Get the key of an hashmap when the values are list-CodePudding

I have a list of orders(order id) List<Integer>order with Integer values in it. I also have a Hashmap of the form <String, List<Integer>> which contains the customer names as keys and for each one a list of orders as values.

What I would like to do is take the values from my first order list and see which customer it belongs to. The result will be a list of customers who have made an order.

For example :

     if we have an first list of orders :  
     order = [1,2,3,4,5,6,7] 
     and after we have  
     hashmap={alice =[1,2,3], bob=[4,5,6], paul=[10,11]}

the output should be
List<String> res = [alice, bob] because "paul" commands are not in the first list.

If someone coud help me for this it would be nice.

CodePudding user response：

Stream over your map entries, filter entries having values having any match with elements of your order list, map to key and collect to list:

List<String> res = hashmap.entrySet().stream()
                          .filter(e -> e.getValue().stream().anyMatch(order::contains))
                          .map(Map.Entry::getKey)
                          .collect(Collectors.toList());

See this code run live at Ideone.com.

CodePudding user response：

You can use another HashMap and use the order id as the key.

    Map<Integer, String> orderNameMapping = new HashMap<>();
    Set<String> keySet = hashmap.keySet();
    for(String key: keySet){
        List<Integer> orderIds = hashmap.get(key);
        for(Integer orderId : orderIds) {
            orderNameMapping.put(orderId, key);
        }
    }

    List<String> res = orders.stream().map(orderNameMapping::get).collect(Collectors.toList());

CodePudding user response：

So the pseudocode would be something like.

Create a string list, to return the result.
l1: Loop though the hashmap of customer/orders
...loop through the orders.
......search for the order in the orderarray. If not found, continue l1
...at this point, all orders have been found. Add customer to return list.
return the list.

hope this helps.