ConcurrentModificationException for ArrayList [duplicate]

Question

I have the following piece of code:

private String toString(List<DrugStrength> aDrugStrengthList) {     StringBuilder str = new StringBuilder();         for (DrugStrength aDrugStrength : aDrugStrengthList) {             if (!aDrugStrength.isValidDrugDescription()) {                 aDrugStrengthList.remove(aDrugStrength);             }         }         str.append(aDrugStrengthList);         if (str.indexOf("]") != -1) {             str.insert(str.lastIndexOf("]"), "\n          " );         }     return str.toString(); } 

When I try to run it, I get ConcurrentModificationException, can anyone explain why it happens, even if the code is running in same thread? And how could I avoid it?

Answer 1

You can't remove from list if you're browsing it with "for each" loop. You can use Iterator. Replace:

for (DrugStrength aDrugStrength : aDrugStrengthList) {     if (!aDrugStrength.isValidDrugDescription()) {         aDrugStrengthList.remove(aDrugStrength);     } } 

With:

for (Iterator<DrugStrength> it = aDrugStrengthList.iterator(); it.hasNext(); ) {     DrugStrength aDrugStrength = it.next();     if (!aDrugStrength.isValidDrugDescription()) {         it.remove();     } } 

Answer 2

Like the other answers say, you can't remove an item from a collection you're iterating over. You can get around this by explicitly using an Iterator and removing the item there.

Iterator<Item> iter = list.iterator(); while(iter.hasNext()) {   Item blah = iter.next();   if(...) {     iter.remove(); // Removes the 'current' item   } }