jdk1.8：stream流式分組groupby

package com.example.apidemo.jdk18;

import java.math.BigDecimal;
import java.util.Arrays;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import org.apache.commons.lang3.tuple.MutablePair;

/**
 * @Author Tim
 * @Date 2021/12/17 16:24
 */
public class GroupBy {

    public static void main(String[] args) {

        //Java 8允許在接口中加入具體方法。接口中的具體方法有兩種，default方法和static方法，identity()就是Function接口的一個靜態方法。
        //Function.identity()返回一個輸出跟輸入一樣的Lambda表達式對象，等價於形如t -> t形式的Lambda表達式

        //按輸入類型，統計分組數量
        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===" +result);

        //按輸入類型，統計分組--排序
        Map<String, List<String>> result2 = items.stream().collect(Collectors.groupingBy(Function.identity(), Collectors.toList()));
        System.out.println("result2===" +result2);

        // result 排序
        Map<String, Long> finalMap = new LinkedHashMap<>();
        result.entrySet().stream().sorted(Map.Entry.<String, Long>comparingByValue().reversed())
            .forEachOrdered(e -> finalMap.put(e.getKey(), e.getValue()));
        System.out.println("finalMap===" + finalMap);


        List<User> users = Arrays.asList(
            new User("apple", 10, new BigDecimal(23)),
            new User("apple", 20, new BigDecimal(32)),

            new User("orange", 30, new BigDecimal(13)),
            new User("orange", 20, new BigDecimal(33)),
            new User("orange", 10, new BigDecimal(63)),

            new User("peach", 20, new BigDecimal(26)),
            new User("peach", 30, new BigDecimal(42)),
            new User("peach", 40, new BigDecimal(24)),
            new User("peach", 10, new BigDecimal(12))
        );

        // 按照name分組，並求出個數
        Map<String, Long> counting = users.stream().collect(Collectors.groupingBy(User::getName, Collectors.counting()));
        System.out.println("counting===="+counting);

        // 按照name分組，求和
        Map<String, Integer> sum = users.stream().collect(Collectors.groupingBy(User::getName, Collectors.summingInt(User::getQty)));
        System.out.println("sum===="+sum);

        int basePoint3 = users.stream().filter(user -> user.getName().equals("orange")).mapToInt(User::getQty).sum();
        System.out.println("basePoint3===="+ basePoint3);

        System.out.println("=================================================================");

        // 按照name分組 轉化List->Set,並且只取 getName作為新的list。
        Map<Integer, List<String>> result_list = users.stream()
            .collect(Collectors.groupingBy(User::getQty, Collectors.mapping(User::getName, Collectors.toList())));
        System.out.println("result_list===="+ result_list);

        Map<String, Set<User>> result_set = users.stream()
            .collect(Collectors.groupingBy(User::getName, Collectors.toSet()));
        System.out.println("result_set===="+ result_set);

        //按照name分組,可以簡寫，不需要,Collectors.toList()
        Map<String, List<User>> groupByPriceMap = users.stream().collect(Collectors.groupingBy(User::getName));
        System.out.println("groupByPriceMap===="+ groupByPriceMap);
    }

}

 

package com.example.apidemo.jdk18;

import java.math.BigDecimal;

public class User {

    private String name;

    private int qty;

    private BigDecimal price;

    public User() {
    }

    public User(String name, int qty, BigDecimal price) {
        this.name = name;
        this.qty = qty;
        this.price = price;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getQty() {
        return qty;
    }

    public void setQty(int qty) {
        this.qty = qty;
    }

    public BigDecimal getPrice() {
        return price;
    }

    public void setPrice(BigDecimal price) {
        this.price = price;
    }

    @Override
    public String toString() {
        return "Item{" +
                "name='" + name + '\'' +
                ", qty=" + qty +
                ", price=" + price +
                '}';
    }
}