Collectors.groupingBy配合Stream流使用，可以对集合中一个或多个属性进行分组，分组后还可以做聚合运算。

首先把数据放入集合：

	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");Product prod6 = new Product(5L, 7, new BigDecimal("25"), "百威啤酒", "啤酒");List<Product> prodList = List.of(prod1, prod2, prod3, prod4, prod5, prod6);

按照类别分组

Map<String, List<Product>> map1 = prodList.stream().collect(Collectors.groupingBy(Product::getCategory));
Set<Map.Entry<String, List<Product>>> entries1 = map1.entrySet();
for (Map.Entry<String, List<Product>> entry : entries1) {System.out.println(entry);
}

输出结果：

啤酒=[Product{id=4, num=3, price=10, name='青岛啤酒', category='啤酒'}, Product{id=5, num=10, price=15, name='百威啤酒', category='啤酒'}, Product{id=5, num=7, price=25, name='百威啤酒', category='啤酒'}]零食=[Product{id=1, num=1, price=15.5, name='面包', category='零食'}, Product{id=2, num=2, price=20, name='饼干', category='零食'}, Product{id=3, num=3, price=30, name='月饼', category='零食'}]

按照多个属性拼接分组

Map<String, List<Product>> map2 = prodList.stream().collect(Collectors.groupingBy(item -> item.getCategory() + "_" + item.getName()));
Set<Map.Entry<String, List<Product>>> entries2 = map2.entrySet();
for (Map.Entry<String, List<Product>> entry : entries2) {System.out.println(entry);
}

输出结果（可以看到百威啤酒那组有两个对象）：

零食_月饼=[Product{id=3, num=3, price=30, name='月饼', category='零食'}]零食_面包=[Product{id=1, num=1, price=15.5, name='面包', category='零食'}]啤酒_百威啤酒=[Product{id=5, num=10, price=15, name='百威啤酒', category='啤酒'}, Product{id=5, num=7, price=25, name='百威啤酒', category='啤酒'}]啤酒_青岛啤酒=[Product{id=4, num=3, price=10, name='青岛啤酒', category='啤酒'}]零食_饼干=[Product{id=2, num=2, price=20, name='饼干', category='零食'}]

根据不同条件分组

这里按照num数值的大小来分

Map<String, List<Product>> map3 = prodList.stream().collect(Collectors.groupingBy(item -> {if (item.getNum() > 3) {return "num大于3";} else if (item.getNum() < 3) {return "num小于3";} else {return "num等于3";}
}));
Set<Map.Entry<String, List<Product>>> entries3 = map3.entrySet();
for (Map.Entry<String, List<Product>> entry : entries3) {System.out.println(entry);
}

输出结果：

num小于3=[Product{id=1, num=1, price=15.5, name='面包', category='零食'}, Product{id=2, num=2, price=20, name='饼干', category='零食'}]num等于3=[Product{id=3, num=3, price=30, name='月饼', category='零食'}, Product{id=4, num=3, price=10, name='青岛啤酒', category='啤酒'}]num大于3=[Product{id=5, num=10, price=15, name='百威啤酒', category='啤酒'}, Product{id=5, num=7, price=25, name='百威啤酒', category='啤酒'}]

多级分组

这里先按照类别分组，再按照num分组

Map<String, Map<String, List<Product>>> map4 = prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.groupingBy(item -> {if (item.getNum() >= 8) {return "num大于等于8";} else {return "num小于8";}})));
Set<Map.Entry<String, Map<String, List<Product>>>> entries4 = map4.entrySet();
for (Map.Entry<String, Map<String, List<Product>>> entry : entries4) {System.out.println(entry);
}

输出结果：

啤酒={num小于8=[Product{id=4, num=3, price=10, name='青岛啤酒', category='啤酒'}, Product{id=5, num=7, price=25, name='百威啤酒', category='啤酒'}], num大于等于8=[Product{id=5, num=10, price=15, name='百威啤酒', category='啤酒'}]}零食={num小于8=[Product{id=1, num=1, price=15.5, name='面包', category='零食'}, Product{id=2, num=2, price=20, name='饼干', category='零食'}, Product{id=3, num=3, price=30, name='月饼', category='零食'}]}

分组后再聚合求总数

这里先按照类别分组，再聚合求总数

Map<String, Long> map5 = prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.counting()));
Set<String> strings5 = map5.keySet();
for (String s : strings5) {System.out.println(s + "---" + "总数：" + map5.get(s));
}

输出结果：

啤酒---总数：3
零食---总数：3

分组后再做相加运算

这里先按照类别分组，再聚合运算（把num相加）

Map<String, Integer> map6 = prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.summingInt(Product::getNum)));
Set<String> strings6 = map6.keySet();
for (String s : strings6) {System.out.println(s + "---" + "num相加后：" + map6.get(s));
}

输出结果：

啤酒---num相加后：20
零食---num相加后：6