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 I\'m not entirely sure WHAT this code does (I\'m trying to get a plugin that\'s out of date to work properly with

Answer 1

List is Interface and you can not Add value in it until it is instance of ArrayList(interface should be implemented by some class)

For Example:

    List test = new ArrayList<>();
    test.add(new Integer(2));

    ArrayList test2 = new ArrayList<>();
    test2.add(new Integer(2));

    List test3 = Collections.EMPTY_LIST;
    test3.add(new Integer(2));

Here Object test and test2 are perfect, because they are object of ArrayList class so addition is possible
While in test3 it is just empty list so you can not add element in it.

I was also doing the same mistake.

Here is my Suggestion Use ArrayList when you have to do operations like add or remove, Use List only for reference purpose.

 Map> itemStockMap = Collections.synchronizedMap(new HashMap>());