenna ant = new Antenna ();}
  

This is the right way.

A very helpful post. Thank you. What you have written is all right, but I think, personally, I would prefer to instantiate the ant object in a constructor.

public class Cellphone
{
   private Antenna ant;
   . . .
   public Cellphone( . . . )
   {
      . . .
      ant = new Antenna( . . . );
      . . . etc etc

And please use correct indentation, and start your class names with CapitalLetters, please.

[srikanth mycherla]

Thanks to punit and vijithakumara..its clear to me now..excellent explanations

[srikanth mycherla]

class MAIN 
{ 
public static void main(String[] args) 
{ 
xyz ob = new xyz(); 
System.out.println(ob.o.x); 
} 
} 

class abc 
{ 
int x = 10; 
} 

class xyz 
{ 
abc o = new abc(); 
} 

This is posted by Kshitij dogra ..it is also a good answer to this question