搜索一个HashMap <String, Arraylist<Users> &gt;或HashMap <String, HashSet<Users> &gt;

Question

I need to search through a HashMap that contains a key/value of either String, ArrayList<Users> or String, HashSet<Users> for a given value. I am unsure how to write the code to do this in java.

I declared..

private HashMap<String, ArrayList<Users>> cm = new HashMap<>();

my method that needs to be called to search looks like this...

public void doSomething(User u, String product) {

}

so I'm unsure of how the code inside the method would work.

if (cm.containsKey(product) {
   //This is where I'm unsure if I find the key on how to check if the ArrayList has a 
   particular user. And then if not how to add them.
}

Answer 1

I'm not sure exactly how to do it, but I asked a question a few days ago concerning map entry sets and using '?' to allow multiple types. The answer I accepted was well explained and comes with an example. Might help you out.

Trouble understanding Java map Entry sets

Answer 2

Your User addition method could look like this:

public void doSomething(User user, String product) {
   if (cm.containsKey(product)) {
      List<User> list = cm.get(product);
      if (!list.contains(user)) {
         list.add(user);
      }
   }
}

This assumes that you define your HashMap with a single User type:

HashMap<String, ArrayList<User>> cm = new HashMap<>();