Can a collection be passed to a method by reference?

Question

If I have a list that I pass into a method to be populated, can I then reference that list variable in the class/method that did the passing, without having to return the list?

Here's an example, where a list with a single account ID is passed to a method, whose purpose is to recursively go through levels of child accounts:

public static void Get_Children(ID mainID, List<ID> ID_Group){        
    for(Account child :[SELECT ID, ParentID FROM Account WHERE ParentID = :mainID]){
        if(child.ParentId == mainID){ /*IF clause to make sure SOQL returned an actual account*/
            ID_Group.add(child.id);
            Get_Children(child.id, ID_Group);
        }
    }       
}

Note that the method doesn't return the list, it simply populates it. If I have a method in which the ID_Group list is declared, then pass that list into this method, and then look at the ID_Group list afterwards, will it have the data loaded in by this method?

Answer 1

In short the answer is yes! To understand why you need to understand the language concept of passing by value and passing by reference. The Apex developers guide has this to say about this concept here.

In Apex, all primitive data type arguments, such as Integer or String, are passed into methods by value. This means that any changes to the arguments exist only within the scope of the method. When the method returns, the changes to the arguments are lost.

Non-primitive data type arguments, such as sObjects, are also passed into methods by value. This means that when the method returns, the passed-in argument still references the same object as before the method call and can't be changed to point to another object. However, the values of the object's fields can be changed in the method.

So while the reference to the list is passed by value, in that changing what ID_Group points to (e.g. another list of Id's) does not change what the caller passed in. You can however change the contents of what it points to, thus you can add more items to the list for the caller to later discover.

Answer 2

Parameters in a method are always passed by reference.

public class TestByReference {

    Public static void TestByReference(List<String> strList){
        strList.add('TestByReference');
    }

}

List<String> strList = new List<String>();
strList.add('outSideMethod');
TestByReference.TestByReference(strList);
system.debug(strList);

OutPut : (outSideMethod, TestByReference)