java8中的Collectors.groupingBy用法

向右看齐 2022-09-05 04:12 197阅读 0赞

转载：https://blog.csdn.net/u014231523/article/details/102535902

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"]}