使用Java8进行分组(多个字段的组合分组)

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摘要

目录java8分组传统写法（单个字段分组）java8分组传统写法（多个字段分组）分析：多个分组条件与单个分组条件两种写法多个字段的优雅写法再度优化在sql中经常会用到分组，

java8分组传统写法（单个字段分组）

场景：根据城市进行分组

使用的是方法引用：User::getCity 来完成分组

import java.time.LocalDateTime;
import java.time.fORMat.DateTimeFormatter;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Demo2 {
    public static void main(String[] args) {
        DateTimeFormatter df = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
        // data list
        List<User> userList = Arrays.asList(
                User.builder().id(123456).name("Zhang, San").city("ShangHai").sex("man").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build(),
                User.builder().id(777777).name("Zhang, San").city("ShangHai").sex("woman").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build(),
                User.builder().id(888888).name("Li, Si").city("ShangHai").sex("man").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build(),
                User.builder().id(999999).name("Zhan, San").city("HangZhou").sex("woman").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build(),
                User.builder().id(555555).name("Li, Si").city("NaJin").sex("man").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build()
        );
        Map<String, List<User>> groupMap = userList.stream()
        	.collect(Collectors.groupingBy(User::getCity));
        groupMap.forEach((k, v) -> {
            System.out.println(k);
            System.out.println(v);
        });
    }
}

java8分组传统写法（多个字段分组）

① 场景：根据 城市,性别进行分组

一般的写法会是下面的这种写法，通过lambda表达式将key的生成逻辑传入进去：u -> u.getCity() + "|" + u.getSex() 来实现分组的效果。

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Demo2 {
    public static void main(String[] args) {
        DateTimeFormatter df = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
        // data list
        List<User> userList = Arrays.asList(
                User.builder().id(123456).name("Zhang, San").city("ShangHai").sex("man").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build(),
                User.builder().id(777777).name("Zhang, San").city("ShangHai").sex("woman").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build(),
                User.builder().id(888888).name("Li, Si").city("ShangHai").sex("man").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build(),
                User.builder().id(999999).name("Zhan, San").city("HangZhou").sex("woman").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build(),
                User.builder().id(555555).name("Li, Si").city("NaJin").sex("man").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build()
        );
        Map<String, List<User>> groupMap = userList.stream()
        	.collect(Collectors.groupingBy(u -> u.getCity() + "|" + u.getSex()));
        groupMap.forEach((k, v) -> {
            System.out.println(k);
            System.out.println(v);
        });
    }
}

分析：多个分组条件与单个分组条件两种写法

单个条件的分组用的比较多，userList.stream().collect(Collectors.groupingBy(User::getCity));
这种方法引用的方式看起来很清爽。
在我们遇到多个字段的分组的时候，我并不太想使用前面那种传统的写法①。

我在想，既然单个字段的分组写法是：

userList.stream().collect(Collectors.groupingBy(User::getCity));

那么多个字段的写法可否是下面这种( 类推 )，传入多个方法引用！

userList.stream().collect(Collectors.groupingBy(User::getCity,User::getSex));

很可惜 jdk 类库中Collectors 没有提供这种写法

多个字段的优雅写法

因为jdk没有提供这种写法，于是自己就想写了一个Util来帮助我们使用多个方法引用的方式完成组合分组

MyBeanUtil groupingBy(userList, User::getSex, User::getCity);

Demo：

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.*;
import java.util.function.Function;
import java.util.stream.Collectors;

public class MyBeanUtil {

    public static void main(String[] args) {
        DateTimeFormatter df = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
        // data list
        List<User> userList = Arrays.asList(
                User.builder().id(123456).name("Zhang, San").city("ShangHai").sex("man").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build(),
                User.builder().id(777777).name("Zhang, San").city("ShangHai").sex("woman").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build(),
                User.builder().id(888888).name("Li, Si").city("ShangHai").sex("man").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build(),
                User.builder().id(999999).name("Zhan, San").city("HangZhou").sex("woman").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build(),
                User.builder().id(555555).name("Li, Si").city("NaJin").sex("man").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build()
        );
        // 进行分组，根据名字和城市分组
        Map<String, List<User>> groupMap = groupingBy(userList, User::getSex, User::getCity);
        //打印分组结果
        groupMap.forEach((k, v) -> {
            System.out.println(k);
            System.out.println(v);
        });
    }
    
    @SafeVarargs
    public static <T, R> Map<String, List<T>> groupingBy(List<T> list, Function<T, R>... functions) {
        return list.stream().collect(Collectors.groupingBy(t -> groupingBy(t, functions)));
    }

    
    @SafeVarargs
    public static <T, R> String groupingBy(T t, Function<T, R>... functions) {
        if (functions == null || functions.length == 0) {
            throw new NullPointerException("functions数组不可以为空");
        } else if (functions.length == 1) {
            return functions[0].apply(t).toString();
        } else {
            return Arrays.stream(functions).map(fun -> fun.apply(t).toString()).reduce((str1, str2) -> str1 + "|" + str2).get();
        }
    }
}

再度优化

依然不是很满足这种写法，因为这种写法需要借助 Util 类，不够接地气！
我更希望是下面这种接地气的写法：能够完全集成在jdk类库中

userList.stream().collect(Collectors.groupingBy(User::getCity,User::getSex));

为了达到上述的效果，那么显然我们是需要修改jdk源代码的；
于是我就将java.util.stream.Collectors源码完整copy出来，然后加入下面3个方法

    public static <T, K> Collector<T, ?, HashMap<K, List<T>>>
    groupingBy(Function<? super T, ? extends K>... classifier) {
        return groupingBy("|", classifier);
    }
    
    public static <T, K> Collector<T, ?, HashMap<K, List<T>>>
    groupingBy(String split, Function<? super T, ? extends K>... classifier) {
        return groupingBy(split, classifier, HashMap::new, toList());
    }
    
    public static <T, K, D, A, M extends Map<? super K, D>>
    Collector<T, ?, M> groupingBy(String split,
                                  Function<? super T, ? extends K>[] classifierArr,
                                  Supplier<M> mapFactory,
                                  Collector<? super T, A, D> downstream) {
        Supplier<A> downstreamSupplier = downstream.supplier();
        BiConsumer<A, ? super T> downstreaMaccumulator = downstream.accumulator();
        BiConsumer<Map<K, A>, T> accumulator = (m, t) -> {
            String key = Arrays.stream(classifierArr).map(classifier -> Objects.requireNonNull(classifier.apply(t))).map(String::valueOf).reduce((s1, s2) -> s1 + split + s2).get();
            A container = m.computeIfAbsent((K) key, k -> downstreamSupplier.get());
            downstreamAccumulator.accept(container, t);
        };
        BinaryOperator<Map<K, A>> merger = Collectors.<K, A, Map<K, A>>mapMerger(downstream.combiner());
        @SuppressWarnings("unchecked")
        Supplier<Map<K, A>> mangledFactory = (Supplier<Map<K, A>>) mapFactory;

        if (downstream.characteristics().contains(Collector.Characteristics.IDENTITY_FINISH)) {
            return new CollectorImpl<>(mangledFactory, accumulator, merger, CH_ID);
        } else {
            @SuppressWarnings("unchecked")
            Function<A, A> downstreamFinisher = (Function<A, A>) downstream.finisher();
            Function<Map<K, A>, M> finisher = intermediate -> {
                intermediate.replaceAll((k, v) -> downstreamFinisher.apply(v));
                @SuppressWarnings("unchecked")
                M castResult = (M) intermediate;
                return castResult;
            };
            return new CollectorImpl<>(mangledFactory, accumulator, merger, finisher, CH_NOID);
        }
    }

就达到了我们预期的效果，为了方便大家也一起体验一下，我已经将demo完整的放到了GitHub上

源码地址：https://github.com/1015770492/CollectorsDemo

下载好源码后，找到下面这个类

在这里插入图片描述

Demo：

import java.io.Serializable;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.*;

public class MultiGroupByDemo {


    public static void main(String[] args) {
        DateTimeFormatter df = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
        // data list
        List<User> userList = Arrays.asList(
                User.builder().id(123456).name("Zhang, San").city("ShangHai").sex("man").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build(),
                User.builder().id(777777).name("Zhang, San").city("ShangHai").sex("woman").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build(),
                User.builder().id(888888).name("Li, Si").city("ShangHai").sex("man").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build(),
                User.builder().id(999999).name("Zhan, San").city("HangZhou").sex("woman").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build(),
                User.builder().id(555555).name("Li, Si").city("NaJin").sex("man").birthDay(LocalDateTime.parse("2022-07-01 12:00:00", df)).build()
        );
        

        // 1.Use the default vertical separator
        System.out.println("Use the default vertical separator:");
        HashMap<String, List<User>> defaultSpilt = userList.stream().collect(Collectors.groupingBy(User::getName, User::getCity));
        printMap(defaultSpilt);
        System.out.println();

        // 2.Use custom delimiters
        System.out.println("Use custom delimiters:");
        userList.stream().collect(Collectors.groupingBy("--", User::getName, User::getCity, User::getId));
        HashMap<? extends Serializable, List<User>> collect = userList.stream().collect(Collectors.groupingBy("--", User::getName, User::getCity, User::getId));
        printMap(collect);
        System.out.println();

        // 3.Use custom delimiters
        System.out.println("Use custom delimiters:");
        userList.stream().collect(Collectors.groupingBy("--", User::getName, User::getCity, User::getId));
        HashMap<? extends Serializable, List<User>> collect2 = userList.stream().collect(Collectors.groupingBy(User::getName, User::getCity, User::getBirthDay));
        printMap(collect2);


    }

    public static <T> void printMap(Map<? extends Serializable, List<T>> map){
        map.forEach((k, v) -> {
            System.out.println(k);
            System.out.println(v);
        });
    }

}

在这里插入图片描述