@Data

@AllArgsConstructor

public class Milk {

    private Integer key;

    private String value;

}

package com.yus.util;

import java.util.*;

import java.util.stream.Collectors;

import static java.util.Comparator.comparing;

import static java.util.Comparator.comparingLong;

import static java.util.stream.Collectors.collectingAndThen;

import static java.util.stream.Collectors.toCollection;

public class main {

    public static void main(String[] args) {

        Milk milk0=new Milk(0,"a");

        Milk milk1=new Milk(1,"b");

        Milk milk2=new Milk(0,"c");

        List<Milk> list=new ArrayList<>();

        list.add(milk0);

        list.add(milk1);

        list.add(milk2);

        //去重

        List<Milk> uniqueMilk = list.stream().collect(

                collectingAndThen(

                        toCollection(() -> new TreeSet<>(comparing(n->n.getKey()))),ArrayList::new)

        );

        for (Milk m:uniqueMilk) {

            System.out.println(m.toString());

        }

    }

}

import java.util.ArrayList;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

import java.util.concurrent.ConcurrentHashMap;

import java.util.function.Function;

import java.util.function.Predicate;

import java.util.stream.Collectors;

public class TestMain {

    public static void main(String[] args) {

        List<Map<String, Object>> list = new ArrayList<>();

        Map<String, Object> map = new HashMap<>();

        map.put("id", "11");

        map.put("orderno", "No.1");

        list.add(map);

        Map<String, Object> map1 = new HashMap<>();

        map1.put("id", "2222");

        map1.put("orderno", "No.2");

        list.add(map1);

        Map<String, Object> map2 = new HashMap<>();

        map2.put("id", "33");

        map2.put("orderno", "No.2");

        list.add(map2);

        Map<String, Object> map3 = new HashMap<>();

        map3.put("id", "444");

        map3.put("orderno", "No.1");

        list.add(map3);

        Map<Object, List<Map<String, Object>>> map0 = list.stream().collect(Collectors.groupingBy(e -> e.get("orderno")));

        List<Map<String, Object>> collect = list.stream().filter(distinctByKey(n -> n.get("orderno")))

                .collect(Collectors.toList());

        collect.forEach(System.out::println);

    }

    private static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {

        ConcurrentHashMap<Object, Boolean> map = new ConcurrentHashMap<>();

        return t -> map.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;

    }

}