java-jdk8的stream 流对List＜map＞和list＜对象＞集合的分组Collectors.groupingBy

 以下以map的k来分组:

public static void main(String[] args) {List<Map<String,Object>> list=new ArrayList<>();Map<String,Object> map=new HashMap<>();map.put("id",1);list.add(map);Map<String,Object> map2=new HashMap<>();map2.put("id",2);list.add(map2);Map<String,Object> map3=new HashMap<>();map3.put("id",3);list.add(map3);Map<String,Object> map4=new HashMap<>();map4.put("id111111",4);list.add(map4);//重点代码:Map<Set<String>, List<Map<String, Object>>> collect = list.stream().collect(Collectors.groupingBy(Map::keySet));collect.forEach((k,v)->{System.err.println("===========");System.err.println("k的值:");k.forEach(System.err::println);System.err.println("v的值:");v.forEach(System.err::println);});}

按照map的其中一个val值分组

public static void main(String[] args) {List<Map<String,Object>> list=new ArrayList<>();Map<String,Object> map=new HashMap<>();map.put("id",1);map.put("name","zhangsan");list.add(map);Map<String,Object> map2=new HashMap<>();map2.put("id",2);map2.put("name","wanger");list.add(map2);Map<String,Object> map3=new HashMap<>();map3.put("id",4);map3.put("name","zhangsan");list.add(map3);Map<String,Object> map4=new HashMap<>();map4.put("id",4);map4.put("name","lisi");list.add(map4);Map<Integer, List<Map<String, Object>>> id = list.stream().collect(Collectors.groupingBy(item -> {return (int)item.get("id");}));id.forEach((k,v)->{System.err.println("===========");System.err.println("k的值:"+k);System.err.println("v的值:");v.forEach(System.err::println);});}

 java对象:

Collectors.groupingBy根据一个或多个属性对集合中的项目进行分组

数据准备：

public Product(Long id, Integer num, BigDecimal price, String name, String category) {this.id = id;this.num = num;this.price = price;this.name = name;this.category = category;
}Product prod1 = new Product(1L, 1, new BigDecimal("15.5"), "面包", "零食");
Product prod2 = new Product(2L, 2, new BigDecimal("20"), "饼干", "零食");
Product prod3 = new Product(3L, 3, new BigDecimal("30"), "月饼", "零食");
Product prod4 = new Product(4L, 3, new BigDecimal("10"), "青岛啤酒", "啤酒");
Product prod5 = new Product(5L, 10, new BigDecimal("15"), "百威啤酒", "啤酒");
List<Product> prodList = Lists.newArrayList(prod1, prod2, prod3, prod4, prod5);

分组

• 按照类目分组：

  Map<String, List<Product>> prodMap= prodList.stream().collect(Collectors.groupingBy(Product::getCategory));//{"啤酒":[{"category":"啤酒","id":4,"name":"青岛啤酒","num":3,"price":10},{"category":"啤酒","id":5,"name":"百威啤酒","num":10,"price":15}],"零食":[{"category":"零食","id":1,"name":"面包","num":1,"price":15.5},{"category":"零食","id":2,"name":"饼干","num":2,"price":20},{"category":"零食","id":3,"name":"月饼","num":3,"price":30}]}

• 按照几个属性拼接分组：

  Map<String, List<Product>> prodMap = prodList.stream().collect(Collectors.groupingBy(item -> item.getCategory() + "_" + item.getName()));//{"零食_月饼":[{"category":"零食","id":3,"name":"月饼","num":3,"price":30}],"零食_面包":[{"category":"零食","id":1,"name":"面包","num":1,"price":15.5}],"啤酒_百威啤酒":[{"category":"啤酒","id":5,"name":"百威啤酒","num":10,"price":15}],"啤酒_青岛啤酒":[{"category":"啤酒","id":4,"name":"青岛啤酒","num":3,"price":10}],"零食_饼干":[{"category":"零食","id":2,"name":"饼干","num":2,"price":20}]}

• 根据不同条件分组

• Map<String, List<Product>> prodMap= prodList.stream().collect(Collectors.groupingBy(item -> {if(item.getNum() < 3) {return "3";}else {return "other";}
  }));//{"other":[{"category":"零食","id":3,"name":"月饼","num":3,"price":30},{"category":"啤酒","id":4,"name":"青岛啤酒","num":3,"price":10},{"category":"啤酒","id":5,"name":"百威啤酒","num":10,"price":15}],"3":[{"category":"零食","id":1,"name":"面包","num":1,"price":15.5},{"category":"零食","id":2,"name":"饼干","num":2,"price":20}]}

 多级分组
要实现多级分组，我们可以使用一个由双参数版本的Collectors.groupingBy工厂方法创 建的收集器，它除了普通的分类函数之外，还可以接受collector类型的第二个参数。那么要进 行二级分组的话，我们可以把一个内层groupingBy传递给外层groupingBy，并定义一个为流 中项目分类的二级标准。

Map<String, Map<String, List<Product>>> prodMap= prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.groupingBy(item -> {if(item.getNum() < 3) {return "3";}else {return "other";}
})));//{"啤酒":{"other":[{"category":"啤酒","id":4,"name":"青岛啤酒","num":3,"price":10},{"category":"啤酒","id":5,"name":"百威啤酒","num":10,"price":15}]},"零食":{"other":[{"category":"零食","id":3,"name":"月饼","num":3,"price":30}],"3":[{"category":"零食","id":1,"name":"面包","num":1,"price":15.5},{"category":"零食","id":2,"name":"饼干","num":2,"price":20}]}}

按子组收集数据

• 求总数

• Map<String, Long> prodMap = prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.counting()));//{"啤酒":2,"零食":3}
  

• 求和

  Map<String, Integer> prodMap = prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.summingInt(Product::getNum)));//{"啤酒":13,"零食":6}
  

• 把收集器的结果转换为另一种类型

• Map<String, Product> prodMap = prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.collectingAndThen(Collectors.maxBy(Comparator.comparingInt(Product::getNum)), Optional::get)));//{"啤酒":{"category":"啤酒","id":5,"name":"百威啤酒","num":10,"price":15},"零食":{"category":"零食","id":3,"name":"月饼","num":3,"price":30}}

• 联合其他收集器

Map<String, Set<String>> prodMap = prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.mapping(Product::getName, Collectors.toSet())));//{"啤酒":["青岛啤酒","百威啤酒"],"零食":["面包","饼干","月饼"]}

List<Long>常量直接使用 Long::longValue

List<Long> list=new ArrayList<>();
Map<Long, List<Long>> collect = list.stream().collect(Collectors.groupingBy(Long::longValue));

分组后获取每组第一条数据 

Employee employee1 = new Employee("松皇","总经理","100000000000");Employee employee2 = new Employee("小赵","助理","100000");Employee employee3 = new Employee("小智","打杂","20000");Employee employee4 = new Employee("小李","打杂","20000");ArrayList<Employee> employees = Lists.newArrayList(employee1, employee2, employee3, employee4);Map<String, Employee> collect =employees.stream().collect(groupingBy(Employee::getRank, Collectors.collectingAndThen(Collectors.toList(), value -> value.get(0))));System.out.println(collect);

去重排序:

 List<Map<String, Object>> Altitude =  v.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(() -> new TreeSet<Map<String, Object>>(Comparator.comparing(e->{return MapUtil.getDate(e, "Altitude");//去重字段}))), ArrayList::new)).stream().sorted(Comparator.comparing(e -> {return MapUtil.getDate(e, "Altitude");})).collect(Collectors.toList());

根据多条件去重

//根据多条件去重
list.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(a->a.getParam1+";"+a.getParam2))), ArrayList::new));