java8中的Collectors.groupingBy用法

Collectors.groupingBy根據一個或多個屬性對集合中的項目進行分組

數據准備：

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"), "餅干", "零食");
Product prod3 = new Product(3L, 3, new BigDecimal("30"), "月餅", "零食");
Product prod4 = new Product(4L, 3, new BigDecimal("10"), "青島啤酒", "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);

分組

• 按照類目分組：

• Map<String, List<Product>> prodMap= prodList.stream().collect(Collectors.groupingBy(Product::getCategory));
  
  {"啤酒":[{"category":"啤酒","id":4,"name":"青島啤酒","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":"餅干","num":2,"price":20},{"category":"零食","id":3,"name":"月餅","num":3,"price":30}]}

• 按照幾個屬性拼接分組：

• Map<String, List<Product>> prodMap = prodList.stream().collect(Collectors.groupingBy(item -> item.getCategory() + "_" + item.getName()));
  
  {"零食_月餅":[{"category":"零食","id":3,"name":"月餅","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_青島啤酒":[{"category":"啤酒","id":4,"name":"青島啤酒","num":3,"price":10}],
  "零食_餅干":[{"category":"零食","id":2,"name":"餅干","num":2,"price":20}]}

• 根據不同條件分組

• 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":"月餅","num":3,"price":30},{"category":"啤酒","id":4,"name":"青島啤酒","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":"餅干","num":2,"price":20}]}

多級分組　　

　　要實現多級分組，我們可以使用一個由雙參數版本的Collectors.groupingBy工廠方法創 建的收集器，它除了普通的分類函數之外，還可以接受collector類型的第二個參數。那么要進 行二級分組的話，我們可以把一個內層groupingBy傳遞給外層groupingBy，並定義一個為流 中項目分類的二級標准。

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":"青島啤酒","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":"月餅","num":3,"price":30}],
"3":[{"category":"零食","id":1,"name":"面包","num":1,"price":15.5},{"category":"零食","id":2,"name":"餅干","num":2,"price":20}]}

按子組收集數據

• 求總數

• 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}

• 把收集器的結果轉換為另一種類型

• 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":"月餅","num":3,"price":30}}

• 聯合其他收集器

• Map<String, Set<String>> prodMap = prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.mapping(Product::getName, Collectors.toSet())));
  
  {"啤酒":["青島啤酒","百威啤酒"],
  "零食":["面包","餅干","月餅"]}