What is difference between null and empty list?

Question

List<Map<String, Object>> pcList = null;
Map<String, Object> pcMap = new HashMap<String, Object>();
ComputerConfigurations tempPC = null;

if (historyList != null) {
    Iterator<ComputerConfigurations> iterator = historyList.iterator();
    while (iterator.hasNext()) {
        tempPC = (ComputerConfigurations) iterator.next();
        pcMap.put(tempPC.getEnvironment(), tempPC);
        pcList.add((Map<String, Object>) pcMap);
    }
}

I am getting null pointer exception on pcList.add((Map<String, Object>)pcMap); line. [Servlet Error]-: java.lang.NullPointerException . Any suggestion ?

Answer 1

In Java, collections won't magically spring into existence just by adding something to them. You have to initialize pcList by creating an empty collection first:

List<Map<String, Object>> pcList = new ArrayList<>();

An empty collection isn't the same as null. An empty collection is actually a collection, but there aren't any elements in it yet. null means no collection exists at all.

Note that an object can't be of type List, because that's an interface; therefore, you have to tell Java what kind of List you really want (such as an ArrayList, as I've shown above, or a LinkedList, or some other class that implements List).