How to instantiate a map <String, List <String>> in Java

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 anyone 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 must be assigned an instance of List as a value. You cannot receive ("myKey"). Add ("some_string_for_this_key"); the first time you get () a list of it.

So, select List, check if it is null.

If it is null, create a new list, add a line to it, return the list. If it's nothing but zero, add to it or do what you want.

Answer 2

You forgot to mention the class. Map here is a reference type and is an interface. HashMap on the other hand of equals indicates the actual type of object created and assigned to the foo reference.

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

The actual type specified here ( HashMap here) must be assigned for the reference type ( Map here), that is, if the type of the link is an interface, the type of the object must implement it. And, if the type of reference is a class, the type of the object must either be the same class or its subtype, that is, it leaves it.

Starting with Java 7, you can use the short name

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

The second method of creation is not recommended . Use 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

A map is an interface. You must tell Java which particular map class you want to create.

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

or

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

and etc.