Java 8 –ストリームコレクターのgroupingByの例
この記事では、Java 8ストリームCollectorsを使用して、Listをグループ化し、カウントし、合計し、並べ替える方法を示します。
1. グループ化、カウント、並べ替え
1.1 Group by a List and display the total count of it.
Java8Example1.java
package com.example.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 items =
Arrays.asList("apple", "apple", "banana",
"apple", "orange", "banana", "papaya");
Map result =
items.stream().collect(
Collectors.groupingBy(
Function.identity(), Collectors.counting()
)
);
System.out.println(result);
}
}
出力
{
papaya=1, orange=1, banana=2, apple=3
}
1.2 Add sorting.
Java8Example2.java
package com.example.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 items =
Arrays.asList("apple", "apple", "banana",
"apple", "orange", "banana", "papaya");
Map result =
items.stream().collect(
Collectors.groupingBy(
Function.identity(), Collectors.counting()
)
);
Map finalMap = new LinkedHashMap<>();
//Sort a map and add to finalMap
result.entrySet().stream()
.sorted(Map.Entry.comparingByValue()
.reversed()).forEachOrdered(e -> finalMap.put(e.getKey(), e.getValue()));
System.out.println(finalMap);
}
}
出力
{
apple=3, banana=2, papaya=1, orange=1
}
2. オブジェクトのリスト
ユーザー定義オブジェクトのリストを「グループ化」する例。
2.1 A Pojo.
Item.java
package com.example.java8;
import java.math.BigDecimal;
public class Item {
private String name;
private int qty;
private BigDecimal price;
//constructors, getter/setters
}
2.2 Group by the name + Count or Sum the Qty.
Java8Examples3.java
package com.example.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- 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
counting = items.stream().collect(
Collectors.groupingBy(Item::getName, Collectors.counting()));
System.out.println(counting);
Map sum = items.stream().collect(
Collectors.groupingBy(Item::getName, Collectors.summingInt(Item::getQty)));
System.out.println(sum);
}
}
出力
//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.example.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- 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
> groupByPriceMap =
items.stream().collect(Collectors.groupingBy(Item::getPrice));
System.out.println(groupByPriceMap);
// group by price, uses 'mapping' to convert List- to Set
Map> result =
items.stream().collect(
Collectors.groupingBy(Item::getPrice,
Collectors.mapping(Item::getName, Collectors.toSet())
)
);
System.out.println(result);
}
}
出力
{
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]
}