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Java Forum / General / August 2007Does the clone() method of ArrayList<> make a copy of the objects in the ArrayList?
anotherArrayList = (ArrayList<Something>) oneArrayList.clone(); anotherArrayList.get(0).makeSomeChange(); Will the change made in the second line make effect on oneArrayList.get(0)? > anotherArrayList = (ArrayList<Something>) oneArrayList.clone(); > anotherArrayList.get(0).makeSomeChange(); > > Will the change made in the second line make effect on > oneArrayList.get(0)? clone is a shallow clone not a deep clone so yes. Arne >> anotherArrayList = (ArrayList<Something>) oneArrayList.clone(); >> anotherArrayList.get(0).makeSomeChange(); [quoted text clipped - 5 lines] > > Arne Arne: I know that it says in the docs that ArrayList.clone() is a shallow copy and that the elements themselves are not copied. Why then does the following code produce the following results (or have I gone completely around the bend today?). import java.util.*; public class test9 { public static void main(String[] args) { ArrayList<Integer> list1 = new ArrayList<Integer>(); list1.add(10); list1.add(11); list1.add(12); ArrayList list2 = (ArrayList)list1.clone(); list1.set(0,20); list1.add(30); for (int i=0; i<list1.size(); i++) System.out.print(list1.get(i)+" "); System.out.println(); for (int i=0; i<list2.size(); i++) System.out.print(list2.get(i)+" "); } } C:\Documents and Settings\Knute Johnson>java test9 20 11 12 30 10 11 12  SignatureKnute Johnson email s/nospam/knute/ On Aug 3, 8:28 pm, Knute Johnson <nos...@rabbitbrush.frazmtn.com> wrote: > Arne Vajh?j wrote: > >> anotherArrayList = (ArrayList<Something>) oneArrayList.clone(); [quoted text clipped - 25 lines] > list1.set(0,20); > list1.add(30); These modify the cloned list, not the Integer objects. Try replacing the Integers with StringBuffers, and change the set line to actually edit the string buffer, i.e. list1.get(0).append("foobar"); Then your first list output should show the first item with "foobar" on the end and your fourth added item; your second list output should still omit the fourth item but also have the first item have "foobar" on the end. >>> anotherArrayList = (ArrayList<Something>) oneArrayList.clone(); >>> anotherArrayList.get(0).makeSomeChange(); [quoted text clipped - 35 lines] > 20 11 12 30 > 10 11 12 The difference between a deep and shallow clone is difficult to see if you only add immutable objects, such as Integer: import java.util.*; public class ListCloneTest { public static void main(String[] args) { ArrayList<StringBuilder> list1 = new ArrayList<StringBuilder>(); list1.add(new StringBuilder("aaa ")); ArrayList list2 = (ArrayList) list1.clone(); StringBuilder builder = list1.get(0); builder .append("Both lists point to the same StringBuilder"); for (int i = 0; i < list1.size(); i++) System.out.print(list1.get(i) + " "); System.out.println(); for (int i = 0; i < list2.size(); i++) System.out.print(list2.get(i) + " "); } } aaa Both lists point to the same StringBuilder aaa Both lists point to the same StringBuilder Patricia On Aug 3, 7:28 pm, Knute Johnson <nos...@rabbitbrush.frazmtn.com> wrote: > Arne Vajh?j wrote: > >> anotherArrayList = (ArrayList<Something>) oneArrayList.clone(); [quoted text clipped - 23 lines] > list1.add(12); > ArrayList list2 = (ArrayList)list1.clone(); this line copies every item in list1 into list2 such that list1.get(i) = list2.get(i) for i = 0, 1, 2, which means, two references (0th item in list1 and 0th item in list2) point to the same thing, but they themselves are not identical. > list1.set(0,20); now you changed the 0th item in list1, in other words, you let the the 0th item in list1, which is a reference, point to another thing (20). This process has nothing to do with list2, meaning 0th item in list2 still points to 10. > list1.add(30); > for (int i=0; i<list1.size(); i++) [quoted text clipped - 9 lines] > 20 11 12 30 > 10 11 12 However, as I konw ArrayList can only deal with classes, e.g. Integer, but not primitive types like int. How come your code compiles? >> ArrayList<Integer> list1 = new ArrayList<Integer>(); >> list1.add(10); >> list1.add(11); >> list1.add(12); > However, as I konw ArrayList can only deal with classes, e.g. Integer, > but not primitive types like int. How come your code compiles? Newer Java versions (1.5+) can auto convert between int and Integer. Arne Do you mean now wherever you can use Integer, you can use int, and vice versa? That's cool. Didn't know that before. On Aug 3, 10:11 pm, Arne Vajh?j <a...@vajhoej.dk> wrote: > >> ArrayList<Integer> list1 = new ArrayList<Integer>(); > >> list1.add(10); [quoted text clipped - 6 lines] > > Arne > Do you mean now wherever you can use Integer, you can use int, and > vice versa? > > That's cool. Didn't know that before. Pretty much. I don't know what the performance hit though but it is handy. SignatureKnute Johnson email s/nospam/knute/ On Aug 4, 6:55 am, Knute Johnson <nos...@rabbitbrush.frazmtn.com> wrote: > > Do you mean now wherever you can use Integer, you can use int, and > > vice versa? > > > That's cool. Didn't know that before. > > Pretty much. I don't know what the performance hit though but it is handy. I'd say that it is as fast as converting it "by hand", since the distinction between int and Integer can be done by compiler at compile time, so there is no runtime overhead. >> clone is a shallow clone not a deep clone so yes. > I know that it says in the docs that ArrayList.clone() is a shallow copy > and that the elements themselves are not copied. Why then does the > following code produce the following results (or have I gone completely > around the bend today?). As other has already stated, then you are changing the references in the lists not the objects pointed to by the references. The docs are correct. Arne >>> clone is a shallow clone not a deep clone so yes. > [quoted text clipped - 9 lines] > > Arne Thanks very much everybody. I was confused when I wrote the test code because it did copy the references. That was not as I expected. Using a mutable object it is immediately obvious though. SignatureKnute Johnson email s/nospam/knute/ >anotherArrayList = (ArrayList<Something>) oneArrayList.clone(); >anotherArrayList.get(0).makeSomeChange(); >Will the change made in the second line make effect on >oneArrayList.get(0)? You have watched Star Trek too many times. The computer will not explode if you try the experiment and guess incorrectly. SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com On Aug 4, 6:53 am, Roedy Green <see_webs...@mindprod.com.invalid> wrote: > >anotherArrayList = (ArrayList<Something>) oneArrayList.clone(); > >anotherArrayList.get(0).makeSomeChange(); [quoted text clipped - 6 lines] > Roedy Green Canadian Mind Products > The Java Glossaryhttp://mindprod.com Excuse me. Star Trek? >>> anotherArrayList = (ArrayList<Something>) oneArrayList.clone(); >>> anotherArrayList.get(0).makeSomeChange(); >>> Will the change made in the second line make effect on >>> oneArrayList.get(0)? Roedy Green wrote: >> You have watched Star Trek too many times. The computer will not >> explode if you try the experiment and guess incorrectly. > Excuse me. Star Trek? Or any other science-fiction work which uses the cliché of a computer exploding by feeding it bad data or code. He's telling you that you should've tried it. Implicitly he's inviting you to examine your actions and ponder the virtues of experimentation as a learning tool generally. The computer will answer your questions more accurately, more ineluctably and more convincingly than anything we can say. SignatureLew >>>> anotherArrayList = (ArrayList<Something>) oneArrayList.clone(); >>>> anotherArrayList.get(0).makeSomeChange(); [quoted text clipped - 9 lines] > Or any other science-fiction work which uses the cliché of a computer > exploding by feeding it bad data or code. Or even non-SF. There was en episode of "The Paper Chase" (generally a fine, intelligent series) that used this same stupid cliche. There was a "super-computer" [1] that had been programmed to solve legal issues, threatening to make lawyers obsolete. But clever Professor Kingsfield asked it a hypothetical question, whereupon the tape drives whirred furiously, the lights dimmed, sparks began to fly, and the computer more or less blew up. 1. Which was every bit as silly as the plot; it was built from racks and racks of obsolete PDP-11 peripherals. > >>> anotherArrayList = (ArrayList<Something>) oneArrayList.clone(); > >>> anotherArrayList.get(0).makeSomeChange(); [quoted text clipped - 15 lines] > -- > Lew :) got it. looks like I need more sense of humor:) Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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