java List type variable

Question

I have been breaking my head with below code which I made. The problem is that when I do tail.child = null; it is also making my childPoint's child as null. tail is instance variable with below definition:

public List tail;
public void removeMultiLinkList() {
    List headPoint = head;
    while (headPoint.next != null) {
        List childPoint = headPoint;
        while (childPoint.child != null) {
            tail.next = childPoint.child;
            tail = tail.next;
            tail.child=null;
            childPoint = childPoint.child;
        }
        headPoint = headPoint.next;
    }
}

I have made this method to solve the problem of multilevel link list and convert it into linear singly link by in non recurssive manner

By @Jeremy : Can you show us the List class you are using? — amit
– amit, Commented Jan 4, 2015 at 10:17

amit · Accepted Answer · 2015-01-04 10:34:44Z

2

Examine what you are doing:

tail.next = childPoint.child; 
tail = tail.next;

In here, tail is childPoint.child (reference identity)

Then, you do:

tail.child=null;

This means, you actually set childPoint.child.child = null; - because chilePoint.child and tail are different names for the same object.

And then, you assign:

childPoint = childPoint.child;

But you assign childPoint to the same object you just changed - so the new childPoint's child, is null!

A very easy workaround is to copy by value (by creating a copy constructor) the elements from one list to the other.

An alternative might be to keep copying references - but without changing child at all. At the end of your algorithm, do some post-processing and set e.child = null for each element e in your list.

edited Jan 4, 2015 at 10:34

answered Jan 4, 2015 at 10:24

amit

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