How to Clone Collection in Java - Deep copy of ArrayList and HashSet

Programmer often mistook copy constructors provided by various collection classes, as a mean to clone Collection e.g. List, Set, ArrayList, HashSet or any other implementation. What is worth remembering is that, copy constructor of Collection in Java only provides shallow copy and not deep copy, which means objects stored in both original List and cloned List will be same and point to same memory location in Java heap. One thing, which adds into this misconception is shallow copy of Collections with Immutable Objects. Since Immutable can't be changed, It's Ok even if two collections are pointing to same object. This is exactly the case of String contained in pool, update on one will not affect other. Problem arise, when we use Copy constructor of ArrayList to create a clone of List of Employees, where Employee is not Immutable. In this case, if original collection modifies an employee, that change will also reflect into cloned collection. Similarly if an employee is modified in cloned collection, it will also appeared as modified in original collection. This is not desirable, in almost all cases, clone should be independent of original object. Solution to avoid this problem is deep cloning of collection, which means recursively cloning object, until you reached to primitive or Immutable. In this article, we will take a look at one approach of deep copying Collection classes e.g. ArrayList or HashSet in Java. By the way, If you know difference between shallow copy and deep copy, it would be very easy to understand how deep cloning of collection works.

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