Get the Distinct and count values from a field of an object in a Collection

Question

I have a List of Pin objects (List<Pin>) where the Pin class has the following attributes:

String pinNumber, String pinType, Date insertDate

I would like to get a HashMap with <String pinNumber, int count> that have the distinct pinNumber telling me how many distinct pinNumber are in the List<Pin> and a count of each.

So the way I know of to do this is to:

• Iterate through the List<Pin>
• Check if the HashMap contains already the key value of the pinNumber and:
• Increase it or add it if it does not exist.

I would like to do the same for each of the fields from the Pin object.

I am sure there should be an easier way to do this?

Maybe Guava has something simpler?

Answer 1

If you have the possibility to use Java 8 (and since what you want to do basically sounds like a "group by" operation), this can be solved in an elegant way using the new Stream API (as hinted by user vallismortis):

import static java.util.stream.Collectors.counting;
import static java.util.stream.Collectors.groupingBy;

import java.util.Arrays;
import java.util.Date;
import java.util.List;
import java.util.Map;

public class Main {

    public static void main(String[] args) {
        List<Pin> pins = Arrays.asList(
                new Pin("PIN-1", "T1", new Date()),
                new Pin("PIN-1", "T2", new Date()),
                new Pin("PIN-1", "T3", new Date()),
                new Pin("PIN-2", "T2", new Date()),
                new Pin("PIN-2", "T2", new Date()),
                new Pin("PIN-3", "T2", new Date())

        );
        Map<String, Long> map = pins.stream().collect(groupingBy(Pin::getPinNumber, counting()));
        System.out.println("map = " + map);
    }
}

class Pin {
    String pinNumber;
    String pinType;
    Date insertDate;

    public Pin(String pinNumber, String pinType, Date insertDate) {
        this.pinNumber = pinNumber;
        this.pinType = pinType;
        this.insertDate = insertDate;
    }

    public String getPinNumber() {
        return pinNumber;
    }

    public String getPinType() {
        return pinType;
    }

    public Date getInsertDate() {
        return insertDate;
    }
}

Output:

map = {PIN-1=3, PIN-3=1, PIN-2=2}