UnsupportedOperationException at java.util.AbstractList.add

Question

I'm having issues getting a block of code to run properly. I'm not entirely sure WHAT this code does (I'm trying to get a plugin that's out of date to work properly with our server), I just know every 20 minutes it runs and throws out an error. Here's the section of code where the issue is happening:

public class DynamicThread extends Thread { private LocalShops plugin = null;   public DynamicThread(ThreadGroup tgroup, String tname, LocalShops plugin) {     super(tgroup, tname);     this.plugin = plugin; }  public void run() {     Map<ItemInfo, List<Integer>> itemStockMap = Collections.synchronizedMap(new HashMap<ItemInfo, List<Integer>>());      //Dump all the shop stock data into the map.     for ( Shop shop : plugin.getShopManager().getAllShops() ) {         for ( InventoryItem item : shop.getItems() ) {             if (itemStockMap.containsKey(item.getInfo()))                 itemStockMap.get(item.getInfo()).add(item.getStock()); //Where error happens             else                 itemStockMap.put(item.getInfo(), Arrays.asList(item.getStock()));              }     }     for(ItemInfo item : itemStockMap.keySet()) {         List<Integer> stockList = GenericFunctions.limitOutliers(itemStockMap.get(item));         //remove the map before re-adding it         if (DynamicManager.getPriceAdjMap().containsKey(item))              DynamicManager.getPriceAdjMap().remove(item);          //Get the overall stock change for a given item and then calculate the adjustment given the volatility         int deltaStock = GenericFunctions.getSum(stockList) - Config.getGlobalBaseStock();         DynamicManager.getPriceAdjMap().put(item, GenericFunctions.getAdjustment(Config.getGlobalVolatility(), deltaStock));      }      Bukkit.getServer().getScheduler().callSyncMethod(plugin, plugin.getShopManager().updateSigns()); } 

}

The error happens from line 42, which is:

                itemStockMap.get(item.getInfo()).add(item.getStock()); 

The error it outputs happens every 20 minutes twice with 2 seconds in between.

2012-02-16 16:53:25 [INFO] Launch Dynamic Thread 2012-02-16 16:53:25 [SEVERE] Exception in thread "dynamic"  2012-02-16 16:53:25 [SEVERE] java.lang.UnsupportedOperationException 2012-02-16 16:53:25 [SEVERE] at java.util.AbstractList.add(AbstractList.java:131) 2012-02-16 16:53:25 [SEVERE] at java.util.AbstractList.add(AbstractList.java:91) 2012-02-16 16:53:25 [SEVERE] at       com.milkbukkit.localshops.threads.DynamicThread.run(DynamicThread.java:42)  2012-02-16 16:53:27 [INFO] Launch Dynamic Thread 2012-02-16 16:53:27 [SEVERE] Exception in thread "dynamic"  2012-02-16 16:53:27 [SEVERE] java.lang.UnsupportedOperationException 2012-02-16 16:53:27 [SEVERE] at java.util.AbstractList.add(AbstractList.java:131) 2012-02-16 16:53:27 [SEVERE] at java.util.AbstractList.add(AbstractList.java:91) 2012-02-16 16:53:27 [SEVERE] at     com.milkbukkit.localshops.threads.DynamicThread.run(DynamicThread.java:42) 

Thanks in advance for any help.

Answer 1

You're using Arrays.asList() to create the lists in the Map here:

itemStockMap.put(item.getInfo(), Arrays.asList(item.getStock()));   

This method returns a non-resizable List backed by the array. From that method's documentation:

Returns a fixed-size list backed by the specified array. (Changes to the returned list "write through" to the array.)

In order to use a resizable List (and actually copy the contents), use the following:

itemStockMap.put(         item.getInfo(),         new ArrayList<Integer>(Arrays.asList(item.getStock())) );  

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Note: in general, when seeing that UnsupportedOperationException is being thrown by add, etc. it's typically an indication that some code is trying to modify a non-resizable or unmodifiable collection.

For example, Collections.emptyList or Collections.singletonList (which return unmodifiable collections) may be used as optimizations but accidentally be passed into methods that try to modify them. For this reason it's good practice for methods to make defensive copies of collections before modifying them (unless of course modifying the collection is a method's intended side effect) - that way callers are free to use the most appropriate collection implementation without worrying about whether it needs to be modifiable.

Answer 2

I think I've worked out your problem. Arrays.asList(item.getStock()) returns a fixed size list based on the Array passed to it.

This means you cannot add more elements to it.

Instead you should do new ArrayList(Arrays.asList(item.getStock())).

This way you are creating a new list that you can add to.