I am unable to add an element to a list? UnsupportedOperationException

问题

This one list object is biting me in the butt..

Any time I try to add an element to it, it produces this:

Caused by: java.lang.UnsupportedOperationException
        at java.util.AbstractList.add(AbstractList.java:148)
        at java.util.AbstractList.add(AbstractList.java:108)

The line producing the error is insignificant, but here it is anyways:

AdventureLobbies.players.add(args[0].toLowerCase());

Should I not be accessing it statically?

Actual declaration of variable:

AdventureLobbies.players = Arrays.asList(rs.getString("players").toLowerCase().split(","));

Any ideas? Can't find anything on Google that's worthwhile.

回答1:

Arrays.asList() will give you back an unmodifiable list, and that is why your add is failing. Try creating the list with:

AdventureLobbies.players = new ArrayList(Arrays.asList(rs.getString("players").toLowerCase().split(",")));

回答2:

The java docs say asList @SafeVarargs public static <T> List<T> asList(T... a) "Returns a fixed-size list backed by the specified array"

Your list is fixed size, meaning it cannot grow or shrink and so when you call add, it throws an unsupported operation exception

回答3:

This exception is very familiar with accessing objects that will not permit the access according to java language rules like accessing immutable objects, for that reason instantiate it in the following way instead:

AdventureLobbies.players = new ArrayList(Arrays.
asList(rs.getString("players").toLowerCase().split(","))); // Perfectly done

来源：https://stackoverflow.com/questions/10059395/i-am-unable-to-add-an-element-to-a-list-unsupportedoperationexception

标签

java

list

object

add

abstract