val map = new java.util.HashMap[String,Int]()
map: java.util.HashMap[String,Int] = {}

map.put("a",1)
res25: Int = 0

map.get("a")
res26: Int = 1

map.get("b")
res27: Int = 0
key 不存在 Int 返回 0


val map = new java.util.HashMap[String,Integer]()
map: java.util.HashMap[String,Integer] = {}

map.put("a",1)
res28: Integer = null

map.get("a")
res29: Integer = 1

map.get("b")
res30: Integer = null