Memory Of the Variable

[Ramesh Sunkara]

Hi Folks,
I have very basic doubt. Please do clarify me.

import java.util.ArrayList;
import java.util.List;

public class MemoryDoubt {

	public List<String> getEmployeeDetails() {
		List<String> empDetailsList = new ArrayList<String>();
		return empDetailsList;
	}
	
	public static void main(String []args) {
		MemoryDoubt vSc = new MemoryDoubt();
		List<String> empDet = vSc.getEmployeeDetails();
	}
}

1) In the method getEmployeeDetails() since am creating an new java.lang.ArrayList it allocates/reserves memory for default capacity i.e., for 10 java.lang.String objects.
( this is my understanding,please let me know if am wrong)

2)So, when that method returns the empDetailsList since empDetailsList is method level variable,it will be eligible for GC.


3) Now, does

empDet

will also consume the same amount of memory since we assigned the value( since Java passes by value) which we got from the method getEmployeeDetails() to it ?.

[Ove Lindström]

Ramesh Sunkara wrote:<cut>

1) In the method getEmployeeDetails() since am creating an new java.lang.ArrayList it allocates/reserves memory for default capacity i.e., for 10 java.lang.String objects.
( this is my understanding,please let me know if am wrong)

2)So, when that method returns the empDetailsList since empDetailsList is method level variable,it will be eligible for GC.


3) Now, does

empDet

will also consume the same amount of memory since we assigned the value( since Java passes by value) which we got from the method getEmployeeDetails() to it ?.

1. You are wrong... ;) The array list in itself are backed by an array of what type you are allocating. But it does NOT allocate memory enough to fit any String (or object) into it, it allocates memory enough for references to String instances (or objects).

A small "memory run" as an example:

Command/Memory allocated
al = new ArrayList<String>()/alloc(10 x 2 bytes)
al.add("Lorem ipsum")/alloc(12 x 2 bytes) put the ref to that memory into the array.


It is impossible to know how much memory the Strings (or objects) will take when you create the Array.

2. Yes, the memory that holds the reference to empDetailsList removed since it belongs to the method stack/context.

3. No. You are not returning a COPY of the ArrayList, you are returning the value of the reference. If bake a Tau (2 pie) and give them to you, they are still taking up the same space, but you have to hold them in your hands and my reference to them are no more.

[Ramesh Sunkara]

The array list in itself are backed by an array of what type you are allocating.

1) I didn't understand what is backed by an array. Can you please clarify me?

al.add("Lorem ipsum")/alloc(12 x 2 bytes) put the ref to that memory into the array.

2) Why it will allocate 12 x 2 when there are only 11 characters (including space) ?

It is impossible to know how much memory the Strings (or objects) will take when you create the Array.

3) thanks a lot. I never knew this.


4) value of the reference --> is the value of reference means the address of the memory space?

5)

If bake a Tau (2 pie) and give them to you, they are still taking up the same space, but you have to hold them in your hands and my reference to them are no more.

i didnt understand this example. what exactly you meant?

6)

return empDetailsList;

will this return the reference i.e., address of the arraylist.
In that case

 List<String> empDet = vSc.getEmployeeDetails();

will just point to that same address?

Thanks a lot for your valuable time.