Java Stream GroupBy 整理[亲测有效]

Java Stream GroupBy 整理[亲测有效]Javastream

大家好,欢迎来到IT知识分享网。

#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);


    }
}

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);


    }
}

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
}

##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);

    }
}

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);

    }
}

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]
}

GroupBy

Map<Integer, Integer> totalNoThings = somethings.stream()
    .collect(Collectors.groupingBy(Something::getParentKey,
        Collectors.summingInt(Something::getNoThings)));

二次排序

List<Something> sorted=somethings.stream().sorted(
    Comparator.comparing((Something x)->totalNoThings.get(x.getParentKey()))
          .thenComparing(Something::getNoThings).reversed())
    .collect(Collectors.toList());

通过多个条件分组

Map<List<String>, Integer> map = 
    myList.stream()
          .collect(Collectors.groupingBy(
             f -> Arrays.asList(f.getType(), f.getCode()),
             Collectors.summingInt(Foo::getQuantity)
          ));

List<Foo> result = 
    map.entrySet()
       .stream()
       .map(e -> new Foo(e.getKey().get(0), e.getValue(), e.getKey().get(1)))
       .collect(Collectors.toList());

高级写法

List<Foo> result = new ArrayList<>(
    myList.stream()
          .collect(Collectors.groupingBy(
              f -> Arrays.<String>asList(f.getType(), f.getCode()),
              MoreCollectors.pairing(
                 Collectors.collectingAndThen(MoreCollectors.first(), Optional::get),
                 Collectors.summingInt(Foo::getQuantity),
                 (f, s) -> new Foo(f.getType(), s, f.getCode())
              )
          )).values());

Compute sum of salaries by department

   Map<Department, Integer> totalByDept
       = employees.stream()
                  .collect(Collectors.groupingBy(Employee::getDepartment,
                                                 Collectors.summingInt(Employee::getSalary)));

我收集了一些java相关的学习资料及架构成长手册,仅供个人学习使用,欢迎大家一起学习探讨

java学习资料及架构成长手册

免责声明:本站所有文章内容,图片,视频等均是来源于用户投稿和互联网及文摘转载整编而成,不代表本站观点,不承担相关法律责任。其著作权各归其原作者或其出版社所有。如发现本站有涉嫌抄袭侵权/违法违规的内容,侵犯到您的权益,请在线联系站长,一经查实,本站将立刻删除。 本文来自网络,若有侵权,请联系删除,如若转载,请注明出处:https://yundeesoft.com/25280.html

(0)
上一篇 2023-07-30 09:00
下一篇 2023-07-30 13:00

相关推荐

发表回复

您的邮箱地址不会被公开。 必填项已用 * 标注

关注微信