数据库中的模式跟基本表、视图、索引的区别
为什么ping不通某网站，但是却可以访问该网站？
A Lifelong Journey:
你就直接说服务端的限制策略是什么，直接贴配置信息，不就完事了。
Java 8 – 分组GroupBy
YuMutianlong:
{
19.99=[
Item{name='banana', qty=20, price=19.99},
Item{name='banana', qty=10, price=19.99}
],
29.99=[
Item{name='orang', qty=10, price=29.99},
Item{name='watermelon', qty=10, price=29.99}
],
9.99=[
Item{name='apple', qty=10, price=9.99},
Item{name='papaya', qty=20, price=9.99},
Item{name='apple', qty=10, price=9.99},
Item{name='apple', qty=20, price=9.99}
]
}
//group by + mapping to Set
{
19.99=[banana],
29.99=[orang, watermelon],
9.99=[papaya, apple]
}
Java 8 – 分组GroupBy
YuMutianlong:
//Group by + Count
{
papaya=1, banana=2, apple=3, orang=1, watermelon=1
}
//Group by + Sum qty
{
papaya=20, banana=30, apple=40, orang=10, watermelon=10
}
Java 8 – 分组GroupBy
YuMutianlong:
{
apple=3, banana=2, papaya=1, orange=1
}
Java 8 – 分组GroupBy
YuMutianlong:
public static void main(String[] args) {
//3 apple, 2 banana, others 1
List
new Item("apple", 10, new BigDecimal("9.99")),
new Item("banana", 20, new BigDecimal("19.99")),
new Item("orang", 10, new BigDecimal("29.99")),
new Item("watermelon", 10, new BigDecimal("29.99")),
new Item("papaya", 20, new BigDecimal("9.99")),
new Item("apple", 10, new BigDecimal("9.99")),
new Item("banana", 10, new BigDecimal("19.99")),
new Item("apple", 20, new BigDecimal("9.99"))
);
//group by price
Map
items.stream().collect(Collectors.groupingBy(Item::getPrice));
System.out.println(groupByPriceMap);
// group by price, uses 'mapping' to convert List
Map
items.stream().collect