Can a Java function save a list before it's changed and recall it when needed?

Question

I have this function and in newList1 I want to have the original l1 that I inserted at the beginning, and the same is true for newList2 and l2 respectively. Any suggestion how I can do it?

 public static List subCombo(List l1, List l2)
    {
    List newList1= new LinkedList();
    List newList2= new LinkedList();    
    List list1= new LinkedList();

    while (!l1.isEmpty())
    {                   
        for (Object o: l2)
        {
            Double product=1.0;

            if ((Double)hd(l1) == 0.0 && (Double)hd(l2)!=0.0)
            {
                product = (1-mySub(newList1))*(Double)hd(l2); 
                list1.add(product);         
            }               
            if ((Double)hd(l1)!=0.0 && (Double)hd(l2) == 0.0)
            {
                product= (1-mySub(newList2))*(Double) hd(l1);
                list1.add(product);

            }
             if ((Double)hd(l2) == 0.0 && (Double)hd(l1)==0.0)
            {                
                product= (1-mySub(newList1))*(1-mySub(newList2));
                list1.add(product);

            }
             if ((Double)hd(l1) != 0.0 && (Double)hd(l2)!=0.0)
            {
                 product=(Double)hd(l1)*(Double)hd(l2);         
                 list1.add(product); 
            }       
            l2=tl(l2);              
        } 
        l1=tl(l1);  
    }   
    return list1;
}

Sure. Just make a copy of the List before you modify it, and store that copy in another variable. Which part of that is giving you trouble? — Kevin Workman, May 14 '15 at 15:19
Use generics and you can get rid of all that `(Double)` casting -- `public static List subCombo(List l1, List l2)` — Stephen P, May 14 '15 at 19:03

score 1 · Answer 1 · edited May 23 '17 at 12:21

1

As long, as you don't modify objects contained in the containers, you may just to copy the lists before modification:

List newList1 = new LinkedList(l1);
List newList2 = new LinkedList(l2);

Otherwise you will need to make a deep copy of the lists. Example

edited May 23 '17 at 12:21

Community

1
1

answered May 14 '15 at 16:53

Arseny Mayorov

98
7

Stephen P · Answer 2 · 2015-05-14T19:27:35.797

0

You can create a List (and other kinds of Collections) from any other Collection simply by passing it in the list constructor:

public static List subCombo(List<Double> l1, List<Double> l2)
{
    // save a copy of the original "l1" list.
    List<Double> savedList = new LinkedList<>( l1 );

    // etc.

}

There is also the addAll() method if you have to, for example, build a list then copy it.

Finally, you can create an unmodifiableList copy of the a list to ensure it never gets changed:

List<Double> savedCopy = Collections.unmodifiableList( l1 );

...but this would be more appropriate for a List you might be returning.

edited May 14 '15 at 19:27

answered May 14 '15 at 19:01

Stephen P

14,422
2
43
67

1

`unmodifiableList` won't work in this case...important to note that it only returns an unmodifiable _view_ of an existing list. Any changes made to the the original list reference will appear in the saved copy as well. – Kevin K May 14 '15 at 19:07
Good point @Kevin - I don't know what the OP's method is really trying to do, but I would not modify the original input param lists and produce a list to return instead, possibly as an unmodifiable, which is why I wanted to mention it. – Stephen P May 14 '15 at 19:25

Can a Java function save a list before it's changed and recall it when needed?

2 Answers2