How to discard objects stored in an ArrayList

[Baiju Mehta]

I have a class called Product:

public class Product {
     public String productId;
}

In my Test class, I have an ArrayList that stores a list of Product.

...
ArrayList aList = new ArrayList()
for(int i=0;i<10;i++) {
     Product prods = new Product()
     prods.productId = i;
     aList.add(prods);
}
...

At some point after I am done using the List, I want to set reference for these objects to "null" so that it is available for garbage collection. How would I do that?

Just calling aList.clear() and setting aList = null, will this mean all the prods stored in aList will be de-referenced and available for GC or do I need to loop over all the products and set them to null explicitly?

Thanks.

[Campbell Ritchie]

Is the List a field, or a local variable? The way you have written it, it looks like a local variable.

[Baiju Mehta]

My bad, sorry about that. aList is a Global variable.

Incase if aList was a local variable, my understanding is that once the method returns, aList is out of scope and hence available for GC. Is that correct?

[Henry Wong]

Just calling aList.clear() and setting aList = null, will this mean all the prods stored in aList will be de-referenced and available for GC or do I need to loop over all the products and set them to null explicitly?

The easy answer here is... an object is eligible for GC, when it is no longer reachable.

For example, if you did a aList = null, and that reference is the only reference to the array list, then the array list object becomes unreachable, and hence, eligible for GC. Furthermore, since the array list is unreachable, if the elements in the list, are not referenced from anywhere else, then the elements too, become unreachable, and hence, eligible for GC.

Henry

[Campbell Ritchie]

We don't talk about global variables in Java. It is a field. In fact, since you didn't write "public" it is not "visible" outside its own package.

[Baiju Mehta]

Thanks Campbell and Henry for your quick replies.

Campbell, yes, I need to access aList outside this class and it is declared as a public field.

Henry, Just to make sure that I understand what you are saying.. Lets take this example.

I have an Order class:

...
public class Order {
        public String orderId;
        public ArrayList products;
...
        public void setProducts(List prod) {
        	this.products = prod;
        }

...
}

Now I have a Product class:

public class Product {
     public String productId;
}

Now I have my Test class that looks like this:

public class Test {

        public void service(...) {
	Order order = new Order();
	ArrayList aList = new ArrayList();

	for(int i=0; i<10;i++) {
	      Product prod = new Product();
	      aList.add(prod);
	}

	order.setProducts(aList);
        
...
	// At some point I replace the products currently in the order object with new products. So the old products are gone and now new products are stored in order object.

	aList.clear();
	for(int i=0; i<10;i++) {
	      Product prod = new Product();
	      aList.add(prod);
	}

	order.setProducts(aList);
	
}

Now, since the old products are unreacheable, they should be available for GC. Is this correct?


Another example:

Everything is similar as above, except that now my Test class is a Servlet. Once I add products to order object, I store order into a session. After setting order in a session I do aList = null;

What will happen in this case? The aList object will be available for GC, but the elements of aList are still referenced (stored in order object which is in a active session). So the products stored in aList will NOT be available for GC. Is my understanding correct?