java8中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：

 

 

 按照項目分組：

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}]}}

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"]}