UnsupportedOperationException When Adding Null to ArrayList

Question

I am trying to add null elements to an ArrayList. This is for the purpose of ignoring columns using supercsv: http://supercsv.sourceforge.net/examples_partial_reading.html I am processing multiple csv files which have different number of header columns.

csvBeanReader.getHeader(true) returns String[]. The line headers.add(null); is throwing an UnsupportedOperationException. Why? What did I do wrong?

List<String> headers = Arrays.asList(csvBeanReader.getHeader(true));

//add null columns to headers
for(int i=0; i<1000; i++){
    headers.add(null);
}

score 3 · Accepted Answer · answered Aug 07 '14 at 17:10

You don't have a java.util.ArrayList, you have something that implements List. This particular List implementation doesn't support modification via changing the size of the List. Even if you add an actual String, you will still get UnsupportedOperationException. From Arrays.asList javadocs:

Returns a fixed-size list backed by the specified array.

To be able to add to that List, wrap it in an actual ArrayList.

List<String> headers = new ArrayList<>(Arrays.asList(csvBeanReader.getHeader(true)));

It looks like Collections.emptyList() is not a good choice as well - it does not let to add to the List too — Andrey M. Stepanov, Jan 27 '19 at 15:42

score 0 · Answer 2 · answered Aug 07 '14 at 17:12

0

This is because Arrays.asList returns an immutable list.

http://docs.oracle.com/javase/7/docs/api/java/util/Arrays.html#asList(T...)

An immutable list will throw an exception when a modification is attempted.

answered Aug 07 '14 at 17:12

Sid

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The list isn't immutable; you can modify the elements. It's just fixed-length, as wrapper around the array. – rgettman Aug 07 '14 at 17:14

UnsupportedOperationException When Adding Null to ArrayList

2 Answers2