Question

When I reverse iterate over an ArrayList I am getting a IndexOutOfBoundsException. I tried doing forward iteration and there is no problem. I expect and know that there are five elements in the list. The code is below:

Collection rtns = absRtnMap.values(); List list = new ArrayList(rtns); Collections.sort(list);  for(int j=list.size();j>0;j=j-1){   System.out.println(list.get(j)); } 

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Forward iteration – which is working fine, but not useful for me:

for(int j=0;j<list.size();j++){     System.out.println(list.isEmpty());     System.out.println(list.get(j)); } // this worked fine 

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The error:

Exception in thread 'Timer-0' java.lang.IndexOutOfBoundsException: Index: 3, Size: 3     at java.util.ArrayList.RangeCheck(Unknown Source)     at java.util.ArrayList.get(Unknown Source)     at model.Return.getReturnMap(Return.java:61)     at controller.Poller$1.run(Poller.java:29)     at java.util.TimerThread.mainLoop(Unknown Source)     at java.util.TimerThread.run(Unknown Source) 

Also if anyone knows of a better idiom for reverse iteration I would be happy to try that out.

Answer 1

Start the iteration at list.size() - 1 because array (or ArrayList) elements are numbered from 0 up through 1 less than the size of the list. This is a fairly standard idiom:

for (int j = list.size() - 1; j >= 0; j--) {     // whatever } 

Note that your forward iteration works because it stops before reaching list.size().