How to clone ArrayList and also clone its contents?
How can I clone an ArrayList
and also clone its items in Java?
For example I have:
ArrayList<Dog> dogs = getDogs();
ArrayList<Dog> clonedList = ....something to do with dogs....
And I would expect that objects in clonedList
are not the same as in dogs list.
You will need to iterate on the items, and clone them one by one, putting the clones in your result array as you go.
public static List<Dog> cloneList(List<Dog> list) {
List<Dog> clone = new ArrayList<Dog>(list.size());
for (Dog item : list) clone.add(item.clone());
return clone;
}
For that to work, obviously, you will have to get your Dog object to implement the Cloneable interface, and the clone() method.
I, personally, would add a constructor to Dog:
class Dog
{
public Dog()
{ ... } // Regular constructor
public Dog(Dog dog) {
// Copy all the fields of Dog.
}
}
Then just iterate (as shown in Varkhan's answer):
public static List<Dog> cloneList(List<Dog> dogList) {
List<Dog> clonedList = new ArrayList<Dog>(dogList.size());
for (Dog dog : dogList) {
clonedList.add(new Dog(dog));
}
return clonedList;
}
I find the advantage of this is you don't need to screw around with the broken Cloneable stuff in Java. It also matches the way that you copy Java collections.
Another option could be to write your own ICloneable interface and use that. That way you could write a generic method for cloning.
All standard collections have copy constructors. Use them.
List<Double> original = // some list
List<Double> copy = new ArrayList<Double>(original); //This does a shallow copy
clone()
was designed with several mistakes (see this question), so it's best to avoid it.
From Effective Java 2nd Edition, Item 11: Override clone judiciously
Given all of the problems associated with Cloneable, it's safe to say that other interfaces should not extend it, and that classes designed for inheritance (Item 17) should not implement it. Because of its many shortcomings, some expert programmers simply choose never to override the clone method and never to invoke it except, perhaps, to copy arrays. If you design a class for inheritance, be aware that if you choose not to provide a well-behaved protected clone method, it will be impossible for subclasses to implement Cloneable.
This book also describes the many advantages copy constructors have over Cloneable/clone.
Consider another benefit of using copy constructors: Suppose you have a HashSet s
, and you want to copy it as a TreeSet
. The clone method can't offer this functionality, but it's easy with a conversion constructor: new TreeSet(s)
.
上一篇: 无法深入克隆链接列表中的对象
下一篇: 如何克隆ArrayList并克隆其内容?