实例化地图的方式 <String, List<String> &gt;在Java中

Question

I would like to instantiate Map<String, List<String>> in Java,

I tried

Map<String, List<String>> foo = new <String, List<String>>();

and

Map<String, List<String>> foo = new <String, ArrayList<String>>();

None of them work. Does any one know how to instantiate this map in Java?

Answer 1

new HashMap<String, List<String>>();

or as gparyani commented:

new HashMap<>(); // type inference

Note: each entry needs to be given an instantiated List as a value. You cannot get("myKey").add("some_string_for_this_key"); the very first time you get() a List from it.

So, fetch a List, check if it's null.

If it's null, make a new list, add the string to it, put the List back. If it's anything but null, add to it, or do what you want.

Answer 2

You forgot to mention the class. Map here is the reference type and is an Interface . HashMap on the other side of equals specifies the actual type of the Object created and assigned to the reference foo .

Map<String, List<String>> foo = new HashMap<String, List<String>>();

The actual type specified ( HashMap here) must be assignable to the reference type ( Map here) ie if the type of reference is an Interface , the Object's type must implement it. And, if the type of the reference is a Class , the Object's type must either be the same class or its subtype ie it extends from it.

From Java 7 onwards, you can use a shorthand like

Map<String, List<String>> foo = new HashMap<>();

Your second way of instantiation is not recommended. Stick to using List which is an Interface .

// Don't bind your Map to ArrayList
new TreeMap<String, ArrayList<String>>();

// Use List interface type instead
new TreeMap<String, List<String>>();

Answer 3

Map is an interface. You have to tell Java which concrete Map class you want to instantiate.

Map<String, List<String>> foo = new HashMap<String, List<String>>();

or

Map<String, List<String>> foo = new TreeMap<String, List<String>>();

etc.