Collections.copy()

[Bill Compton]

I'm sure I'm doing something silly: can't get Collections.copy() to work:

>type CopyTest.java
import java.util.* ;
public class CopyTest
{
    public static void main( String args[] )
    {
        ArrayList source = new ArrayList();
        source.add( "one" );
        ArrayList destination = new ArrayList( 1000 );
        Collections.copy( destination , source );
    }
}
>java CopyTest
Exception in thread "main" java.lang.IndexOutOfBoundsException: Source does not fit in dest.
        at java.util.Collections.copy(Unknown Source)
        at CopyTest.main(CopyTest.java:10)

Any ideas? In case it matters, java -version says:
java version "1.3.0"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.0-C)
Java HotSpot(TM) Client VM (build 1.3.0-C, mixed mode)
Thanks.

[Bill Compton]

Ok, so I think I figured it out. Collections.copy() requires not only the capacity for an equivalent number of members in the destination, but also that at least that many members already exist in the destination! The javadoc could-a been a little clearer on this point. Reading the Collections.java brought this to light. So, to do a copy that doesn't require pre-existing items to be over-written, something like this is needed:

    public static void copyList( java.util.List dest ,
        java.util.List source ) 
    {
        dest.clear();
        for( int i = 0 ; i < source.size() ; i++ )
        {
            dest.add( source.get( i ));
        }
    }

[Brian Jackson]

Here's another way to accomplish what you needed:

List source = new ArrayList();
source.add("one");
List destination = Arrays.asList(new Object[source.size()]);
Collections.copy(destination, source);

[thushara wijeratna]

You could use List.addAll(Collection c) to do this as well.

[Campbell Ritchie]

Welcome to JavaRanch, both of you

I am afraid addAll will have a slightly different effect; old elements in the destination List will still be there.

[thushara wijeratna]

yes true. one should clear the dest list first. normally, people who want to use Collections.copy has an empty destination list in mind. Alas, Collections.copy doesn't really do a copy, it is an "element replacer". so we fall back on List.addAll().

[Zongjian Feng]

thushara wijeratna wrote:yes true. one should clear the dest list first. normally, people who want to use Collections.copy has an empty destination list in mind. Alas, Collections.copy doesn't really do a copy, it is an "element replacer". so we fall back on List.addAll().

 
List<String> src = new ArrayList<String>();
src.add("1");
src.add("2");
src.add("3");
List<String> dest = new ArrayList<String>(Arrays.asList(new String[src.size()]));
dest.add("4");
Collections.copy(dest, src);
System.out.println(dest); // output is: [1, 2, 3, 4]