What kind of Java collection should I use for this?

Question

I am fairly new to Java and am having trouble understanding the differences between different collections. I have a list of 19 resources. Each resource is assigned a hex color. I want to get a random resource with its color and use that resource with my code. Once I am finished with the current resource, I want to remove it from the list so that only a certain number of resources are used.

Should I use a dictionary, map, or hashtable? Or any others that I am missing.

Answer 1

What you could do, is store your resources into a List, then randomly permutes it thanks to Collections.shuffle(List<?> list) or shuffle(List<?> list, Random rnd), and finally call iterator() on the resulting list to get an instance of Iterator that you could store into a member field to be able to iterate over your list (thanks to hasNext()/next()) and remove your resource once done with remove().

Here is a pseudo code using String as resource just to show the idea:

// Create a read/write list (Arrays.asList returns a read-only list)
List<String> resources = new ArrayList<>(Arrays.asList("1", "2", "3"));
System.out.printf("Initial state = %s%n", resources);
Collections.shuffle(resources);
System.out.printf("Permuted List = %s%n", resources);
Iterator<String> iterator = resources.iterator();
if (iterator.hasNext()) {
    String resource = iterator.next();
    // do something
    iterator.remove();
    System.out.printf("After remove = %s%n", resources);
}

Output:

Initial state = [1, 2, 3]
Permuted List = [3, 1, 2]
After remove = [1, 2]

NB: This approach makes sense in your case as you have a small list, please note that if you have a big list and you intend to retrieve only a small part of it, you should consider using a Random to get randomly the index of the next element (using nextInt(int bound) with list.size() as parameter) to get (using get(int index)) and remove (using remove(int index)) instead of using Collections.shuffle(List<?> list) as it would cause an overhead.

---

ArrayList wouldn't work because I need to assign the color(value) to the resource(key)

Yes it can work if you use a List of a wrapper class that will contain both, your color and your resource (for example AbstractMap.SimpleImmutableEntry or simply a custom class). It is good enough as you don't seem to need to retrieve the color by resource. If you do, you could simply have a Map with resource as key and color as value and use new ArrayList<>(map.keySet()) to initialize your list of resources, then you will be able to apply what is proposed previously in this answer.

Answer 2

If you want to lookup (get) resource based on its hex use following

// Initialize
Map<String, Resource> resourceMap = new HashMap<>();
resourceMap.put("hex1", hex1Resource);
resourceMap.put("hex2", hex3Resource);
resourceMap.put("hex3", hex3Resource);

// Get specific
Resource hex2Resource = resourceMap.get("hex2");
resourceMap.remove("hex2");

If you want to lookup resource randomly, there are 2 options

• Use List (this allows duplicates)
• Use Set (stores only unique values)

Using Lists

// Initialize
List<Resource> list = new ArrayList<>();
list.add(resource1);
list.add(resource2);
list.add(resource3);

// Get random
Random rand = new Random();
Resource randomResource = list.get(rand.nextInt(list.size()));

// Delete the element from list
list.remove(randomResource);

Using Sets

// Initialize
Set<Resource> set = new HashSet<>();
set.add(resource1);
set.add(resource2);
set.add(resource3);

// Convert to List, since Set does not have get(int) method. 
List<Resource> list = new ArrayList<>(set);

// Get random
Random rand = new Random();
Resource randomResource = list.get(rand.nextInt(list.size()));

// Delete the element from list
list.remove(randomResource);

Note: For both cases above, Resource class will need to implement methods equals and hashcode so that list/set can compare elements and work correctly. See Java equals and hashcode

Update: Sets do not have get(int) method. Updated the code to fix this.

Answer 3

The following should work. The steps are:

1. Create a list of Resource
2. Generate a random number and get the item
3. Do what you want to do with your random item
4. Remove it from list

Example Code:

 //1
List<Resource> resources= getList();

//2
Random generator = new Random();
Resource randomResource = resources.get(generator.nextInt(resources.size() -1));

//3
//do your stuff

//4
resources.remove(randomResource);

Resource can be a class which wraps your data

class Resource{
    String color;
    String otherProperties;
    //..


    @Override
    public boolean equals(Object obj) {
        //generate from IDE or write one
    }

    @Override
    public int hashCode() {
        //generate from IDE or write one
    }
}

Answer 4

Blockquote

if u need random order better you can go for List object.

List<String> resourceMap = new CopyOnWriteArrayList<>();
          resourceMap.add("hex1");
          resourceMap.add("hex2");
          resourceMap.add("hex3");
          resourceMap.add("hex4");
          Collections.shuffle(resourceMap);
          resourceMap.forEach(resource->{
              resourceMap.remove(resource);
          });