ArrayIndexOutOfBoundsException when using the ArrayList's iterator

Question

Right now, I have a program containing a piece of code that looks like this:

while (arrayList.iterator().hasNext()) {
     //value is equal to a String value
     if( arrayList.iterator().next().equals(value)) {
          // do something 
     }
}

Am I doing that right, as far as iterating through the ArrayList goes?

The error I am getting is:

java.lang.ArrayIndexOutOfBoundsException: -1
    at java.util.ArrayList.get(Unknown Source)
    at main1.endElement(main1.java:244)
    at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.endElement(Unknown Source)
    at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanEndElement(Unknown Source)
    at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(Unknown Source)
    at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(Unknown Source)
    at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
    at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
    at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
    at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(Unknown Source)
    at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(Unknown Source)
    at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(Unknown Source)
    at javax.xml.parsers.SAXParser.parse(Unknown Source)
    at javax.xml.parsers.SAXParser.parse(Unknown Source)
    at main1.traverse(main1.java:73)
    at main1.traverse(main1.java:102)
    at main1.traverse(main1.java:102)
    at main1.main(main1.java:404)

I would show the rest of the code, but it's pretty extensive, and if I am not doing the iteration correctly, I would assume the only possibility is that I am not initializing the ArrayList properly.

Answer 1

Am I doing that right, as far as iterating through the Arraylist goes?

No: by calling iterator twice in each iteration, you're getting new iterators all the time.

The easiest way to write this loop is using the for-each construct:

for (String s : arrayList)
    if (s.equals(value))
        // ...

As for

java.lang.ArrayIndexOutOfBoundsException: -1

You just tried to get element number -1 from an array. Counting starts at zero.

Answer 2

While I agree that the accepted answer is usually the best solution and definitely easier to use, I noticed no one displayed the proper usage of the iterator. So here is a quick example:

Iterator<Object> it = arrayList.iterator();
while(it.hasNext())
{
    Object obj = it.next();
    //Do something with obj
}

Answer 3

List<String> arrayList = new ArrayList<String>();
for (String s : arrayList) {
    if(s.equals(value)){
        //do something
    }
}

or

for (int i = 0; i < arrayList.size(); i++) {
    if(arrayList.get(i).equals(value)){
        //do something
    }
}

But be carefull ArrayList can hold null values. So comparation should be

value.equals(arrayList.get(i))

when you are sure that value is not null or you should check if given element is null.

Answer 4

You can also use like this:

for(Iterator iterator = arrayList.iterator(); iterator.hasNext();) {
x = iterator.next();
//do some stuff
}

Its a good practice to cast and use the object. For example, if the 'arrayList' contains a list of 'Object1' objects. Then, we can re-write the code as:

for(Iterator iterator = arrayList.iterator(); iterator.hasNext();) {
x = (Object1) iterator.next();
//do some stuff
}

Answer 5

You could also do a for loop as you would for an array but instead of array[i] you would use list.get(i)

for (int i = 0; i < list.size(); i++) {
    System.out.println(list.get(i));
}

Answer 6

Apart of larsmans answer (who is indeed correct), the exception in a call to a get() method, so the code you have posted is not the one that is causing the error.

Answer 7

Efficient way to iterate your ArrayList followed by this link. This type will improve the performance of looping during iteration

int size = list.size();

for(int j = 0; j < size; j++) {
    System.out.println(list.get(i));
}

Answer 8

iterating using iterator is not fail-safe for example if you add element to the collection after iterator's creation then it will throw concurrentmodificaionexception. Also it's not thread safe, you have to make it thread safe externally.

So it's better to use for-each structure of for loop. It's atleast fail-safe.