Java8 Stream 之groupingBy 分组详解

喜欢ヅ旅行 2023-09-30 17:00 28阅读 0赞

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<String, List<Employee>> 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<String, Long> 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<String, Double> 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<String, Long> 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<String, Long> sortedMap = new LinkedHashMap<>();
    		map.entrySet().stream().sorted(Map.Entry.<String, Long> 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<String, String> 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<String, List<String>> 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<String, Set<String>> 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<Manage, List<Employee>> 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<String, Map<String, List<Employee>>> 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<Employee> 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 <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
        
    		Set<Object> seen = ConcurrentHashMap.newKeySet();
    		return t -> seen.add(keyExtractor.apply(t));
    	}

完整源代码：

package com.stream;
     
    import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.LinkedHashMap;
    import java.util.List;
    import java.util.Map;
    import java.util.Random;
    import java.util.Set;
    import java.util.concurrent.ConcurrentHashMap;
    import java.util.function.Function;
    import java.util.function.Predicate;
    import java.util.stream.Collectors;
     
    /**
     * Java 8 Stream 之groupingBy 分组讲解
     * 
     * @author zzg
     *
     */
    public class Java8GroupBy {
        
     
    	List<Employee> employees = new ArrayList<Employee>();
     
    	/**
    	 * 数据初始化
    	 */
    	public void init() {
        
    		List<String> citys = Arrays.asList("湖南", "湖北", "江西", "广西 ");
    		for (int i = 0; i < 10; i++) {
        
    			Random random = new Random();
    			Integer index = random.nextInt(4);
    			Employee employee = new Employee(citys.get(index), "姓名" + i, (random.nextInt(4) * 10 - random.nextInt(4)),
    					(random.nextInt(4) * 1000 - random.nextInt(4)));
    			employees.add(employee);
    		}
    	}
     
    	/**
    	 * 使用java8 stream groupingBy操作,按城市分组list
    	 */
    	public void groupingByCity() {
        
    		Map<String, List<Employee>> map = employees.stream().collect(Collectors.groupingBy(Employee::getCity));
     
    		map.forEach((k, v) -> {
        
    			System.out.println(k + " = " + v);
    		});
    	}
     
    	/**
    	 * 使用java8 stream groupingBy操作,按城市分组list统计count
    	 */
    	public void groupingByCount() {
        
    		Map<String, Long> map = employees.stream()
    				.collect(Collectors.groupingBy(Employee::getCity, Collectors.counting()));
     
    		map.forEach((k, v) -> {
        
    			System.out.println(k + " = " + v);
    		});
    	}
     
    	/**
    	 * 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<String, Double> map = employees.stream()
    				.collect(Collectors.groupingBy(Employee::getCity, Collectors.averagingInt(Employee::getAge)));
     
    		map.forEach((k, v) -> {
        
    			System.out.println(k + " = " + v);
    		});
    	}
     
    	/**
    	 * 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<String, Long> 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<String, Long> sortedMap = new LinkedHashMap<>();
    		map.entrySet().stream().sorted(Map.Entry.<String, Long> comparingByValue().reversed())
    				.forEachOrdered(e -> sortedMap.put(e.getKey(), e.getValue()));
     
    		sortedMap.forEach((k, v) -> {
        
    			System.out.println(k + " = " + v);
    		});
    	}
     
    	/**
    	 * 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<String, String> map = employees.stream().collect(Collectors.groupingBy(Employee::getCity,
    				Collectors.mapping(Employee::getName, Collectors.joining(", ", "Names: [", "]"))));
     
    		map.forEach((k, v) -> {
        
    			System.out.println(k + " = " + v);
    		});
    	}
     
    	/**
    	 * 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<String, List<String>> 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);
    			});
    		});
    	}
     
    	/**
    	 * 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<String, Set<String>> 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);
    			});
    		});
    	}
     
    	/**
    	 * 使用java8 stream groupingBy操作,通过Object对象的成员分组List
    	 */
    	public void groupingByObject() {
        
    		Map<Manage, List<Employee>> map = employees.stream().collect(Collectors.groupingBy(item -> {
        
    			return new Manage(item.getName());
    		}));
     
    		map.forEach((k, v) -> {
        
    			System.out.println(k + " = " + v);
    		});
    	}
     
    	/**
    	 * 使用java8 stream groupingBy操作, 基于city 和name 实现多次分组
    	 */
    	public void groupingBys() {
        
    		Map<String, Map<String, List<Employee>>> 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);
    			});
    		});
    	}
     
    	/**
    	 * 使用java8 stream groupingBy操作, 基于Distinct 去重数据
    	 */
    	public void groupingByDistinct() {
        
    		List<Employee> 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 <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
        
    		Set<Object> seen = ConcurrentHashMap.newKeySet();
    		return t -> seen.add(keyExtractor.apply(t));
    	}
     
    	public static void main(String[] args) {
        
    		// TODO Auto-generated method stub
    		Java8GroupBy instance = new Java8GroupBy();
    		instance.init();
    		instance.groupingByCity();
    		instance.groupingByCount();
    		instance.groupingByAverage();
    		instance.groupingBySum();
    		instance.groupingByString();
    		instance.groupingByList();
    		instance.groupingBySet();
    		instance.groupingByObject();
    		instance.groupingBys();
    		instance.groupingByDistinct();
     
    	}
     
    	class Employee {
        
    		private String city;
    		private String name;
    		private Integer age;
    		private Integer amount;
     
    		public String getCity() {
        
    			return city;
    		}
     
    		public void setCity(String city) {
        
    			this.city = city;
    		}
     
    		public String getName() {
        
    			return name;
    		}
     
    		public void setName(String name) {
        
    			this.name = name;
    		}
     
    		public Integer getAge() {
        
    			return age;
    		}
     
    		public void setAge(Integer age) {
        
    			this.age = age;
    		}
     
    		public Integer getAmount() {
        
    			return amount;
    		}
     
    		public void setAmount(Integer amount) {
        
    			this.amount = amount;
    		}
     
    		public Employee(String city, String name, Integer age, Integer amount) {
        
    			super();
    			this.city = city;
    			this.name = name;
    			this.age = age;
    			this.amount = amount;
    		}
     
    		public Employee() {
        
    			super();
    		}
    	}
     
    	class Manage {
        
    		private String name;
     
    		public String getName() {
        
    			return name;
    		}
     
    		public void setName(String name) {
        
    			this.name = name;
    		}
     
    		public Manage(String name) {
        
    			super();
    			this.name = name;
    		}
     
    		public Manage() {
        
    			super();
    		}
    	}
     
    }