Printing a List backwards

[archu sweet]

I have a doubt in the following code :


List list = new ArrayList();
list.add("1"); list.add("2"); list.add("3");
for (Object obj: reverse(list)) // can't we declare like this
System.out.print(obj + ", ");

[Campbell Ritchie]

What happens when you try it? Where is the reverse() method?

[Campbell Ritchie]

. . . and your original thread title bore no relationship to its contents. I have changed it.

[archu sweet]

@campbell :

 public static Iterator reverse(List list) {
 Collections.reverse(list);
 return list.iterator();
 }
 public static void main(String[] args) {
 List list = new ArrayList();
 list.add("1"); list.add("2"); list.add("3");
 for (Object obj: reverse(list))
System.out.print(obj + ", ");
 }

[Callum Ludwig]

Hi Archu,

Wrapping source code examples in code blocks makes for a nicer easier to read output for forum posts.

 
List list = new ArrayList(); 
list.add("1"); list.add("2"); list.add("3"); 
for (Object obj: reverse(list)) // can't we declare like this 
System.out.print(obj + ", ");

Where is the reverse method? Do you mean the static method found in the java.util.Collections class?

Collections.reverse(list);
for (Object obj : list)
	//print out here

You can't call the static method reverse within the enhanced for loop as it returns void. It actually updates the loop passed in by the reference. Hence why I called reverse and then was able to use the list directly in the enhanced for loop.

You should get to know the classes and interfaces found in the java.util.* package, as there are heaps and heaps of awesome stuff already implemented to help Java developers.

Hope all this helps.

[archu sweet]

@callum : thanks a ton for your explanation...

[Rob Spoor]

archu sweet wrote:@campbell :

 public static Iterator reverse(List list) {
 Collections.reverse(list);
 return list.iterator();
 }
 public static void main(String[] args) {
 List list = new ArrayList();
 list.add("1"); list.add("2"); list.add("3");
 for (Object obj: reverse(list))
System.out.print(obj + ", ");
 }

The for-each loop works with Iterable, not Iterator. There's a difference. Make the reverse method return List (and so "return list" instead of "return list.iterator()") and that code should work, since List extends Iterable (indirectly).

[Campbell Ritchie]

Thank you everybody for answering my questions.

[archu sweet]

Thanks all