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.mkyong.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 Java8Example1 {
public static void main(String[] args) {
//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);
}
}
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output
{
papaya=1, orange=1, banana=2, apple=3
}
1.2 Add sorting.
Java8Example2.java
package com.mkyong.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 Java8Example2 {
public static void main(String[] args) {
//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);
}
}
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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.mkyong.java8;
import java.math.BigDecimal;
public class Item {
private String name;
private int qty;
private BigDecimal price;
//constructors, getter/setters
}
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2.2 Group by the name + Count or Sum the Qty.
Java8Examples3.java
package com.mkyong.java8;
import java.math.BigDecimal;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class Java8Examples3 {
public static void main(String[] args) {
//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);
}
}
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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
}
2.2 Group by Price – Collectors.groupingBy and Collectors.mapping example.
Java8Examples4.java
package com.mkyong.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;
public class Java8Examples4 {
public static void main(String[] args) {
//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);
}
}
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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]
}
