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);
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'}]
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'}]
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'}]
map实体示例:
List<Map> deviceList = ToLowerCaseForList(iBoardDataDao.queryDeviceProduce(params));
Map<String, List<Map>> deviceMap = deviceList.stream().collect(Collectors.groupingBy(map->{
Map result = map;
if("".equals(String.valueOf(result.get("usestate")))){
return "osNoOrder";
}else{
return String.valueOf(result.get("usestate"));
}
}));
// Map> deviceMap = deviceList.stream().collect(Collectors.groupingBy(map-> map.get("usestate"));
Map<String,Object> resultMap = new HashMap<>();
deviceMap.forEach((key,list)->{
resultMap.put(key,list.size());
});
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'}]}
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
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