CODEWITHSUNDEEP
javajava-8java-stream
Ibrahim
Ibrahim

Reputation: 1471

Java 8 Stream: Extract map from list of objects and sum specific key

I have a list of Customer objects

class Customer {
     //Other properties
     Map<String, Object> additionalData;
}

When I got a List<Customer> customers, I want to sum a key called transactions in additionalData. How can I do that with java 8 streams?

Upvotes: 4

Views: 2342

Answers (1)

Eran
Eran

Reputation: 393841

You can map each element of your list to the value of the "transactions" key in the corresponding map, and then sum these values:

int sum = customers.stream()
                   .map(c->(Integer)c.additionalData.get("transactions"))
                   .filter(Objects::nonNull)
                   .mapToInt (Integer::intValue)
                   .sum();

For example:

Customer c1 = new Customer();
c1.additionalData = new LinkedHashMap<> ();
c1.additionalData.put ("transactions", 14);
Customer c2 = new Customer();
c2.additionalData = new LinkedHashMap<> ();
c2.additionalData.put ("transactions", 7);
Customer c3 = new Customer();
c3.additionalData = new LinkedHashMap<> ();
List<Customer> customers = Arrays.asList (c1,c2,c3);
int sum = customers.stream()
                   .map(c->(Integer)c.additionalData.get("transactions"))
                   .filter(Objects::nonNull)
                   .mapToInt (Integer::intValue)
                   .sum();
System.out.println ("sum is " + sum);

Output:

sum is 21

This code is assuming the additionalData member is never null, and the value of the "transactions" key (if it exists in the Map) is always an Integer. If these assumptions are incorrect, the code should be adjusted accordingly.

Upvotes: 5

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