Java Hashmap的键入系统是什么？

Question

I want to store attributes of an entity with a hashmap. The value is either an built-in int or a List of String .

name : "John Smith"
attributes:
   "seniority" : (int) 7
   "tags" : List<String>("asst_prof","cs_dept")
   "another_attrib" : (int) 3

I am confused about the typing system of the Map, after reading diverging tutorials Google gives. The closest I came to was something that used String keys and Object values.

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Question: How do I create a Hashmap and insert values of int or List<String> , so that when I fetch the value, it is typecast (identified as a member of type) as either an int or a List<String> , not an Object .

I am depending on Drools Expert package, which accesses values from maps by itself , so the typecasting is not in my control .

// Same as attributes.get("jsmith").isValid()
Person( attributes["jsmith"].valid )

Answer 1

You can't. Either you use the basic form of Map that stores and returns the values as Objects, then you have to cast them yourself:

Object value = map.get(key);

if (value instanceof List<String>) {
    List<String> myList = (List<String>) value;
}

With ints, you can't store the primitive type int, but it will be auto-boxed to an Integer. So you would have to check for instanceof Integer, then call .intValue() on the Integer Object.

To get the Objects returned as the Objects they are then you have to use Generics, but you can't mix types. So you would have to create a Map of List<String> attributes and another for int attributes.

Answer 2

What you are proposing is an example of an algebraic data type . Unfortunately, these are not supported in Java.

You'll need to use Map and cast the value to either Integer (int), or List yourself.

Answer 3

In Drools, you can disable the compile time type safety for specific types if you want. In this case, Drools will work as a dynamically typed language and will resolve the types at runtime for the given type. Example:

declare Person
    @typesafe(false)
end

rule X
when
    Person( attributes["seniority"] == 7 ) // resolving seniority to Number
...

rule Y
when
    Person( attributes["tags"].size() > 1 ) // resolving tags to List
...