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Java8 分组

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在SQL中经常会用到分组,我们也常常遇到一些组合分组的场景。

有下面的一个User类

import lombok.Builder;
import lombok.Data;
import java.time.LocalDateTime;

@Data
@Builder
public class User {
    private String name;
    private int id;
    private String city;
    private String sex;
    private LocalDateTime birthDay;
}

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)

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);
        });
    }
    /**
     * 将数据分组,根据方法引用(bean的get方法)
     *
     * @param list      为分组的数据
     * @param functions get方法数组
     */
    @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)));
    }

    /**
     * 分组工具根据函数式接口使用分组,将数据根据分组结果进行拆分
     */
    @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()
        );
        /*
         * maybe we can
         */

        // 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

最后我希望这个特性能被JDK所吸收,这样可以方便大家更好的使用这些好用的特性

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