Why was Map<> in Java designed to require two types; Map<> and HashMap<>?

Question

My primary language has been C#, though lately I've been doing more Java development. In C#, I can define a Dictionary like this:

using System.Collections.Generic;

...

Dictionary<string, string> myDict = new Dictionary<string, string>();

However, if I want to create a similar object in Java, I need to do this:

import java.utils.Map;
import java.utils.HashMap;

...

Map<String, String> myMap = new HashMap<String, String>();

Why is Java designed so that Map<> is created with a HashMap<> and two different imports are required to use it?

Just curious.

Update

It never even crossed my mind that Map could be an interface. It doesn't follow the convention of prefixing the interface name with an I. I'm surprised that such a convention isn't used there.

• user330315: You could just write HashMap<String, String> myMap = new HashMap<String, String>(); and then you only need one import.
• Pshemo: Take a look at What does it mean to "program to an interface"?
• Svetlin Zarev: I'm pretty sure that he knows what's an interface and how it's used :)
• Kevin Panko: @yshavit Yes, in C# you would use the IDictionary interface just that way. The statement using System.Collections.Generic makes both IDictionary and Dictionary available, because they are both in that namespace, so only one using is needed. It is similar to the way that import java.utils.* does the job for both Map and HashMap.
• Chris Dodd: @quakkels: Because Java wasn't created by Microsoft, so doesn't use MS notation. So called Hungarian notation is pretty much only used in the Microsoft ecosystem.

Answer 1

Map is an interface, while HashMap is a concrete implementation, just like TreeMap

BTW You can use only HashMap if you like:

HashMap<k,v> hashmap = new HashMap<k,v>();

Answer 2

Map is an interface that HashMap implements. enter image description here

We can do ,

HashMap<String, String> map = new HashMap<String, String>();

and

Map<String, String> map = new HashMap<String, String>();

The advantage to using Map<String, String> is that you can change the underlying object to be a different kind of map without breaking your contract with any code that's using it. If you declare it as HashMap<String, String>, you have to change your contract if you want to change the underlying implementation.

Answer 3

It wasn't 'designed to require two types', but it is an interface, and any interface requires an implementing class somewhere.

Answer 4

As others said, Map is an interface that HashMap implements.

Java contains three general-purpose Map implementations: HashMap, TreeMap, and LinkedHashMap

Map<String, String> myMap = new HashMap<String, String>();

will only allow the use of functions defined in the Map interface, while

HashMap<String, String> myMap = new HashMap<String, String>();

will allow the use of all public functions in HashMap (Map interface methods + hashMap methods).

update from the oracle website: http://docs.oracle.com/javase/tutorial/collections/interfaces/map.html

The Java platform contains three general-purpose Map implementations: HashMap, TreeMap, and LinkedHashMap. Their behavior and performance are precisely analogous to HashSet, TreeSet, and LinkedHashSet, as described in The Set Interface section.

but as mentioned in the comments below, java has actually more Map implementations: http://docs.oracle.com/javase/7/docs/api/java/util/Map.html

• user207421: There are more than three.
• yshavit: ... and EnumMap, and ConcurrentHashMap, and Properties, and others...
• Rami.Q: @EJP: see this link: docs.oracle.com/javase/tutorial/collections/interfaces/map.html
• yshavit: @Rami.Q Those are three of the "newbie-oriented" implementations, but there are more implementations. The javadoc for Map lists all of the ones that come with the JDK.
• Rami.Q: @yshavit, i know, but i wrote what oracle said, i 'll update my answer to include the others