Why do I get an UnsupportedOperationException when trying to remove an element from a List?

Question

I have this code:

public static String SelectRandomFromTemplate(String template,int count) {    String[] split = template.split("|");    List<String> list=Arrays.asList(split);    Random r = new Random();    while( list.size() > count ) {       list.remove(r.nextInt(list.size()));    }    return StringUtils.join(list, ", "); } 

I get this:

06-03 15:05:29.614: ERROR/AndroidRuntime(7737): java.lang.UnsupportedOperationException 06-03 15:05:29.614: ERROR/AndroidRuntime(7737):     at java.util.AbstractList.remove(AbstractList.java:645) 

How would be this the correct way? Java.15

Answer 1

Quite a few problems with your code:

On Arrays.asList returning a fixed-size list

From the API:

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

You can't add to it; you can't remove from it. You can't structurally modify the List.

Fix

Create a LinkedList, which supports faster remove.

List<String> list = new LinkedList<String>(Arrays.asList(split)); 

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On split taking regex

From the API:

String.split(String regex): Splits this string around matches of the given regular expression.

| is a regex metacharacter; if you want to split on a literal |, you must escape it to \|, which as a Java string literal is "\\|".

Fix:

template.split("\\|") 

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On better algorithm

Instead of calling remove one at a time with random indices, it's better to generate enough random numbers in the range, and then traversing the List once with a listIterator(), calling remove() at appropriate indices. There are questions on stackoverflow on how to generate random but distinct numbers in a given range.

With this, your algorithm would be O(N).

Answer 2

This one has burned me many times. Arrays.asList creates an unmodifiable list. From the Javadoc: Returns a fixed-size list backed by the specified array.

Create a new list with the same content:

newList.addAll(Arrays.asList(newArray)); 

This will create a little extra garbage, but you will be able to mutate it.