我应该使用什么样的Java集合？

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);
          });