Question

I have a problem where I retrieve and element from a list (list1), and modify one of the parameters in the element and then add it to another list (list2). When I do this to the final item in list1, it will sometimes modify the parameters of the elements in list2.

This function is called once per generation, but it is not until about the 8th generation when I start to see this happen. It has affect anywhere from 2 to 16 elements in list2.

Any ideas where my screw up might be? Here’s a block of code I wrote to illustrate the problem. The problem occurs in the section where I check count==0.

    public void sampleCode (List list) {
    List differentList = new ArrayList();
    Individual element;
    Individual differentelement;
    int i = 0;
    int count = 0;
    for(i = 0; i < list.size(); i++) {

        element = (Individual) list.get(i)  ;


        // does some checking to see if this meets criteria
        // this is sorta pseudo code
        if(probability == true) { 
            element.setDoMutation(true);
            count++;
        }
        //always add this element to differentList
                    //even if no changes are made to the element
        differentList.add(i,element);
    }
        //need to make sure one elements has mutation=true;
    if(count == 0) { 
        differentelement = (Individual) list.get((list.size()-1));

        //setting this element field changes the contents of
        //different list.
        differentelement.setDoMutation(true);
        differentList.set((list.size()-1), differentelement);
    }   
}

Answer 1

In Java, a variable doesn’t hold an object. It holds an object reference (i.e. a pointer to an object). Getting an object fom a list and putting it in another list doesn’t make a copy of the object. Both lists simply have a pointer to the same object. So, of course, if you modify the contents of the object, both lists will have the object modified.

Side note: You should use parameterized types (i.e. List<Individual> rather than List), and avoid declaring variables at the beginning of your methods as you would do in C. Only declare a variable when you need it. This will make the code much clearer, and reduce the scope of your variables.