Question

I have a HashSet that I created and this is what it contains. It will contain more later on, this is pasted from standard out when I did a toString on it. Just to show the contents.

foo.toString(): Abstractfoo [id=2, serial=1d21d, value=1.25, date=2012-09-02 12:00:00.0]
INFO  [STDOUT] price.toString(): Abstractfoo [id=1, serial=1d24d, value=1.30, date=2012-09-19 12:00:00.0]

I have a List that I also have and I need to compare the two. One of the elements in List is:

Bar.toString(): Bar [id=1d21d, name=Dell, description=Laptop, ownerId=null]

Here is what I am trying to do…

Bar contains all of the elements I want foo to have. There will only be one unique serial. I would like my program to see if an element in the list that is in HashSet contains the id for bar. So serial == id.

Here is what I’ve been trying to do

Removed code and added clearer code below

I’ve verified the data is getting entered into the HashSet and List correctly by viewing it through the debugger.

foo is being pulled from a database through hibernate, and bar is coming from a different source. If there is an element in bar I need to add it to a list and I’m passing it back to my UI where I’ll enter some additional data and then commit it to the database.

Let me know if this makes sense and if I can provide anymore information.
Thanks

EDIT: Here is the class

@RequestMapping(value = "/system", method = RequestMethod.GET)
    public @ResponseBody
    List<AbstractSystem> SystemList() {

        // Retrieve system list from database
        HashSet<AbstractSystem> systemData = new HashSet<AbstractSystem>(
                systemService.getSystemData());

        // Retrieve system info from cloud API
        List<SystemName> systemName= null;
        try {
            systemName = cloudClass.getImages();
        } catch (Exception e) {
            LOG.warn("Unable to get status", e);
        }
            
                // Tried this but, iter2 only has two items and iter has many more.
                // In production it will be the other way around, but I need to not 
                // Have to worry about that
        Iterator<SystemName> iter = systemName.iterator();
        Iterator<AbstractSystem> iter2 = systemData .iterator();
        while(iter.hasNext()){
            Image temp = iter.next();
            while(iter2.hasNext()){
                AbstractPricing temp2 = iter2.next();
                System.out.println("temp2.getSerial(): " + temp2.getSerial());
                System.out.println("temp.getId(): " + temp.getId());
                if(temp2.getSerial().equals(temp.getId())){
                    System.out.println("This will be slow...");
                }
            }
        }

        return systemData;
    }

Answer 1

If N is the number of items in systemName and M is the number of items in systemData, then you’ve effectively built an O(N*M) method.

If you instead represent your systemData as a HashMap of AbstractSystem by AbstractSystem.getSerial() values, then you just loop through the systemName collection and lookup by systemName.getId(). This becomes more like O(N+M).

(You might want to avoid variables like iter, iter2, temp2, etc., since those make the code harder to read.)

EDIT – here’s what I mean:

// Retrieve system list from database
HashMap<Integer, AbstractSystem> systemDataMap = new HashMap<AbstractSystem>(
            systemService.getSystemDataMap());

// Retrieve system info from cloud API
List<SystemName> systemNames = cloudClass.getImages();
for (SystemName systemName : systemNames) {
    if (systemDataMap.containsKey(systemName.getId()) {
        System.out.println("This will be slow...");
    }
}

I used Integer because I can’t tell from your code what the type of AbstractSystem.getSerial() or SystemName.getId() are. This assumes that you store the system data as a Map elsewhere. If not, you could construct the map yourself here.