In this article, we will show you how to use Java 8 Stream Collectors to group by, count, sum and sort a List.
1. Group By, Count and Sort
1.1 Group by a List and display the total count of it.
Java8Example1.java
package com.favtuts.java8;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
public class CollectorsGroupingMapping {
public static void main(String[] args) {
groupListAndDisplayTotalCount();
}
static void groupListAndDisplayTotalCount() {
//3 apple, 2 banana, others 1
List<String> items =
Arrays.asList("apple", "apple", "banana",
"apple", "orange", "banana", "papaya");
Map<String, Long> result =
items.stream().collect(
Collectors.groupingBy(
Function.identity(), Collectors.counting()
)
);
System.out.println(result);
}
}
Output
{papaya=1, orange=1, banana=2, apple=3}1.2 Add sorting.
Java8Example2.java
package com.favtuts.java8;
import java.util.Arrays;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
public class CollectorsGroupingMapping {
public static void main(String[] args) {
groupListAndDisplayTotalCountAddSorting();
}
static void groupListAndDisplayTotalCountAddSorting() {
// 3 apple, 2 banana, others 1
List<String> items =
Arrays.asList("apple", "apple", "banana",
"apple", "orange", "banana", "papaya");
Map<String, Long> result =
items.stream().collect(
Collectors.groupingBy(
Function.identity(), Collectors.counting()
)
);
Map<String, Long> finalMap = new LinkedHashMap<>();
// Sort a map and add to finalMap
result.entrySet().stream()
.sorted(Map.Entry.<String, Long>comparingByValue().reversed())
.forEachOrdered(e -> finalMap.put(e.getKey(), e.getValue()));
System.out.println(finalMap);
}
}
Output
{apple=3, banana=2, papaya=1, orange=1}2. List Objects
Examples to ‘group by’ a list of user defined Objects.
2.1 A Pojo.
Item.java
package com.favtuts.java8.stream.grouping;
import java.math.BigDecimal;
public class Item {
public Item(String name, int qty, BigDecimal price) {
this.name = name;
this.qty = qty;
this.price = price;
}
public Item() {
}
private String name;
private int qty;
private BigDecimal price;
//constructors, getter/setters
@Override
public String toString() {
return "Item{" +
" name='" + getName() + "'" +
", qty='" + getQty() + "'" +
", price='" + getPrice() + "'" +
"}";
}
public String getName() {
return this.name;
}
public void setName(String name) {
this.name = name;
}
public int getQty() {
return this.qty;
}
public void setQty(int qty) {
this.qty = qty;
}
public BigDecimal getPrice() {
return this.price;
}
public void setPrice(BigDecimal price) {
this.price = price;
}
}
2.2 Group by the name + Count or Sum the Qty.
Java8Examples3.java
package com.favtuts.java8;
import java.math.BigDecimal;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import com.favtuts.java8.stream.grouping.Item;
public class CollectorsGroupingMapping {
public static void main(String[] args) {
groupListPOJOandCountSumQty();
}
static void groupListPOJOandCountSumQty() {
//3 apple, 2 banana, others 1
List<Item> items = Arrays.asList(
new Item("apple", 10, new BigDecimal("9.99")),
new Item("banana", 20, new BigDecimal("19.99")),
new Item("orang", 10, new BigDecimal("29.99")),
new Item("watermelon", 10, new BigDecimal("29.99")),
new Item("papaya", 20, new BigDecimal("9.99")),
new Item("apple", 10, new BigDecimal("9.99")),
new Item("banana", 10, new BigDecimal("19.99")),
new Item("apple", 20, new BigDecimal("9.99"))
);
Map<String, Long> counting = items.stream().collect(
Collectors.groupingBy(Item::getName, Collectors.counting()));
System.out.println(counting);
Map<String, Integer> sum = items.stream().collect(
Collectors.groupingBy(Item::getName, Collectors.summingInt(Item::getQty)));
System.out.println(sum);
}
}
Output
//Group by + Count
{
papaya=1, banana=2, apple=3, orang=1, watermelon=1
}
//Group by + Sum qty
{
papaya=20, banana=30, apple=40, orang=10, watermelon=10
}2.3 Group by Price – Collectors.groupingBy and Collectors.mapping example.
Java8Examples4.java
package com.favtuts.java8;
import java.math.BigDecimal;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;
import com.favtuts.java8.stream.grouping.Item;
public class CollectorsGroupingMapping {
public static void main(String[] args) {
groupListPOJObyPriceAndMapping();
}
static void groupListPOJObyPriceAndMapping() {
//3 apple, 2 banana, others 1
List<Item> items = Arrays.asList(
new Item("apple", 10, new BigDecimal("9.99")),
new Item("banana", 20, new BigDecimal("19.99")),
new Item("orang", 10, new BigDecimal("29.99")),
new Item("watermelon", 10, new BigDecimal("29.99")),
new Item("papaya", 20, new BigDecimal("9.99")),
new Item("apple", 10, new BigDecimal("9.99")),
new Item("banana", 10, new BigDecimal("19.99")),
new Item("apple", 20, new BigDecimal("9.99"))
);
//group by price
Map<BigDecimal, List<Item>> groupByPriceMap =
items.stream().collect(Collectors.groupingBy(Item::getPrice));
System.out.println(groupByPriceMap);
// group by price, uses 'mapping' to convert List<Item> to Set<String>
Map<BigDecimal, Set<String>> result =
items.stream().collect(
Collectors.groupingBy(Item::getPrice,
Collectors.mapping(Item::getName, Collectors.toSet())
)
);
System.out.println(result);
}
}
Output
{
19.99=[
Item{name='banana', qty=20, price=19.99},
Item{name='banana', qty=10, price=19.99}
],
29.99=[
Item{name='orang', qty=10, price=29.99},
Item{name='watermelon', qty=10, price=29.99}
],
9.99=[
Item{name='apple', qty=10, price=9.99},
Item{name='papaya', qty=20, price=9.99},
Item{name='apple', qty=10, price=9.99},
Item{name='apple', qty=20, price=9.99}
]
}
//group by + mapping to Set
{
19.99=[banana],
29.99=[orang, watermelon],
9.99=[papaya, apple]
}Download Source Code
$ git clone https://github.com/favtuts/java-core-tutorials-examples
$ cd java-basic/java8