How to sum values from List, Set and Map using stream in Java 8?
Following examples shows how you can use java 8 Stream api to do summation of Integer, Double and Long in List, Set and Map.
Source code (Item.java)
public class Item {
private Long id;
private String name;
private Double price;
public Item(String name, Double price) {
this.name = name;
this.price = price;
}
/* getter - setter */
public Double getPrice() {
return price;
}
public void setPrice(Double price) {
this.price = price;
}
}
List
- Summation of Integers in List.
- Summation of Price from List of beans(Item).
Source code (ListStreamSum.java)
import com.javaquery.bean.Item;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
/**
* Summation of element in List using Stream api in java 8.
*
* @author javaQuery
* @date 17th October, 2016
* @Github: https://github.com/javaquery/Examples
*/
public class ListStreamSum {
public static void main(String[] args) {
/* Summation of Integers in List */
List<Integer> integers = new ArrayList<>(Arrays.asList(10, 20, 30));
Integer integerSum = integers.stream().mapToInt(Integer::intValue).sum();
System.out.println("summation: " + integerSum);
/* Summation when you have list of beans */
Item motoG = new Item("MotoG", 100.12);
Item iPhone = new Item("iPhone", 200.12);
List<Item> listBeans = new ArrayList<>(Arrays.asList(motoG, iPhone));
Double doubleSum = listBeans.stream().mapToDouble(Item::getPrice).sum();
System.out.println("summation: " + doubleSum);
}
}
Output
summation: 60 summation: 300.24
Set
- Summation of Integers in Set.
- Summation of Price from Set of beans(Item).
Source code (SetStreamSum.java)
import com.javaquery.bean.Item;
import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;
/**
* Summation of element in Set using Stream api in java 8.
*
* @author javaQuery
* @date 17th October, 2016
* @Github: https://github.com/javaquery/Examples
*/
public class SetStreamSum {
public static void main(String[] args) {
/* Summation of Integers in Set */
Set<Integer> integers = new HashSet<>(Arrays.asList(10, 20, 30));
Integer integerSum = integers.stream().mapToInt(Integer::intValue).sum();
System.out.println("summation: " + integerSum);
/* Summation when you have set of beans */
Item motoG = new Item("MotoG", 100.12);
Item iPhone = new Item("iPhone", 200.12);
Set<Item> listBeans = new HashSet<>(Arrays.asList(motoG, iPhone));
Double doubleSum = listBeans.stream().mapToDouble(Item::getPrice).sum();
System.out.println("summation: " + doubleSum);
}
}
Output
summation: 60 summation: 300.24
Map
- Summation of Integers in Map as a value.
- Summation of Integers in List as a value.
- Summation of Price from List of beans(Item) in Map.
Source code (MapStreamSum.java)
import com.javaquery.bean.Item;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
/**
* Summation of element in Map using Stream api in java 8.
*
* @author javaQuery
* @date 17th October, 2016
* @Github: https://github.com/javaquery/Examples
*/
public class MapStreamSum {
public static void main(String[] args) {
/* Summation of Integers in Map */
Map<String, Integer> integers = new HashMap<>();
integers.put("A", 10);
integers.put("B", 20);
integers.put("C", 30);
Integer integerSum = integers.values().stream().mapToInt(Integer::intValue).sum();
System.out.println("summation: " + integerSum);
/* Summation when you have List/Set in Map */
Map<String, List<Integer>> listInMap = new HashMap<>();
listInMap.put("even_numbers", new ArrayList<>(Arrays.asList(2, 4, 6)));
integerSum = listInMap.values().stream()
.flatMapToInt(list -> list.stream().mapToInt(Integer::intValue))
.sum();
System.out.println("summation: " + integerSum);
/* Summation when you have List/Set of beans in Map */
Item motoG = new Item("MotoG", 100.12);
Item iPhone = new Item("iPhone", 200.12);
List<Item> items = new ArrayList<>(Arrays.asList(motoG, iPhone));
Map<String, List<Item>> itemMap = new HashMap<>();
itemMap.put("items", items);
Double doubleSum = itemMap.values().stream()
.flatMapToDouble(list -> list.stream().mapToDouble(Item::getPrice))
.sum();
System.out.println("summation: " + doubleSum);
}
}
Output
summation: 60 summation: 12 summation: 300.24
