How to clone two-dimensional List?

Question

I want to clone list:

public List<List<Test>> a = new ArrayList<List<Test>>();
public List<List<Test>> b = new ArrayList<List<Test>>();

But if I do:

b = (ArrayList<List<Test>>)a.clone();

Occurs an error:

The method clone() is undefined for the type List<List<Test>>

Can you help me?

You must override clone() from Object. A related question is http://stackoverflow.com/questions/869033/how-do-i-copy-an-object-in-java. — Mihai8, Feb 03 '13 at 12:42

score 1 · Answer 1 · answered Feb 03 '13 at 12:41

java.util.List does not implement Cloneable, so the compiler cannot ensure that the concrete implementation you are using does. So, it won't allow you to write a code that calls a method that may not be there.

If you know the concrete implementation, you can cast it. V.g., if it is an ArrayList<List<Test>> (since ArrayList does implement Cloneable), then you do

  b = (List<List<Test>>) ((ArrayList<List<Test>>) a).clone();

If you do not, implement your own method.

Remember that the default clone makes shallow copies. That is, the object returned will be a new List<List<Test>>, but the inner List<Test> and Test will not get copied and you will get references to the original objects.

score 1 · Answer 2 · answered Feb 03 '13 at 12:42

1

The clone() is not available for the abstract list, only for ArrayList. However it will not work as you expect as it only returns a shallow copy. To make a deep copy, you need to clone in a loop:

   for (List<Test> item: a)
      b.add(new ArrayList(item);

answered Feb 03 '13 at 12:42

Audrius Meškauskas

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score 0 · Answer 3 · answered Feb 03 '13 at 12:41

0

Use the constructor or the addAll method:

new ArrayList<>(a);
//or
b.addAll(a);

answered Feb 03 '13 at 12:41

tb-

1,240
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score 0 · Answer 4 · answered May 19 '15 at 09:48

If you're using Java 8, you can create a new list using the same objects as the previous one simply using streams:

List<Double> secondList = firstList.stream().collect(Collectors.toList());

If you want a new list that doesn't share the same objects with the first one, you can do this (supposing your objects implements Cloneable):

If you're using Java 8, you can create a new list using the same objects as the previous one simply using streams:

List<Double> secondList = firstList.stream().collect(Collectors.toList());

If you want a new list that doesn't share the same objects with the first one, you can do this (supposing your objects implements Cloneable):

List<Double> secondList = firstList.stream().map(v -> v.clone()).collect(Collectors.toList());

How to clone two-dimensional List?

4 Answers4