Java8 Stream 之groupingBy 分组讲解

本文主要讲解：Java 8 Stream之Collectors.groupingBy()分组示例

Collectors.groupingBy() 分组之常见用法

功能代码:

/**
	 * 使用java8 stream groupingBy操作,按城市分组list
	 */
	public void groupingByCity() {
		Map> map = employees.stream().collect(Collectors.groupingBy(Employee::getCity));

		map.forEach((k, v) -> {
			System.out.println(k + " = " + v);
		});
	}

Collectors.groupingBy() 分组之统计每个分组的count

功能代码:

/**
	 * 使用java8 stream groupingBy操作,按城市分组list统计count
	 */
	public void groupingByCount() {
		Map map = employees.stream()
				.collect(Collectors.groupingBy(Employee::getCity, Collectors.counting()));

		map.forEach((k, v) -> {
			System.out.println(k + " = " + v);
		});
	}

Collectors.groupingBy() 分组之统计分组平均值

功能代码:

/**
	 * 使用java8 stream groupingBy操作,按城市分组list并计算分组年龄平均值
	 */
	public void groupingByAverage() {
		Map map = employees.stream()
				.collect(Collectors.groupingBy(Employee::getCity, Collectors.averagingInt(Employee::getAge)));

		map.forEach((k, v) -> {
			System.out.println(k + " = " + v);
		});
	}

Collectors.groupingBy() 分组之统计分组总值

功能代码:

/**
	 * 使用java8 stream groupingBy操作,按城市分组list并计算分组销售总值
	 */
	public void groupingBySum() {
		Map map = employees.stream()
				.collect(Collectors.groupingBy(Employee::getCity, Collectors.summingLong(Employee::getAmount)));

		map.forEach((k, v) -> {
			System.out.println(k + " = " + v);
		});

		// 对Map按照分组销售总值逆序排序
		Map sortedMap = new LinkedHashMap<>();
		map.entrySet().stream().sorted(Map.Entry. comparingByValue().reversed())
				.forEachOrdered(e -> sortedMap.put(e.getKey(), e.getValue()));

		sortedMap.forEach((k, v) -> {
			System.out.println(k + " = " + v);
		});
	}

Collectors.groupingBy() 分组之Join分组List

功能代码:

/**
	 * 使用java8 stream groupingBy操作,按城市分组list并通过join操作连接分组list中的对象的name 属性使用逗号分隔
	 */
	public void groupingByString() {
		Map map = employees.stream().collect(Collectors.groupingBy(Employee::getCity,
				Collectors.mapping(Employee::getName, Collectors.joining(", ", "Names: [", "]"))));

		map.forEach((k, v) -> {
			System.out.println(k + " = " + v);
		});
	}

Collectors.groupingBy() 分组之转换分组结果List -> List

功能代码:

/**
	 * 使用java8 stream groupingBy操作,按城市分组list,将List转化为name的List
	 */
	public void groupingByList() {
		Map> map = employees.stream().collect(
				Collectors.groupingBy(Employee::getCity, Collectors.mapping(Employee::getName, Collectors.toList())));

		map.forEach((k, v) -> {
			System.out.println(k + " = " + v);
			v.stream().forEach(item -> {
				System.out.println("item = " + item);
			});
		});
	}

Collectors.groupingBy() 分组之转换分组结果List -> Set

功能代码:

/**
	 * 使用java8 stream groupingBy操作,按城市分组list,将List转化为name的Set
	 */
	public void groupingBySet() {
		Map> map = employees.stream().collect(
				Collectors.groupingBy(Employee::getCity, Collectors.mapping(Employee::getName, Collectors.toSet())));

		map.forEach((k, v) -> {
			System.out.println(k + " = " + v);
			v.stream().forEach(item -> {
				System.out.println("item = " + item);
			});
		});
	}

Collectors.groupingBy() 分组之使用对象分组List

功能代码:

/**
	 * 使用java8 stream groupingBy操作,通过Object对象的成员分组List
	 */
	public void groupingByObject() {
		Map> map = employees.stream().collect(Collectors.groupingBy(item -> {
			return new Manage(item.getName());
		}));

		map.forEach((k, v) -> {
			System.out.println(k + " = " + v);
		});
	}

Collectors.groupingBy() 分组之使用两个成员分组List

功能代码:

/**
	 * 使用java8 stream groupingBy操作, 基于city 和name 实现多次分组
	 */
	public void groupingBys() {
		Map>> map = employees.stream()
				.collect(Collectors.groupingBy(Employee::getCity, Collectors.groupingBy(Employee::getName)));

		map.forEach((k, v) -> {
			System.out.println(k + " = " + v);
			v.forEach((i, j) -> {
				System.out.println(i + " = " + j);
			});
		});
	}

自定义Distinct对结果去重

功能代码

/**
	 * 使用java8 stream groupingBy操作, 基于Distinct 去重数据
	 */
	public void groupingByDistinct() {
		List list = employees.stream().filter(distinctByKey(Employee :: getCity))
				.collect(Collectors.toList());;

		list.stream().forEach(item->{
			System.out.println("city = " + item.getCity());
		});
		
		
	}

	/**
	 * 自定义重复key 规则
	 * @param keyExtractor
	 * @return
	 */
	private static  Predicate distinctByKey(Function keyExtractor) {
		Set